Haploid eukaryotes?

Haploid eukaryotes?

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I was wondering whether there are any eukaryotes which never have a diploid phase. I can't think of any. Fungi have diploid stages, and I know any sexually reproducing organisms will have at least transient diploid stages (I am not referring to such transient stages- I mean extended diploid stages so I am counting as a 'haploid' organism one that only ever has two sets of chromosomes for a very short time while it undergoes meiosis.)

Update: There are plenty of eukaryotes that occur in haploid stage as the dominant life cycle stage. See metagenesis in cnidarian animals and "alternation of generation" in algae, protists and fungi…

I am not referring to such transient stages.- I mean extended diploid stages so I am counting as a 'haploid' organism one that only ever has two sets of chromosomes for a very short time while it undergoes meiosis.

  • Yes there exist a vast lot; and such life cycle pattern is called haplontic life cycle (also called haploid lifecycle)

Outline of haploid life-cycle

Most of the Green algae like Chlamydomonas, Volvox, Spirogyra etc. are prominent example.

Among fungi; Mucor, Schizosaccharomyces are some prominent example.

Among Protozoans; an example of haploid life-cycle is seen in Wood-roach(Cryptocercus)'s some gut - microfauna such as Trichonympha

(Table from Google Books, Protozoology by Karl Gottlieb Grell)

Among slime-molds, Dictyostelium shows haploid lifecycle.

Dictyostelium life cycle diagram from Biology- A dynamic Science, by Russell et al (Google Books)


  1. Advanced BIOLOGY, principles and applications, C.J. Clegg and D. G. Mackean, First Edition (1994), John Murray publications.

  2. College Botany, Vol-2, By Gangulee and Kar, New Central Book Agency, Kolkata.

Some apicomplexans such as Plasmodium spp. are haploid during their asexual stages. The organism spends more time (at least in the human host) in the asexual stage than sexual. source

Plasmodium spp. are haploid in both human and mosquito hosts except for a brief stage in the mosquito midgut where two haploid malaria gametes present in the mosquito's blood meal fuse to form a diploid zygote. This zygote immediately undergoes meiosis to generate four haploid cells that continue in the haploid form, reproducing through mitosis, until several thousand are present in a small sac, the oocyst, attached to the gut wall.

“Sexual” Population Structure and Genetics of the Malaria Agent P. falciparum

Are there single-celled (haploïd) eukaryotes with multiple chromosomes?

so I'm working on a project for my study and me and my colleague made the observation that as you go to higher levels of organisation, there are more methods of genetic variation. But then I stumbled upon the following question:

Are there single-celled (haploïd) eukaryotes with multiple chromosomes?

I know there are some single-celled eukaryotes that can fuse and perform crossover to increase genetic variability, like yeast, but yeast only has 1 chromosome. If it would have more then 1 chromosome, it could also shuffle those around. But does anyone know of a species that actually does that?

Also for anyone wondering:

omega 3(+): multicellular organisms

omega 3++ : humans (we made an exception for humans as we now use genetic engineering since a few weeks, what changes things from an evolutionary perspective)

Chromosome and Eukaryotic Chromosome: Difference

1. Structural organization of bacterial chromosome is simple and is represented mainly by double-stranded DNA molecule. Although there are specific proteins associated with bacterial chromosome (not the histories) that help stabilize its supercoiled domains. Compared to eukaryotic chromosome, one can consider bacterial chromosome to be naked DNA.

2. Bacterial chromosome is covalently closed circular structure consisting of only a single molecule of DNA (with few exceptions such as Borellia burgdorfii and Streptomyces the chromosomes of which are linear).

3. Only one bacterial chromosome occurs per bacterial cell (with few exceptions such as Rhodobacter sphaeroides, a gram-negative phototroph that possesses 2 chromosomes per cell).

4. Bacterial chromosome contains only a single copy of each gene and is therefore genetically haploid.

5. Bacterial chromosomes are shorter and contain lesser number of genes.

6. Bacterial chromosome lies free in the cell cytoplasm without any membrane to separate the chromosome from the cytoplasm., Since the ribosomes also occur free in cell cytoplasm, the process of transcription and translation are not spatially separated.

7. Except few, the bacterial DNAs do not contain introns, the noncoding sequences. As a result, the protein coding genes are not interrupted by introns and synthesize a single mRNA often containing more than one coding region. Each coding region independently synthesizes one or more proteins depending upon the number of operons (Fig. 5.37).

Difference # Eukaryotic Chromosome:

1. Structural organization of eukaryotic chromosome is complex as it contains more than just DNA. In addition to DNA large amount of histone proteins wound around DNA molecule in a very regular fashion to form structures called nucleosome. The nucleosomes aggregate to form a fibres material called chromatin, which itself further compact by folding and looping to eventually form very dense structure called chromosome.

2. Each eukaryotic chromosome is linear and consists of several pieces of DNA.

3. Eukaryotic chromosomes are more than one per cell, and this number varies with the organism. For example, Saccharomyces cerevisiae, a single-called Baker’s yeast, contains 16 chromosomes arranged in eight pairs, while human cells contain 46 chromosomes arranged in 23 pairs.

4. Eukaryotic chromosome typically contains 2 copies of each gene and is therefore genetically diploid. The diploid eukaryotic genome is halved to haploid via the process called meiosis.

5. Eukaryotic chromosomes are larger and contain greater number of genes.

6. Eukaryotic chromosome occurs in the cell nucleus, which is surrounded by nuclear membrane that separates chromosome from the cytoplasm while the ribosomes are in the cytoplasm, the processes of transaction and translation are spatially separated.

7. Eukaryotic chromosome contains both introns (noncoding sequence) and exons (coding sequence). As a result, the protein coding genes are interrupted by introns. Both introns and exons are transcribed into the primary RNA transcript from which the nature (functional) mRNA is formed by excision of introns and transported to the cytoplasm for translation (protein synthesis) (Fig. 5.38).

The Zebrafish: Cellular and Developmental Biology, Part A

A Haploidy in Evolution

In most bisexual animals, haploidy oscillates with diploidy. Diploidy ensures mitotic divisions and pluripotency throughout life, whereas haploidy exists only in the post-meiotic germline and lacks the mitotic ability, and thus represents the dead end of the life unless fertilization to restore diploidy in a zygote. Haploidy is the ancestral status of evolution. In a broad sense, all viruses and prokaryotic organisms are haploid. They often possess a single RNA or DNA molecule as their genome. In single-celled eukaryotic organisms such as yeast, the genome usually comprises several DNA molecules that are compacted with proteins into individual chromosomes. In these single-celled organisms, haploidy prevails, and cell divisions by fission or budding lead to individual propagation. This is asexual reproduction. Under defined conditions such as starvation, these haploid organisms fuse together to form transient diploid cells, which immediately undergo meiosis and become haploid cells. In plants, haploid callus and pollens in culture can form plantlets. It is well known in certain invertebrates such as the honeybee ( Heimpel and de Boer, 2008 ) that parthenogenetic haploid embryos can develop into adult animals. Therefore, haploidy in these unicellular organisms, in plants, and in invertebrates is associated with the intrinsic ability for continuous cell divisions, pluripotency in vivo, fertility, and heredity. However, haploid organisms have so far been absent in vertebrates.

Living things fall into three large groups:

, Bacteria, and Eukarya. The first two groups include non-nucleated cells, and the third contains all eukaryotes. A relatively sparse fossil record is available to help us discern what the first members of each of these lineages looked like, so it is possible that all the events that led up to the last common ancestor of extant eukaryotes will remain unknown. However, comparative biology of extant organisms and the limited fossil record provide some insights into the history of Eukarya.

The earliest fossils found appear to be bacteria, most likely cyanobacteria. They are about 3.5 billion years old and are recognizable because of their relatively complex structure and, for bacteria, relatively large cells. Most other bacteria and archaea have small cells, 1 or 2

in size, and would be difficult to pick out as fossils. Most living eukaryotes have cells measuring 10

or greater. Structures this size, which might be fossils, appear in the geological record about 2.1 billion years ago.

Characteristics of eukaryotes

Data from these fossils have led biologists to the conclusion that living eukaryotes are all descendants of a single common ancestor. Mapping the characteristics found in all major groups of eukaryotes reveals that the following characteristics must have been present in the last common ancestor, because these characteristics are present in at least

members of each major lineage.

    Cells with nuclei surrounded by a nuclear envelope with nuclear pores. This is the single characteristic that is both necessary and sufficient to define an organism as

a linear DNA molecule coiled around basic (alkaline) proteins called histones. The few eukaryotes with chromosomes lacking histones

about eukaryotes&rsquo cell walls and their development to know how much homology exists among them. If the last common ancestor could make cell walls,

this ability must have been lost

Endosymbiosis and the evolution of eukaryotes

understand eukaryotic organisms fully, it is necessary to understand that all extant eukaryotes are descendants of a chimeric organism that was a composite of a host cell and the cell

that &ldquotook up residence&rdquo inside it.

This major theme in the origin of

as , one cell engulfing another such that the engulfed cell survives and both cells benefit. Over many generations, a symbiotic relationship can

two organisms that depend on each other so completely that neither could survive on its own.

events likely contributed to the origin of the last common ancestor of today&rsquos eukaryotes and to later diversification in certain lineages of eukaryotes. Before explaining this further, it is necessary to consider metabolism in bacteria and archaea.

Bacterial and archaeal metabolism

Many important metabolic processes arose in bacteria and archaea, and some of these, such as nitrogen fixation,

in eukaryotes. The process of aerobic respiration

in all major lineages of eukaryotes, and

Aerobic respiration is also found

in many lineages of bacteria and archaea, but it is not present in all of them, and many forms of evidence suggest that such anaerobic microbes never carried out aerobic respiration nor did their ancestors.

While today&rsquos atmosphere is about one-fifth molecular oxygen (O2), geological evidence shows that it originally lacked O2. Without oxygen, aerobic respiration would not

, and living things would have relied on fermentation instead. Around 3.5 billion years ago, some bacteria and archaea began using energy from sunlight to power anabolic processes that reduce carbon dioxide to form organic compounds. They evolved the ability to photosynthesize. Hydrogen, derived from various sources,

using light-powered reactions to reduce fixed carbon dioxide in the Calvin cycle. The group of Gram-negative bacteria that gave rise to cyanobacteria used water as the hydrogen source and released O2 as a waste product.

Eventually, the amount of photosynthetic oxygen built up in some environments to levels that posed a risk to living organisms, since it can damage many organic compounds. Various metabolic processes evolved that protected organisms from oxygen one of which, aerobic respiration, also generated high levels of ATP. It became widely present among microbes, including in a group we now call alpha-

. Organisms that did not gain aerobic respiration had to remain in oxygen-free environments. Originally, oxygen-rich environments were likely localized around places where cyanobacteria were active, but by about 2 billion years ago, geological evidence shows that oxygen was building up to higher concentrations in the atmosphere. Oxygen levels similar to today&rsquos levels only arose within the last 700 million years.

Recall that the first fossils that we believe to be eukaryotes are about 2 billion years old, so they appeared as oxygen levels were increasing. Also, recall that all extant eukaryotes descended from an ancestor with mitochondria. These organelles were first observed by light microscopists in the late 1800s, where they appeared to be worm-shaped structures that seemed to move around in the cell. Some early observers suggested that they might be bacteria living inside host cells, but these hypotheses remained unknown or rejected in most scientific communities.

As cell biology developed in the twentieth century,

mitochondria were the organelles responsible for producing ATP using aerobic respiration. In the 1960s, American biologist Lynn Margulis developed , which states that eukaryotes may have been a product of one cell engulfing another (one living within another) and evolving until the separate cells were no longer recognizable

. In 1967, Margulis introduced new work on the theory and substantiated her findings through microbiological evidence. Although Margulis&rsquo work initially

with resistance, this once-revolutionary hypothesis is now widely (but not completely) accepted, with work progressing on uncovering the steps involved in this evolutionary process and the key players involved. Much remains to

about the origins of the cells that now make up the cells in all living eukaryotes.

Broadly, it has become clear that many of our nuclear genes and the molecular machinery responsible for replication and expression appear closely related to those in

metabolic organelles and genes responsible for

-harvesting processes had their origins in bacteria. Much remains to

about how this relationship occurred this continues to be an exciting field of discovery in biology. For instance, it

whether the endosymbiotic event that led to mitochondria occurred before or after the host cell had a nucleus. Such organisms would be among the extinct precursors of the last common ancestor of eukaryotes.


One of the major features distinguishing bacteria and archaea from eukaryotes is mitochondria. Eukaryotic cells may contain anywhere from one to several thousand mitochondria, depending on the cell&rsquos level of energy consumption. Each mitochondrion measures 1 to 10 or greater micrometers

and exists in the cell as an organelle that can be ovoid, worm-shaped, or intricately branched. Mitochondria arise from the division of existing mitochondria they may fuse and

around inside the cell by interactions with the cytoskeleton. However, mitochondria cannot survive outside the cell. As the atmosphere

by photosynthesis, and as successful aerobic microbes evolved, evidence suggests that an ancestral cell with some membrane compartmentalization engulfed a free-living aerobic bacterium, specifically an alpha-

giving the host cell the ability to use oxygen to release energy stored in nutrients. Alpha-

are a large group of bacteria that includes species symbiotic with plants, disease organisms that can infect humans via ticks, and many free-living species that use light for energy. Several lines of evidence support that mitochondria

from this endosymbiotic event. Most mitochondria

by two membranes, which would result when one membrane-bound organism

into a vacuole by another membrane-bound organism. The mitochondrial inner membrane is extensive and involves substantial

called cristae that resemble the textured, outer surface of alpha-

. The matrix and inner membrane are rich with the enzymes necessary for aerobic respiration.

Intraspecific Variation in DNA Content

Genomes also vary dramatically among individuals within species of diverse eukaryotic lineages (♦ fig. 1). The portion of the genome that varies ranges from polyploidization of the entire genome to insertions and deletions of megabase stretches of genomic DNA ( fig. 4). Such variation contrasts markedly with the SNP variants that are the focus of many current studies of genomic variation within species.

Substantial levels of among individual variation in DNA content have recently been found in humans, where the phenomenon is called copy number variation ( Freeman et al. 2006, Redon et al. 2006). Redon et al. (2006) found insertions and deletions of kilobase to megabase segments of DNA that lead to polymorphisms in the presence/absence of chromosome regions and genes contained within them. The scale of this variation is enormous a survey of 270 individuals found that 360 Mb (13% of the genome) varied ( Redon et al. 2006), presenting a marked contrast to the genomic conservation symbolized by the 99% similarity in orthologous sequences between humans and chimps ( Mikkelsen et al. 2005). Geneticists are struggling to conceptualize the species genome and redefine “normal” in this context as they search for the disease implications of this variability ( Kehrer-Sawatzki 2007).

The ciliate macronucleus displays intraspecific variability on the level of whole chromosomes. The macronucleus is inherited by amitosis during asexual reproduction, an imprecise mechanism resembling binary fission or budding in which chromosomes are replicated and distributed to daughter nuclei without a mitotic spindle. Amitosis can lead to differential inheritance of alleles or paralogs ( Robinson and Katz 2007) and contribute to overall elevated rates of protein evolution seen in ciliates ( Katz et al. 2004, Zufall and Katz 2007). In both cases, epigenetic mechanisms likely play a role in regulating genome dynamics. Hence, ciliates within a population may have identical, or very similar, germ line nuclei, whereas their somatic nuclei can vary in the presence/absence of chromosomes.

Populations of Entamoeba, the causative agent of amoebic dysentery in humans ( fig. 2e Stauffer and Ravdin 2003), demonstrate heterogeneity in nuclear ploidy due to varying levels of endomitosis. Entamoeba alternates between infective resting cysts with 4 haploid nuclei and metabolically active trophozoites with one or more nuclei. DNA levels within a population of trophozoites exhibit continuous variation from 4N to 40N, and this variation is present both within multinucleate individuals and among the nuclei of separate individuals ( Lohia 2003). Populations can be synchronized to 4N by starvation but achieve the same 10-fold range of variation within 2 h of addition of serum ( Lohia 2003).

The diplomonad Giardia, a causative agent of diarrhea in humans, is an anaerobic flagellate with a hypervariable karyotype whereby the number and lengths of chromosomes vary among isolates ( fig. 2f Hou et al. 1995, Adam 2000). Though Giardia is a putative asexual lineage, it has a ploidy cycle due to endoreplication and reduction ( Bernander et al. 2001). Analysis of Giardia chromosomes by pulsed field gel electrophoresis reveals several size variants for each chromosome, and there is no fixed karyotype for this species ( Le Blancq and Adam 1998). Whereas the core portions of the chromosomes appear to be stable, there is considerable variation in the subtelomeric region ( Le Blancq and Adam 1998, Adam 2000). Heterogeneity in karyotypes contrasts starkly with the almost complete lack of nucleotide heterogeneity in protein-coding genes between strains ( Morrison et al. 2007 Teodorovic et al. 2007 Lasek-Nesselquist E, personal communication). As Giardia is asexual, one would expect large amounts of allelic variation to accumulate.

Intraspecific DNA sequence variation has also been found in arbuscular mycorrhizal fungi (AMF), which supply essential nutrients to plant roots ( Smith and Read 1997). Vegetative AMF cells contain numerous nuclei, as do fungi in general however, AMF are unusual in that hundreds to thousands of nuclei appear to be transferred to each spore ( Pawlowska 2005). Within individual and within spore, genetic variation is documented for ribosomal DNA and protein-coding genes in several species of AMF (reviewed in Pawlowska [2005]). These results contradict the expected clonal population structure and haploid genome expected for fungi considered ancient asexuals ( Judson and Normark 1996). It remains to be seen whether this variation is harbored in each nuclei as either duplicated genes or polyploid genomes ( Pawlowska and Taylor 2004) or in genetically distinct nuclei (each presumably haploid) that are passed to each spore ( Kuhn et al. 2001, Hijri and Sanders 2005).


The study of reproduction and development in organisms was carried out by many botanists and zoologists.

Wilhelm Hofmeister demonstrated that alternation of generations is a feature that unites plants, and published this result in 1851 (see plant sexuality).

Some terms (haplobiont and diplobiont) used for the description of life cycles were proposed initially for algae by Nils Svedelius, and then became used for other organisms. [4] [5] Other terms (autogamy and gamontogamy) used in protist life cycles were introduced by Karl Gottlieb Grell. [6] The description of the complex life cycles of various organisms contributed to the disproof of the ideas of spontaneous generation in the 1840s and 1850s. [7]

A zygotic meiosis is a meiosis of a zygote immediately after karyogamy, which is the fusion of two cell nuclei. This way, the organism ends its diploid phase and produces several haploid cells. These cells divide mitotically to form either larger, multicellular individuals, or more haploid cells. Two opposite types of gametes (e.g., male and female) from these individuals or cells fuse to become a zygote.

In the whole cycle, zygotes are the only diploid cell mitosis occurs only in the haploid phase.

The individuals or cells as a result of mitosis are haplonts, hence this life cycle is also called haplontic life cycle. Haplonts are:

  • In archaeplastidans: some green algae (e.g., Chlamydomonas, Zygnema, Chara) [8]
  • In stramenopiles: some golden algae[8]
  • In alveolates: many dinoflagellates, e.g., Ceratium, Gymnodinium, some apicomplexans (e.g., Plasmodium) [9]
  • In rhizarians: some euglyphids, [10]ascetosporeans
  • In excavates: some parabasalids[11]
  • In amoebozoans: Dictyostelium[8]
  • In opisthokonts: most fungi (some chytrids, zygomycetes, some ascomycetes, basidiomycetes) [8][12] : 15

In gametic meiosis, instead of immediately dividing meiotically to produce haploid cells, the zygote divides mitotically to produce a multicellular diploid individual or a group of more unicellular diploid cells. Cells from the diploid individuals then undergo meiosis to produce haploid cells or gametes. Haploid cells may divide again (by mitosis) to form more haploid cells, as in many yeasts, but the haploid phase is not the predominant life cycle phase. In most diplonts, mitosis occurs only in the diploid phase, i.e. gametes usually form quickly and fuse to produce diploid zygotes.

In the whole cycle, gametes are usually the only haploid cells, and mitosis usually occurs only in the diploid phase.

The diploid multicellular individual is a diplont, hence a gametic meiosis is also called a diplontic life cycle. Diplonts are:

  • In archaeplastidans: some green algae (e.g., Cladophora glomerata, [13]Acetabularia[8] )
  • In stramenopiles: some brown algae (the Fucales, however, their life cycle can also be interpreted as strongly heteromorphic-diplohaplontic, with a highly reduced gametophyte phase, as in the flowering plants), [12] : 207 some xanthophytes (e.g., Vaucheria), [12] : 124 most diatoms, [11] some oomycetes (e.g., Saprolegnia, Plasmopara viticola), [8]opalines, [11] some "heliozoans" (e.g., Actinophrys, Actinosphaerium) [11][14]
  • In alveolates: ciliates[11]
  • In excavates: some parabasalids[11]
  • In opisthokonts: animals, some fungi (e.g., some ascomycetes) [8]

In sporic meiosis (also commonly known as intermediary meiosis), the zygote divides mitotically to produce a multicellular diploid sporophyte. The sporophyte creates spores via meiosis which also then divide mitotically producing haploid individuals called gametophytes. The gametophytes produce gametes via mitosis. In some plants the gametophyte is not only small-sized but also short-lived in other plants and many algae, the gametophyte is the "dominant" stage of the life cycle.

  • In archaeplastidans: red algae (which have two sporophyte generations), some green algae (e.g., Ulva), land plants[8]
  • In stramenopiles: most brown algae[8]
  • In rhizarians: many foraminiferans, [11]plasmodiophoromycetes[8]
  • In amoebozoa: myxogastrids
  • In opisthokonts: some fungi (some chytrids, some ascomycetes like the brewer's yeast) [8]
  • Other eukaryotes: haptophytes[11]

Some animals have a sex-determination system called haplodiploid, but this is not related to the haplodiplontic life cycle.

Some red algae (such as Bonnemaisonia [15] and Lemanea) and green algae (such as Prasiola) have vegetative meiosis, also called somatic meiosis, which is a rare phenomenon. [12] : 82 Vegetative meiosis can occur in haplodiplontic and also in diplontic life cycles. The gametophytes remain attached to and part of the sporophyte. Vegetative (non-reproductive) diploid cells undergo meiosis, generating vegetative haploid cells. These undergo many mitosis, and produces gametes.

A different phenomenon, called vegetative diploidization, a type of apomixis, occurs in some brown algae (e.g., Elachista stellaris). [16] Cells in a haploid part of the plant spontaneously duplicate their chromosomes to produce diploid tissue.

Parasites depend on the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles. Dirofilaria immitis, or the heartworm, has an indirect life cycle, for example. The microfilariae must first be ingested by a female mosquito, where it develops into the infective larval stage. The mosquito then bites an animal and transmits the infective larvae into the animal, where they migrate to the pulmonary artery and mature into adults. [17]

Those parasites that infect a single species have direct life cycles. An example of a parasite with a direct life cycle is Ancylostoma caninum, or the canine hookworm. They develop to the infective larval stage in the environment, then penetrate the skin of the dog directly and mature to adults in the small intestine. [18]

If a parasite has to infect a given host in order to complete its life cycle, then it is said to be an obligate parasite of that host sometimes, infection is facultative—the parasite can survive and complete its life cycle without infecting that particular host species. Parasites sometimes infect hosts in which they cannot complete their life cycles these are accidental hosts.

A host in which parasites reproduce sexually is known as the definitive, final or primary host. In intermediate hosts, parasites either do not reproduce or do so asexually, but the parasite always develops to a new stage in this type of host. In some cases a parasite will infect a host, but not undergo any development, these hosts are known as paratenic [19] or transport hosts. The paratenic host can be useful in raising the chance that the parasite will be transmitted to the definitive host. For example, the cat lungworm (Aelurostrongylus abstrusus) uses a slug or snail as an intermediate host the first stage larva enters the mollusk and develops to the third stage larva, which is infectious to the definitive host—the cat. If a mouse eats the slug, the third stage larva will enter the mouse's tissues, but will not undergo any development.

The primitive type of life cycle probably had haploid individuals with asexual reproduction. [11] Bacteria and archaea exhibit a life cycle like this, and some eukaryotes apparently do too (e.g., Cryptophyta, Choanoflagellata, many Euglenozoa, many Amoebozoa, some red algae, some green algae, the imperfect fungi, some rotifers and many other groups, not necessarily haploid). [20] However, these eukaryotes probably are not primitively asexual, but have lost their sexual reproduction, or it just was not observed yet. [21] [22] Many eukaryotes (including animals and plants) exhibit asexual reproduction, which may be facultative or obligate in the life cycle, with sexual reproduction occurring more or less frequently. [23]

Individual organisms participating in a biological life cycle ordinarily age and die, while cells from these organisms that connect successive life cycle generations (germ line cells and their descendants) are potentially immortal. The basis for this difference is a fundamental problem in biology. The Russian biologist and historian Zhores A. Medvedev [24] considered that the accuracy of genome replicative and other synthetic systems alone cannot explain the immortality of germ lines. Rather Medvedev thought that known features of the biochemistry and genetics of sexual reproduction indicate the presence of unique information maintenance and restoration processes at the gametogenesis stage of the biological life cycle. In particular, Medvedev considered that the most important opportunities for information maintenance of germ cells are created by recombination during meiosis and DNA repair he saw these as processes within the germ line cells that were capable of restoring the integrity of DNA and chromosomes from the types of damage that cause irreversible ageing in non-germ line cells, e.g. somatic cells.

The ancestry of each present day cell presumably traces back, in an unbroken lineage for over 3 billion years to the origin of life. It is not actually cells that are immortal but multi-generational cell lineages. [25] The immortality of a cell lineage depends on the maintenance of cell division potential. This potential may be lost in any particular lineage because of cell damage, terminal differentiation as occurs in nerve cells, or programmed cell death (apoptosis) during development. Maintenance of cell division potential of the biological life cycle over successive generations depends on the avoidance and the accurate repair of cellular damage, particularly DNA damage. In sexual organisms, continuity of the germline over successive cell cycle generations depends on the effectiveness of processes for avoiding DNA damage and repairing those DNA damages that do occur. Sexual processes in eukaryotes, as well as in prokaryotes, provide an opportunity for effective repair of DNA damages in the germ line by homologous recombination. [25] [26]

Unicellular eukaryotes as models in cell and molecular biology: critical appraisal of their past and future value

Unicellular eukaryotes have been appreciated as model systems for the analysis of crucial questions in cell and molecular biology. This includes Dictyostelium (chemotaxis, amoeboid movement, phagocytosis), Tetrahymena (telomere structure, telomerase function), Paramecium (variant surface antigens, exocytosis, phagocytosis cycle) or both ciliates (ciliary beat regulation, surface pattern formation), Chlamydomonas (flagellar biogenesis and beat), and yeast (S. cerevisiae) for innumerable aspects. Nowadays many problems may be tackled with "higher" eukaryotic/metazoan cells for which full genomic information as well as domain databases, etc., were available long before protozoa. Established molecular tools, commercial antibodies, and established pharmacology are additional advantages available for higher eukaryotic cells. Moreover, an increasing number of inherited genetic disturbances in humans have become elucidated and can serve as new models. Among lower eukaryotes, yeast will remain a standard model because of its peculiarities, including its reduced genome and availability in the haploid form. But do protists still have a future as models? This touches not only the basic understanding of biology but also practical aspects of research, such as fund raising. As we try to scrutinize, due to specific advantages some protozoa should and will remain favorable models for analyzing novel genes or specific aspects of cell structure and function. Outstanding examples are epigenetic phenomena-a field of rising interest.

Keywords: Chlamydomonas Ciliate Dictyostelium Epigentics Eukaryote Paramecium Prion Protist Tetrahymena.

Haploid eukaryotes? - Biology

The Eukaryotes: Fungi, Algae, Protozoa and Helminths

  • The DNA in eukaryotes is complexed with histones which are proteins. Eukaryote chromosomes and complexed proteins form chromatin and is located in the cells nucleus.
  • Some eukaryotic DNA can be found in chloroplasts and mitochondria. These organelles reproduce by binary fission.
  • Eukaryote asexual reproduction includes: binary fission, budding, fragmentation, spore formation and schizogony.
  • Sexual reproduction involves the formation of gametes. Fusions of gametes and formation of a zygote.
  • Algae, fungi and some protozoa reproduce both sexually and asexually
  • In meiosis a diploid parent cell creates four haploid daughter cells. The DNA has usually undergone some crossing over so the chromosomes are not only halved but are also changed through rearrangement.
  • Mitosis: Cells have two main stages in the life cycle. Interphase : the cells grow and duplicate their DNA, in the second stage the cell’s nucleus divides. In mitosis nuclear division starts after the cell has duplicated its DNA. The result is two exact copies of the DNA.
  • One of each copy goes to the new cells nucleus. Mitosis maintains the ploidy of a cell. That is a haploid nucleus undergoing mitosis creates two identical cells each with a haploid nucleus. A diploid nucleus undergoing mitosis creates two identical cells each with a haploid nucleus.
  • Asexual reproduction, in which a cell undergoes multiple mitoses creating a multinucleate cell is called a schizont. This multinucleate cell then simultaneously releases many uninucleate daughter cells called merozoites. Plasmodium, which causes malaria, reproduces this way inside red blood cells and within the liver. The huge release of merozoites results in the cyclic fever and chills associated with malaria.
  • Modern (early) 21st century classification is based on DNA and protein sequence information. This information combined with existing taxonomic information has resulted in new divisions and classifications. The kingdom Protista was reclassified into several new kingdoms: Alveolata, Euglenozoa, Diplomonadida and Parabasala. In the current classification scheme Algae are distributed in the kingdoms: Stramenophila and Rhodophyta and Plantae. Water molds belong to the kingdom of Stramonophila and slime molds are in the kingdom Mycetozoa.

The taxonomy and classification of the eukaryotes: fungi, algae, protozoa and helminths (parasitic worms e.g. tapeworms, flukes) are introduce. Meiosis (sexual reproduction) is presented in a step by step process and contrasted with mitosis.

21st and 20th Century classification methods are compared and contrasted and applied to eukaryotes.

  • Meiosis and Mitosis are described in a detailed step by step process.
  • Protozoa nutrition reproduction and classification is described.
  • Fungus morphology, nutrition, reproduction and classification is examined and described.
  • Algae as a group are examined and described in detail. The classification and unique niches these organisms live in are explored. The essential contribution that algae make to oxygen is introduced.
  • Eukaryotes have great reproductive variability and benefit from genetic recombination.
  • Concept map showing inter-connections of new concepts in this tutorial and those previously introduced.
  • Definition slides introduce terms as they are needed.
  • Visual representation of concepts
  • Animated examples—of concepts are used to step wise breakdown a concepts.
  • A concise summary is given at the conclusion of the tutorial.

In sexual reproduction two gametes having a single copy of each chromosome are united. The resultant cell is 2N or diploid. The gametes are formed in a process known as meiosis. Mitosis takes place in somatic cells, (cells not involved in sexual reproduction). In mitosis the newly made cells are an exact copy of its DNA. The cell then divides with each new cell having a diploid (2N copy of the DNA).

The taxonomy and classification of the eukaryotes: fungi, algae, protozoa and helminths (parasitic worms e.g. tapeworms, flukes) are introduce.

Eukaryote DNA and genome structure and function are presented.
Classification of eukaryotes are described in terms of traditional and modern methods.

See all 24 lessons in Anatomy and Physiology, including concept tutorials, problem drills and cheat sheets: Teach Yourself Microbiology Visually in 24 Hours


"Saccharomyces" derives from Latinized Greek and means "sugar-mold" or "sugar-fungus", saccharon (σάκχαρον) being the combining form "sugar" and myces (μύκης) being "fungus". [4] [5] cerevisiae comes from Latin and means "of beer". [6] Other names for the organism are:

  • Brewer's yeast, though other species are also used in brewing [7]
  • Ale yeast
  • Top-fermenting yeast
  • Baker's yeast[7]
  • Ragi yeast, in connection to making tapai
  • Budding yeast

This species is also the main source of nutritional yeast and yeast extract.

In the 19th century, bread bakers obtained their yeast from beer brewers, and this led to sweet-fermented breads such as the Imperial "Kaisersemmel" roll, [8] which in general lacked the sourness created by the acidification typical of Lactobacillus. However, beer brewers slowly switched from top-fermenting (S. cerevisiae) to bottom-fermenting (S. pastorianus) yeast. The Vienna Process was developed in 1846. [9] While the innovation is often popularly credited for using steam in baking ovens, leading to a different crust characteristic, it is notable for including procedures for high milling of grains (see Vienna grits [10] ), cracking them incrementally instead of mashing them with one pass as well as better processes for growing and harvesting top-fermenting yeasts, known as press-yeast. [ citation needed ]

Refinements in microbiology following the work of Louis Pasteur led to more advanced methods of culturing pure strains. In 1879, Great Britain introduced specialized growing vats for the production of S. cerevisiae, and in the United States around the turn of the century centrifuges were used for concentrating the yeast, [11] making modern commercial yeast possible, and turning yeast production into a major industrial endeavor. The slurry yeast made by small bakers and grocery shops became cream yeast, a suspension of live yeast cells in growth medium, and then compressed yeast, the fresh cake yeast that became the standard leaven for bread bakers in much of the Westernized world during the early 20th century. [ citation needed ]

During World War II, Fleischmann's developed a granulated active dry yeast for the United States armed forces, which did not require refrigeration and had a longer shelf-life and better temperature tolerance than fresh yeast it is still the standard yeast for US military recipes. The company created yeast that would rise twice as fast, cutting down on baking time. Lesaffre would later create instant yeast in the 1970s, which has gained considerable use and market share at the expense of both fresh and dry yeast in their various applications. [ citation needed ]

Ecology Edit

In nature, yeast cells are found primarily on ripe fruits such as grapes (before maturation, grapes are almost free of yeasts). [12] Since S. cerevisiae is not airborne, it requires a vector to move. [ citation needed ]

Queens of social wasps overwintering as adults (Vespa crabro and Polistes spp.) can harbor yeast cells from autumn to spring and transmit them to their progeny. [13] The intestine of Polistes dominula, a social wasp, hosts S. cerevisiae strains as well as S. cerevisiae × S. paradoxus hybrids. Stefanini et al. (2016) showed that the intestine of Polistes dominula favors the mating of S. cerevisiae strains, both among themselves and with S. paradoxus cells by providing environmental conditions prompting cell sporulation and spores germination. [14]

The optimum temperature for growth of S. cerevisiae is 30–35 °C (86–95 °F). [13]

Life cycle Edit

Two forms of yeast cells can survive and grow: haploid and diploid. The haploid cells undergo a simple lifecycle of mitosis and growth, and under conditions of high stress will, in general, die. This is the asexual form of the fungus. The diploid cells (the preferential 'form' of yeast) similarly undergo a simple lifecycle of mitosis and growth. The rate at which the mitotic cell cycle progresses often differs substantially between haploid and diploid cells. [15] Under conditions of stress, diploid cells can undergo sporulation, entering meiosis and producing four haploid spores, which can subsequently mate. This is the sexual form of the fungus. Under optimal conditions, yeast cells can double their population every 100 minutes. [16] [17] However, growth rates vary enormously both between strains and between environments. [18] Mean replicative lifespan is about 26 cell divisions. [19] [20]

In the wild, recessive deleterious mutations accumulate during long periods of asexual reproduction of diploids, and are purged during selfing: this purging has been termed "genome renewal". [21] [22]

Nutritional requirements Edit

All strains of S. cerevisiae can grow aerobically on glucose, maltose, and trehalose and fail to grow on lactose and cellobiose. However, growth on other sugars is variable. Galactose and fructose are shown to be two of the best fermenting sugars. The ability of yeasts to use different sugars can differ depending on whether they are grown aerobically or anaerobically. Some strains cannot grow anaerobically on sucrose and trehalose.

All strains can use ammonia and urea as the sole nitrogen source, but cannot use nitrate, since they lack the ability to reduce them to ammonium ions. They can also use most amino acids, small peptides, and nitrogen bases as nitrogen sources. Histidine, glycine, cystine, and lysine are, however, not readily used. S. cerevisiae does not excrete proteases, so extracellular protein cannot be metabolized.

Yeasts also have a requirement for phosphorus, which is assimilated as a dihydrogen phosphate ion, and sulfur, which can be assimilated as a sulfate ion or as organic sulfur compounds such as the amino acids methionine and cysteine. Some metals, like magnesium, iron, calcium, and zinc, are also required for good growth of the yeast.

Concerning organic requirements, most strains of S. cerevisiae require biotin. Indeed, a S. cerevisiae-based growth assay laid the foundation for the isolation, crystallisation, and later structural determination of biotin. Most strains also require pantothenate for full growth. In general, S. cerevisiae is prototrophic for vitamins.

Mating Edit

Yeast has two mating types, a and α (alpha), which show primitive aspects of sex differentiation. [23] As in many other eukaryotes, mating leads to genetic recombination, i.e. production of novel combinations of chromosomes. Two haploid yeast cells of opposite mating type can mate to form diploid cells that can either sporulate to form another generation of haploid cells or continue to exist as diploid cells. Mating has been exploited by biologists as a tool to combine genes, plasmids, or proteins at will. [ citation needed ]

The mating pathway employs a G protein-coupled receptor, G protein, RGS protein, and three-tiered MAPK signaling cascade that is homologous to those found in humans. This feature has been exploited by biologists to investigate basic mechanisms of signal transduction and desensitization. [ citation needed ]

Cell cycle Edit

Growth in yeast is synchronised with the growth of the bud, which reaches the size of the mature cell by the time it separates from the parent cell. In well nourished, rapidly growing yeast cultures, all the cells have buds, since bud formation occupies the whole cell cycle. Both mother and daughter cells can initiate bud formation before cell separation has occurred. In yeast cultures growing more slowly, cells lacking buds can be seen, and bud formation only occupies a part of the cell cycle. [ citation needed ]

Cytokinesis Edit

Cytokinesis enables budding yeast Saccharomyces cerevisiae to divide into two daughter cells. S. cerevisiae forms a bud which can grow throughout its cell cycle and later leaves its mother cell when mitosis has completed. [24]

S. cerevisiae is relevant to cell cycle studies because it divides asymmetrically by using a polarized cell to make two daughters with different fates and sizes. Similarly, stem cells use asymmetric division for self-renewal and differentiation. [25]

Timing Edit

For many cells, M phase does not happen until S phase is complete. However, for entry into mitosis in S. cerevisiae this is not true. Cytokinesis begins with the budding process in late G1 and is not completed until about halfway through the next cycle. The assembly of the spindle can happen before S phase has finished duplicating the chromosomes. [24] Additionally, there is a lack of clearly defined G2 in between M and S. Thus, there is a lack of extensive regulation present in higher eukaryotes. [24]

When the daughter emerges, the daughter is two-thirds the size of the mother. [26] Throughout the process, the mother displays little to no change in size. [27] The RAM pathway is activated in the daughter cell immediately after cytokinesis is complete. This pathway makes sure that the daughter has separated properly. [26]

Actomyosin ring and primary septum formation Edit

Two interdependent events drive cytokinesis in S. cerevisiae. The first event is contractile actomyosin ring (AMR) constriction and the second event is formation of the primary septum (PS), a chitinous cell wall structure that can only be formed during cytokinesis. The PS resembles in animals the process of extracellular matrix remodeling. [26] When the AMR constricts, the PS begins to grow. Disrupting AMR misorients the PS, suggesting that both have a dependent role. Additionally, disrupting the PS also leads to disruptions in the AMR, suggesting both the actomyosin ring and primary septum have an interdependent relationship. [28] [27]

The AMR, which is attached to the cell membrane facing the cytosol, consists of actin and myosin II molecules that coordinate the cells to split. [24] The ring is thought to play an important role in ingression of the plasma membrane as a contractile force. [ citation needed ]

Proper coordination and correct positional assembly of the contractile ring depends on septins, which is the precursor to the septum ring. These GTPases assemble complexes with other proteins. The septins form a ring at the site where the bud will be created during late G1. They help promote the formation of the actin-myosin ring, although this mechanism is unknown. It is suggested they help provide structural support for other necessary cytokinesis processes. [24] After a bud emerges, the septin ring forms an hourglass. The septin hourglass and the myosin ring together are the beginning of the future division site. [ citation needed ]

The septin and AMR complex progress to form the primary septum consisting of glucans and other chitinous molecules sent by vesicles from the Golgi body. [29] After AMR constriction is complete, two secondary septums are formed by glucans. How the AMR ring dissembles remains poorly unknown. [25]

Microtubules do not play as significant a role in cytokinesis compared to the AMR and septum. Disruption of microtubules did not significantly impair polarized growth. [30] Thus, the AMR and septum formation are the major drivers of cytokinesis. [ citation needed ]

Differences from fission yeast Edit
  • Budding yeast form a bud from the mother cell. This bud grows during the cell cycle and detaches fission yeast divide by forming a cell wall [24]
  • Cytokinesis begins at G1 for budding yeast, while cytokinesis begins at G2 for fission yeast. Fission yeast “select” the midpoint, whereas budding yeast “select” a bud site [31]
  • During early anaphase the actomyosin ring and septum continues to develop in budding yeast, in fission yeast during metaphase-anaphase the actomyosin ring begins to develop [31]

Model organism Edit

When researchers look for an organism to use in their studies, they look for several traits. Among these are size, generation time, accessibility, manipulation, genetics, conservation of mechanisms, and potential economic benefit. The yeast species S. pombe and S. cerevisiae are both well studied these two species diverged approximately 600 to 300 million years ago , and are significant tools in the study of DNA damage and repair mechanisms. [32]

S. cerevisiae has developed as a model organism because it scores favorably on a number of these criteria.

  • As a single-cell organism, S. cerevisiae is small with a short generation time (doubling time 1.25–2 hours [33] at 30 °C or 86 °F) and can be easily cultured. These are all positive characteristics in that they allow for the swift production and maintenance of multiple specimen lines at low cost.
  • S. cerevisiae divides with meiosis, allowing it to be a candidate for sexual genetics research.
  • S. cerevisiae can be transformed allowing for either the addition of new genes or deletion through homologous recombination. Furthermore, the ability to grow S. cerevisiae as a haploid simplifies the creation of gene knockout strains.
  • As a eukaryote, S. cerevisiae shares the complex internal cell structure of plants and animals without the high percentage of non-coding DNA that can confound research in higher eukaryotes.
  • S. cerevisiae research is a strong economic driver, at least initially, as a result of its established use in industry.

In the study of aging Edit

For more than five decades S. cerevisiae has been studied as a model organism to better understand aging and has contributed to the identification of more mammalian genes affecting aging than any other model organism. [34] Some of the topics studied using yeast are calorie restriction, as well as in genes and cellular pathways involved in senescence. The two most common methods of measuring aging in yeast are Replicative Life Span (RLS), which measures the number of times a cell divides, and Chronological Life Span (CLS), which measures how long a cell can survive in a non-dividing stasis state. [34] Limiting the amount of glucose or amino acids in the growth medium has been shown to increase RLS and CLS in yeast as well as other organisms. [35] At first, this was thought to increase RLS by up-regulating the sir2 enzyme, however it was later discovered that this effect is independent of sir2. Over-expression of the genes sir2 and fob1 has been shown to increase RLS by preventing the accumulation of extrachromosomal rDNA circles, which are thought to be one of the causes of senescence in yeast. [35] The effects of dietary restriction may be the result of a decreased signaling in the TOR cellular pathway. [34] This pathway modulates the cell's response to nutrients, and mutations that decrease TOR activity were found to increase CLS and RLS. [34] [35] This has also been shown to be the case in other animals. [34] [35] A yeast mutant lacking the genes sch9 and ras2 has recently been shown to have a tenfold increase in chronological lifespan under conditions of calorie restriction and is the largest increase achieved in any organism. [36] [37]

Mother cells give rise to progeny buds by mitotic divisions, but undergo replicative aging over successive generations and ultimately die. However, when a mother cell undergoes meiosis and gametogenesis, lifespan is reset. [38] The replicative potential of gametes (spores) formed by aged cells is the same as gametes formed by young cells, indicating that age-associated damage is removed by meiosis from aged mother cells. This observation suggests that during meiosis removal of age-associated damages leads to rejuvenation. However, the nature of these damages remains to be established.

During starvation of non-replicating S. cerevisiae cells, reactive oxygen species increase leading to the accumulation of DNA damages such as apurinic/apyrimidinic sites and double-strand breaks. [39] Also in non-replicating cells the ability to repair endogenous double-strand breaks declines during chronological aging. [40]

Meiosis, recombination and DNA repair Edit

S. cerevisiae reproduces by mitosis as diploid cells when nutrients are abundant. However, when starved, these cells undergo meiosis to form haploid spores. [41]

Evidence from studies of S. cerevisiae bear on the adaptive function of meiosis and recombination. Mutations defective in genes essential for meiotic and mitotic recombination in S. cerevisiae cause increased sensitivity to radiation or DNA damaging chemicals. [42] [43] For instance, gene rad52 is required for both meiotic recombination [44] and mitotic recombination. [45] Rad52 mutants have increased sensitivity to killing by X-rays, Methyl methanesulfonate and the DNA cross-linking agent 8-methoxypsoralen-plus-UVA, and show reduced meiotic recombination. [43] [44] [46] These findings suggest that recombination repair during meiosis and mitosis is needed for repair of the different damages caused by these agents.

Ruderfer et al. [42] (2006) analyzed the ancestry of natural S. cerevisiae strains and concluded that outcrossing occurs only about once every 50,000 cell divisions. Thus, it appears that in nature, mating is likely most often between closely related yeast cells. Mating occurs when haploid cells of opposite mating type MATa and MATα come into contact. Ruderfer et al. [42] pointed out that such contacts are frequent between closely related yeast cells for two reasons. The first is that cells of opposite mating type are present together in the same ascus, the sac that contains the cells directly produced by a single meiosis, and these cells can mate with each other. The second reason is that haploid cells of one mating type, upon cell division, often produce cells of the opposite mating type with which they can mate. The relative rarity in nature of meiotic events that result from outcrossing is inconsistent with the idea that production of genetic variation is the main selective force maintaining meiosis in this organism. However, this finding is consistent with the alternative idea that the main selective force maintaining meiosis is enhanced recombinational repair of DNA damage, [47] since this benefit is realized during each meiosis, whether or not out-crossing occurs.

Genome sequencing Edit

S. cerevisiae was the first eukaryotic genome to be completely sequenced. [48] The genome sequence was released to the public domain on April 24, 1996. Since then, regular updates have been maintained at the Saccharomyces Genome Database. This database is a highly annotated and cross-referenced database for yeast researchers. Another important S. cerevisiae database is maintained by the Munich Information Center for Protein Sequences (MIPS). The S. cerevisiae genome is composed of about 12,156,677 base pairs and 6,275 genes, compactly organized on 16 chromosomes. [48] Only about 5,800 of these genes are believed to be functional. It is estimated at least 31% of yeast genes have homologs in the human genome. [49] Yeast genes are classified using gene symbols (such as sch9) or systematic names. In the latter case the 16 chromosomes of yeast are represented by the letters A to P, then the gene is further classified by a sequence number on the left or right arm of the chromosome, and a letter showing which of the two DNA strands contains its coding sequence. [50]

Systematic gene names for Baker's yeast
Example gene name YGL118W
Y the Y to show this is a yeast gene
G chromosome on which the gene is located
L left or right arm of the chromosome
118 sequence number of the gene/ORF on this arm, starting at the centromere
W whether the coding sequence is on the Watson or Crick strand

  • YBR134C (aka SUP45 encoding eRF1, a translation termination factor) is located on the right arm of chromosome 2 and is the 134th open reading frame (ORF) on that arm, starting from the centromere. The coding sequence is on the Crick strand of the DNA.
  • YDL102W (aka POL3 encoding a subunit of DNA polymerase delta) is located on the left arm of chromosome 4 it is the 102nd ORF from the centromere and codes from the Watson strand of the DNA.

Gene function and interactions Edit

The availability of the S. cerevisiae genome sequence and a set of deletion mutants covering 90% of the yeast genome [51] has further enhanced the power of S. cerevisiae as a model for understanding the regulation of eukaryotic cells. A project underway to analyze the genetic interactions of all double-deletion mutants through synthetic genetic array analysis will take this research one step further. The goal is to form a functional map of the cell's processes.

As of 2010 [update] a model of genetic interactions is most comprehensive yet to be constructed, containing "the interaction profiles for

75% of all genes in the Budding yeast". [52] This model was made from 5.4 million two-gene comparisons in which a double gene knockout for each combination of the genes studied was performed. The effect of the double knockout on the fitness of the cell was compared to the expected fitness. Expected fitness is determined from the sum of the results on fitness of single-gene knockouts for each compared gene. When there is a change in fitness from what is expected, the genes are presumed to interact with each other. This was tested by comparing the results to what was previously known. For example, the genes Par32, Ecm30, and Ubp15 had similar interaction profiles to genes involved in the Gap1-sorting module cellular process. Consistent with the results, these genes, when knocked out, disrupted that process, confirming that they are part of it. [52]

From this, 170,000 gene interactions were found and genes with similar interaction patterns were grouped together. Genes with similar genetic interaction profiles tend to be part of the same pathway or biological process. [53] This information was used to construct a global network of gene interactions organized by function. This network can be used to predict the function of uncharacterized genes based on the functions of genes they are grouped with. [52]

Other tools in yeast research Edit

Approaches that can be applied in many different fields of biological and medicinal science have been developed by yeast scientists. These include yeast two-hybrid for studying protein interactions and tetrad analysis. Other resources, include a gene deletion library including

4,700 viable haploid single gene deletion strains. A GFP fusion strain library used to study protein localisation and a TAP tag library used to purify protein from yeast cell extracts. [ citation needed ]

Stanford University's yeast deletion project created knockout mutations of every gene in the S. cerevisiae genome to determine their function. [54]

Synthetic yeast genome project Edit

The international Synthetic Yeast Genome Project (Sc2.0 or Saccharomyces cerevisiae version 2.0) aims to build an entirely designer, customizable, synthetic S. cerevisiae genome from scratch that is more stable than the wild type. In the synthetic genome all transposons, repetitive elements and many introns are removed, all UAG stop codons are replaced with UAA, and transfer RNA genes are moved to a novel neochromosome. As of March 2017 [update] , 6 of the 16 chromosomes have been synthesized and tested. No significant fitness defects have been found. [55]

Astrobiology Edit

Among other microorganisms, a sample of living S. cerevisiae was included in the Living Interplanetary Flight Experiment, which would have completed a three-year interplanetary round-trip in a small capsule aboard the Russian Fobos-Grunt spacecraft, launched in late 2011. [56] [57] The goal was to test whether selected organisms could survive a few years in deep space by flying them through interplanetary space. The experiment would have tested one aspect of transpermia, the hypothesis that life could survive space travel, if protected inside rocks blasted by impact off one planet to land on another. [56] [57] [58] Fobos-Grunt's mission ended unsuccessfully, however, when it failed to escape low Earth orbit. The spacecraft along with its instruments fell into the Pacific Ocean in an uncontrolled re-entry on January 15, 2012. The next planned exposure mission in deep space using S. cerevisiae is BioSentinel. (see: List of microorganisms tested in outer space)

Brewing Edit

Saccharomyces cerevisiae is used in brewing beer, when it is sometimes called a top-fermenting or top-cropping yeast. It is so called because during the fermentation process its hydrophobic surface causes the flocs to adhere to CO2 and rise to the top of the fermentation vessel. Top-fermenting yeasts are fermented at higher temperatures than the lager yeast Saccharomyces pastorianus, and the resulting beers have a different flavor than the same beverage fermented with a lager yeast. "Fruity esters" may be formed if the yeast undergoes temperatures near 21 °C (70 °F), or if the fermentation temperature of the beverage fluctuates during the process. Lager yeast normally ferments at a temperature of approximately 5 °C (41 °F), where Saccharomyces cerevisiae becomes dormant. A variant yeast known as Saccharomyces cerevisiae var. diastaticus is a beer spoiler which can cause secondary fermentations in packaged products. [59]

In May 2013, the Oregon legislature made S. cerevisiae the official state microbe in recognition of the impact craft beer brewing has had on the state economy and the state's identity. [60]

Baking Edit

S. cerevisiae is used in baking the carbon dioxide generated by the fermentation is used as a leavening agent in bread and other baked goods. Historically, this use was closely linked to the brewing industry's use of yeast, as bakers took or bought the barm or yeast-filled foam from brewing ale from the brewers (producing the barm cake) today, brewing and baking yeast strains are somewhat different.

Nutritional yeast Edit

Saccharomyces cerevisiae is the main source of nutritional yeast, which is sold commercially as a food product. It is popular with vegans and vegetarians as an ingredient in cheese substitutes, or as a general food additive as a source of vitamins and minerals, especially amino acids and B-complex vitamins.

Uses in aquaria Edit

Owing to the high cost of commercial CO2 cylinder systems, CO2 injection by yeast is one of the most popular DIY approaches followed by aquaculturists for providing CO2 to underwater aquatic plants. The yeast culture is, in general, maintained in plastic bottles, and typical systems provide one bubble every 3–7 seconds. Various approaches have been devised to allow proper absorption of the gas into the water. [61]

Saccharomyces cerevisiae is used as a probiotic in humans and animals. Especially, a strain Saccharomyces cerevisiae var. boulardii is industrially manufactured and clinically used as a medication.

Several clinical and experimental studies have shown that Saccharomyces cerevisiae var. boulardii is, to lesser or greater extent, useful for prevention or treatment of several gastrointestinal diseases. [62] Moderate quality evidence shown Saccharomyces cerevisiae var. boulardii to reduce risk of antibiotic-associated diarrhea both in adults [63] [62] [64] and in children [63] [62] and to reduce risk of adverse effects of Helicobacter pylori eradication therapy. [65] [62] [64] Also some limited evidence support efficacy of Saccharomyces cerevisiae var. boulardii in prevention (but not treatment) of traveler's diarrhea [62] [64] and, at least as an adjunct medication, in treatment of acute diarrhea in adults and children and of persistent diarrhea in children. [62] It may also reduce symptoms of allergic rhinitis. [66]

Administration of S. cerevisiae var. boulardii is considered generally safe. [64] In clinical trials it was well tolerated by patients, and adverse effects rate was similar to that in control groups (i. e. groups with placebo or no treatment). [63] No case of S. cerevisiae var. boulardii fungemia has been reported during clinical trials. [64]

In clinical practice, however, cases of fungemia, caused by Saccharomyces cerevisiae var. boulardii are reported. [64] [62] Patients with compromised immunity or those with central vascular catheters are at special risk. Some researchers have recommended not to use Saccharomyces cerevisiae var. boulardii for treatment of such patients. [64] Others suggest only that caution must be exercised with its use in risk group patients. [62]

Saccharomyces cerevisiae is proven to be an opportunistic human pathogen, though of relatively low virulence. [67] Despite widespread use of this microorganism at home and in industry, contact with it very rarely leads to infection. [68] Saccharomyces cerevisiae was found in the skin, oral cavity, oropharinx, duodenal mucosa, digestive tract, and vagina of healthy humans [69] (one review found it to be reported for 6% of samples from human intestine [70] ). Some specialists consider S. cerevisiae to be a part of the normal microbiota of the gastrointestinal tract, the respiratory tract, and the vagina of humans, [71] while others believe that the species cannot be called a true commensal because it originates in food. [70] [72] Presence of S. cerevisiae in the human digestive system may be rather transient [72] for example, experiments show that in the case of oral administration to healthy individuals it is eliminated from the intestine within 5 days after the end of administration. [70] [68]

Under certain circumstances, such as degraded immunity, Saccharomyces cerevisiae can cause infection in humans. [68] [67] Studies show that it causes 0.45-1.06% of the cases of yeast-induced vaginitis. In some cases, women suffering from S. cerevisiae-induced vaginal infection were intimate partners of bakers, and the strain was found to be the same that their partners used for baking. As of 1999, no cases of S. cerevisiae-induced vaginitis in women, who worked in bakeries themselves, were reported in scientific literature. Some cases were linked by researchers to the use of the yeast in home baking. [67] Cases of infection of oral cavity and pharynx caused by S. cerevisiae are also known. [67]

Invasive and systemic infections Edit

Occasionally Saccharomyces cerevisiae causes invasive infections (i. e. gets into the bloodstream or other normally sterile body fluid or into a deep site tissue, such as lungs, liver or spleen) that can go systemic (involve multiple organs). Such conditions are life-threatening. [67] [72] More than 30% cases of S. cerevisiae invasive infections lead to death even if treated. [72] S. cerevisiae invasive infections, however, are much rarer than invasive infections caused by Candida albicans [67] [73] even in patients weakened by cancer. [73] S. cerevisiae causes 1% to 3.6% nosocomial cases of fungemia. [72] A comprehensive review of S. cerevisiae invasive infection cases found all patients to have at least one predisposing condition. [72]

Saccharomyces cerevisiae may enter the bloodstream or get to other deep sites of the body by translocation from oral or enteral mucosa or through contamination of intravascular catheters (e. g. central venous catheters). [71] Intravascular catheters, antibiotic therapy and compromised immunity are major predisposing factors for S. cerevisiae invasive infection. [72]

A number of cases of fungemia were caused by intentional ingestion of living S. cerevisiae cultures for dietary or therapeutic reasons, including use of Saccharomyces boulardii (a strain of S. cerevisiae which is used as a probiotic for treatment of certain forms of diarrhea). [67] [72] Saccharomices boulardii causes about 40% cases of invasive Saccharomyces infections [72] and is more likely (in comparison to other S. cerevisiae strains) to cause invasive infection in humans without general problems with immunity, [72] though such adverse effect is very rare relative to Saccharomices boulardii therapeutic administration. [74]

S. boulardii may contaminate intravascular catheters through hands of medical personnel involved in administering probiotic preparations of S. boulardii to patients. [72]

Systemic infection usually occurs in patients who have their immunity compromised due to severe illness (HIV/AIDS, leukemia, other forms of cancer) or certain medical procedures (bone marrow transplantation, abdominal surgery). [67]

A case was reported when a nodule was surgically excised from a lung of a man employed in baking business, and examination of the tissue revealed presence of Saccharomyces cerevisiae. Inhalation of dry baking yeast powder is supposed to be the source of infection in this case. [75] [72]

Virulence of different strains Edit

Not all strains of Saccharomyces cerevisiae are equally virulent towards humans. Most environmental strains are not capable of growing at temperatures above 35 °C (i. e. at temperatures of living body of humans and other mammalian). Virulent strains, however, are capable of growing at least above 37 °C and often up to 39 °C (rarely up to 42 °C). [69] Some industrial strains are also capable of growing above 37 °C. [67] European Food Safety Authority (as of 2017) requires that all S. cerevisiae strains capable of growth above 37 °C that are added to the food or feed chain in viable form must, as to be qualified presumably safe, show no resistance to antimycotic drugs used for treatment of yeast infections. [76]

The ability to grow at elevated temperatures is an important factor for strain's virulence but not the sole one. [69]

Watch the video: Haploid vs Diploid cell and Cell division (January 2023).