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What is the death mechanism of an electrocuted Mosquito?

What is the death mechanism of an electrocuted Mosquito?


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While using the Electric bat, does the mosquito get killed by primary electrocution, or it secondarily by burning?


How Does Electrocution Kill You?

Power is everywhere in the modern world, and installations are present everywhere we turn. While that means electricity is at our fingertips at all times, it also means individuals are injured and killed in electrocution accidents. In fact, it's estimated that electric shocks account for at least 300 deaths per year in the US workforce alone.

Electrocution is a sudden and involuntary introduction of large and/or continuous amount of electricity into the human body. The word “electrocution” is derived from “electro” and “execution” and was coined by US newspapers as a result of the first electric chair used in 1890. Since no English word was available for non-judicial deaths due to electric shock, the word "electrocution" ultimately took over as a description of all circumstances of electrical death from the new commercial electricity.

Before the introduction of the electric chair, there had been numerous accidental deaths from electric shock, but it wasn’t until 1879, that the first accidental death by electricity (besides lighting strikes) was recorded – when a French stage carpenter in Lyon touched a 250-volt wire.

Today, electrical hazards cause more than 300 deaths and 4,000 injuries in the U.S. workforce alone. The Occupational Safety and Health Administration (OSHA), estimates approximately one fatality per day as a result of electrocution. And the National Institute for Occupational Safety and Health (NIOSH) names electrocution as the third-leading cause of death at work amongst 16 and 17 year old workers (accounting for 12% of all workplace deaths). But the general public and even pets need to be aware of the potential dangers surrounding them.

It’s also estimated that there are more than 30,000 nonfatal electrical shock accidents annually, with some accidents going unreported. Between 2003 and 2014, fourteen deaths were registered in swimming pool electrocutions, and between 2012 and 2016, eight deaths were registered in lake electrocutions.

We know that a 3-volt battery is safe, but outlets are dangerous enough that they should be covered in order to protect small children. We also know not to use a hair dryer in the bathtub. But why? Why is water dangerous around electricity? And how does electricity actually kill you?

Volts and amperes (amps) are words used to explain the most basic electrical phenomena in our everyday surroundings. Amperage (A) is a measure of current flow, for example, what number of electrons flow through something every second. One amp is approximately 6 million trillion electrons in a second. This flow of electrons is what causes tissue or nervous system damage. Each one of those electrons going through a body either heat and burn tissues or obstruct fundamental electrical signs, for example, those that cause a heart to beat.

The latter phenomenon is the reason an electric shock over a specific amperage will make your muscles tighten and make letting go of the current source unfeasible. Being unable to let go of the current source due to a live wire is known as a tetanic contraction.

Voltage (V) is a force from an electrical circuit’s power source that pushes charged electrons (current) through a conducting loop. A quick comparison is that current resembles water atoms, and voltage resembles a slope. The more extreme the slant, the more the water wants to flow down. When there is no voltage between the two it resembles a plateau and, thus, there is no current flow.

An object's electrical resistance (measured in ohms) restricts the amount of current that any voltage can drive through it. The more grounded the resistance, the more voltage you have to push a similar measure of current. The body's normal resistance is its guard against electricity. Internal tissue has a low resistance in comparison to the skin. Consequently, a shock is not as big of an issue, but once it gets to the skin, the rest of the body is vulnerable. That clarifies why a 3-volt battery is safe, yet the electrical chair is lethal.

Dry skin is far more resistant than wet or damaged skin. That is why it can be fatal to drop an electrical appliance into the bathtub. You are one hundred times more likely to be killed if you drop a 120-volt hair dryer in the bathtub than grabbing the end units of a 12-volt car battery with dry hands. But either way, always steer clear of doing either.

Alternating current (AC) and direct current (DC) also come into play. A constant voltage between two points can drive a current that does not diverge (DC). On the other hand, a fluctuating voltage drives an alternating current (AC), in which electrons are actually being spit out of an electrical outlet and then sucked back into it roughly 60 times per second.

The oscillation rate of 60 Hz makes these currents particularly suitable to jumble up the nerves that control the heartbeat. This can cause the heart to beat erratically and lead to death.

The heart is the most susceptible organ to electrical injury. If you are interested in reading more, several mechanisms of death from electric injury, include: ventricular fibrillation, bradycardia, respiratory arrest, hyperthermia, fluid loss, metabolic acidosis, direct injury to vital structures, burns, blasts and explosions, secondary trauma, and sepsis.

That makes AC wall current a big threat to humans, yet direct current is only dangerous as the voltage and current levels increase.

Even though the warning adverts read, “Danger! High Voltage”, it’s the amperage traveling through the body that kills you. Even though the amps needed to kill you vary, any electrical device used on a home wiring circuit can, under several aspects, transmit a fatal current. Know that any amount of current over 10 milliamps (0.01 amp) is capable of producing painful to severe shock, and currents between 100 and 200 mA (0.1 to 0.2 amp) are fatal.

Finally, these are just some basic guidelines. The truth is, electrical current enters the body at the point of contact with the power source, usually a hand or the head, and it travels through the body until it exits at the nearest point of ground, generally taking the most direct route.


Key Mechanism Of DDT Resistance Found In Malarial Mosquitoes

University of Illinois researchers have identified a key detoxifying protein in Anopheles mosquitoes that metabolizes DDT, a synthetic insecticide used since World War II to control the mosquitoes that spread malaria.

The new findings, described the week of June 16 in the Proceedings of the National Academy of Sciences, reveal that a protein produced at elevated levels in DDT-resistant Anopheles gambiae mosquitoes actually metabolizes the insecticide.

Anopheles gambiae as a species includes many closely related mosquito strains that transmit the malarial parasite to humans and other animals. The A. gambiae genome, isolated from an insecticide-susceptible strain, was first published in 2002.

The protein that metabolized DDT, CYP6Z1, belongs to a class of cytochrome P450 monooxygenases (P450s) that are known to be important detoxifying agents in many species. Many studies in a variety of insect species have shown that P450s play key roles in insect defenses against plant toxins.

Using molecular modeling techniques based on the three-dimensional structure of a similar protein found in humans, principal investigator Mary A. Schuler and postdoctoral researchers Ting-Lan Chiu and Sanjeewa Rupasinghe were able to visualize the likely orientation of the molecules that allowed CYP6Z1 to bind to, and inactivate, DDT.

The researchers' model predicted that the active site of CYP6Z1 could accommodate a single molecule of DDT and inactivate it by adding oxygen to a chlorinated side group on the DDT molecule.

Their model of a similar protein, CYP6Z2, which is also produced at elevated levels in some DDT-resistant Anopheles mosquito strains, predicted that it was structurally incapable of binding -- and hence inactivating -- DDT.

Biochemical studies conducted by postdoctoral researcher Zhimou Wen confirmed that CYP6Z1 did in fact inactivate DDT while CYP6Z2 did not.

"To understand the relationship of different P450s, you really need to look at three-dimensional active site predictions in order to see what are critical variations between evolutionarily related P450s," Schuler said.

"The configuration of the CYP6Z1 active site is open enough so that DDT can come in close enough to the reactive center to be oxygenated and, therefore, disabled."

Schuler is a professor of cell and developmental biology, of biochemistry, of plant biology and of entomology and is affiliated with the Institute for Genomic Biology.

Malaria infects between 300 million and 500 million people a year, according to the World Health Organization, and is the leading cause of disease-related sickness and death in the world. Although banned in the United States, DDT is used in mosquito-control programs in many other parts of the world.

Schuler chose the CYP6Z1 protein for further study from a list of P450 genes that were transcriptionally elevated in resistant mosquitoes because its gene structure closely resembled other P450s that she and entomology department head May Berenbaum had studied in pest insects in the United States.

Much earlier work by Schuler, Berenbaum and their colleagues had identified the CYP6 family of related P450s as an important part of insects' defense against plant toxins and some insecticides. Efficient expression of these proteins allows insects to survive on host plants normally toxic to other species, and confers resistance to some insecticides.

"In the mosquito genome you've got somewhat over a hundred P450 genes, and if you can identify which ones are responsible for DDT resistance, there are many things you can do to control this pest species," Schuler said. "And if you can effectively block the actions of proteins that metabolize DDT then you can prevent the resistance levels from becoming elevated in natural populations."

By comparing models developed for the CYP6Z1 proteins in "sensitive" and "resistant" strains of A. gambiae mosquitoes, the researchers found that, from a three-dimensional perspective, the CYP6Z1 proteins were not appreciably different from one another. Variations dID occur, but often these were on the surface of the protein in regions not important for DDT metabolism.

"With biochemical analysis showing that the CYP6Z1 protein can metabolize DDT quite efficiently, you have to ask: What's the difference between the sensitive strain and the resistant strain?" Schuler said. "It has to be that these transcripts and their proteins are over-expressed in the resistant strains and, as a consequence, are allowing them to exhibit this resistance."

It is probable that exposure to potent, naturally occurring plant toxins or to synthetic insecticides causes the insects to step up production of certain P450 proteins, such as CYP6Z1, that subsequently aid in the detoxification of these compounds, Schuler said. Other studies have shown that insects encountering high levels of plant toxins in their food sources have higher levels of detoxifying proteins in their bodies, allowing them to withstand exposure to a broad range of insecticides, she said.

"There's a lot out there that still has to be learned about mosquito populations in the wild," she said.


Heritable reverse genetics in mosquitoes

Characterization of tissue-specific promoters in mosquitoes

Many of the strategies for developing parasite-resistant transgenic mosquitoes rely on the use of mosquito endogenous promoters to control when,where, and how much an engineered exogenous gene product is expressed. Genes specifically expressed in the midgut, the fat body (which produces most hemolymph proteins) and the salivary glands have been the most studied(Coates et al., 1999 Kokoza et al., 2000, 2001a,2001b Moreira et al., 2000). The Maltase-like I (MalI James et al., 1989) and Apy(Smartt et al., 1995) genes are expressed specifically in the adult salivary glands. MalI encodes an α-glucosidase that is expressed in both males and females, but only in the proximal-lateral lobes of the female salivary gland. The Apygene encodes a potential platelet anti-aggregating factor that is expressed only in the distal-lateral and medial salivary gland lobes in females. Putative promoter fragments have been cloned to the 5′-end of the luciferase gene, placed in Hermes vectors, and transformed into mosquitoes (Coates et al.,1999). Both promoters showed the proper sex-, stage- and tissue-specific expressions in transgenic animals, but their expression levels were low, which limits their use for driving effector molecules. In contrast,Moreira et al. (2000) reported a robust gut-specific expression in the 1.4 kb promoter region of the Ae. aegypti midgut carboxypeptidase (CP) gene in transgenic Ae. aegypti. Interestingly, the 3.4-kb An. gambiae CP promoter also exhibited a high level of expression in transgenic Ae. aegypti. In transgenic Anopheles stephensi, the 3.9-kb An. gambiae CPpromoter also showed high levels of expression(Ito et al., 2002 Moreira et al., 2002). However, no information is available on the composition of the Ae. aegypti and An. gambiae CP regulatory regions.

Our interest is to explore hemolymph-borne antipathogen factors. We have centered our efforts on the detailed analysis of the vitellogenin(Vg) gene as a female- and fat body-specific gene that could drive an effector molecule to a high level of expression, but only exclusively after a blood meal. Understanding the internal structure of a promoter for use in transgenic research is essential in order to reduce possible negative effects associated with the engineered gene. Knowledge of the presence and location of precise enhancer elements in the promoter allows its manipulation in order to achieve desired and predicted expression levels. We undertook a detailed analysis of the Vg promoter(Kokoza et al., 2001b). This analysis revealed three regulatory regions in the 2.1-kb upstream portion of the Vg gene. The proximal region (-121/-619) is required for the correct tissue- and stage-specific expression at a low level. The median region (-619/-1071) is responsible for a stage-specific hormonal enhancement of the Vg expression. Finally, the distal region (-1071/-2015) is necessary for the extremely high expression levels characteristic of the Vg gene (Fig. 1). The development of the Vg gene-based expression cassette that can drive the fat body-specific expression in response to a blood meal permits the testing of numerous effector molecules for their antibacterial and anti-pathogen properties.

Schematic illustration of the regulatory regions of Ae. aegypi Vitellogenin (Vg) gene. Numbers refer to nucleotide positions relative to the transcription start site, and letters refer to restriction enzyme sites: E, EcoRI R, EcoRV K, KpnI S,Sau3A. C/EBP, response element of C/EBP transcription factor EcRE,ecdysteroid response element E74 and E75, response elements for the respective early gene product of the ecdysone hierarchy GATA, response element for GATA transcription factor HNF-3/fkh, response element for HNF/forkhead factor Vg, coding region of the Vg gene. (Reproduced from Kokoza et al., 2001a,with permission.)

Schematic illustration of the regulatory regions of Ae. aegypi Vitellogenin (Vg) gene. Numbers refer to nucleotide positions relative to the transcription start site, and letters refer to restriction enzyme sites: E, EcoRI R, EcoRV K, KpnI S,Sau3A. C/EBP, response element of C/EBP transcription factor EcRE,ecdysteroid response element E74 and E75, response elements for the respective early gene product of the ecdysone hierarchy GATA, response element for GATA transcription factor HNF-3/fkh, response element for HNF/forkhead factor Vg, coding region of the Vg gene. (Reproduced from Kokoza et al., 2001a,with permission.)

Utilization of transgenesis for overexpression of antimicrobial peptides

The sequencing of the An. gambiae genome has revealed that Anopheles mosquitoes utilize fewer antimicrobial peptide (AMP)families than Drosophila(Christophides et al., 2002). The major AMP families in Anopheles are Defensins and Cecropins, each consisting of four genes. For Ae. aegypti, Defensins and Cecropins also appear to be the major AMPs(Lowenberger, 2001).

Our research has focused primarily on achieving an understanding of the mosquito immune system utilizing the reverse genetic approach through stable transformation. We have elected to focus on Ae. aegypti because this species is more amenable to transgenesis. Furthermore, Ae. aegyptieggs can be stored over 6 months, which makes it possible to maintain a large genetic stock. Ae. aegypti has long been used to study malaria because it can transmit the avian parasite Plasmodium gallinaceum. Moreover, Ae. aegypti genomic and EST projects soon will bring abundant information concerning immune genes and effector molecules in this vector mosquito (D. Severson, personal communication). Therefore, Ae.aegypti represents an outstanding model system for research on vector-pathogen interactions in mosquitoes.

To express Aedes Defensin A, we employed the Hermestransposable element as a vector and the Drosophila cinnabar gene as a marker to transform the white-eye Ae. aegypti host strain(Kokoza et al., 2000). We used the 2.1 kb 5′ upstream region of the vitellogenin (Vg)gene to drive expression of the Defensin A (DefA) gene. The Vg-DefA transgene insertion was stable and that the Vg-DefAtransgene was strongly activated in the fat body after a blood meal. The mRNA levels reached a maximum at 24 h post-blood meal, corresponding to the expression peak of the endogenous Vg gene. High levels of transgenic Defensin A were accumulated in the hemolymph of blood-fed female mosquitoes and persisted for 20-22 days after a single blood feeding(Fig. 2C). Purified transgenic Defensin A showed antibacterial activity similar to that of Defensin isolated from bacterially challenged control mosquitoes. This work made it possible to use a tissue-specific inducible promoter for the overexpression of immune genes in the center of innate humoral immunity, the fat body.

Transgenic mosquitoes with the 3xP3-EGFP selectable marker and structure of the transformation vector overexpressing the immune effector molecules and its expression in transgenic mosquitoes. (A) Expression of the 3xP3-EGFP selectable marker was observed in the eyes of the larval, pupal and adult stages of transgenic Ae. aegypti. (B) Schematic diagram of the pBac[3xP3-EGFP afm, DefA or CecA] transformation vector that was transformed into the Ae. aegypti germ line. (C) Developmental profiles of Vg-DefA and Vg mRNA expression in the fat bodies of the transgenic mosquitoes. The DefA peptide level of hemolymph was detected by western analysis. PV, previtellogenic stage. (D) The increased resistance to Enterobacter cloacae was shown by the survival test of transgenic mosquitoes. Survival rates (%) of the parental wild-type and transgenic blood-fed mosquitoes at 24 h after the injection of E. cloacae are shown. UGAL, the parental wild type. Vg-DefA, transgenic mosquitoes with the Vg-DefA transgene Vg-CefA, transgenic mosquitoes with the Vg-CefA transgene.

Transgenic mosquitoes with the 3xP3-EGFP selectable marker and structure of the transformation vector overexpressing the immune effector molecules and its expression in transgenic mosquitoes. (A) Expression of the 3xP3-EGFP selectable marker was observed in the eyes of the larval, pupal and adult stages of transgenic Ae. aegypti. (B) Schematic diagram of the pBac[3xP3-EGFP afm, DefA or CecA] transformation vector that was transformed into the Ae. aegypti germ line. (C) Developmental profiles of Vg-DefA and Vg mRNA expression in the fat bodies of the transgenic mosquitoes. The DefA peptide level of hemolymph was detected by western analysis. PV, previtellogenic stage. (D) The increased resistance to Enterobacter cloacae was shown by the survival test of transgenic mosquitoes. Survival rates (%) of the parental wild-type and transgenic blood-fed mosquitoes at 24 h after the injection of E. cloacae are shown. UGAL, the parental wild type. Vg-DefA, transgenic mosquitoes with the Vg-DefA transgene Vg-CefA, transgenic mosquitoes with the Vg-CefA transgene.

Further progress has depended on developing an efficient, routine gene transformation for vector insects by using the piggyBac transposable vector pBac[3xP3-EGFP afm] and the selectable marker EGFP under the 3xP3 promoter for transformation of Ae. aegypti(Kokoza et al., 2001a). Selection was performed from immediately hatched, first-instar larvae of G1 progeny, and this larval selection, in combination with the use of a vigorous wild-type mosquito strain, significantly improved the efficiency of the labor-intensive transgenic technique(Fig. 2A). Based on developed transgenic techniques, we have successfully generated transgenic mosquitoes containing the AMPs Defensin A and recently Cecropin A(Fig. 2B)(Kokoza et al., 2001a A. Ahmed, S. W. Shin, I. Lobkov, V. Kokoza and A. S. Raikhel, manuscript in preparation). In survival tests, transgenic mosquitoes carrying either Vg-DefA or Vg-CecA transgenes exhibited resistance to the Gram-negative bacterium Enterobacter cloacae that was nearly twice as high as that of the wild-type mosquitoes(Fig. 2D) (A. Ahmed, S. W. Shin, I. Lobkov, V. Kokoza, and A. S. Raikhel, manuscript in preparation).

Anti-malarial activities have been described for the natural cationic AMPs,Cecropins and Defensins. Studies using exogenous Defensins and Cecropins have demonstrated that these antibacterial peptides possess potent anti-Plasmodium activity(Shahabuddin et al., 1998 Gwadz et al., 1989). Furthermore, Defensin has been implicated in the local innate immune response of An. gambiae midgut to Plasmodium infection(Richman et al., 1997 Tahar et al., 2002). These observations suggest that some AMPs could be involved in anti-malarial defense and therefore could be explored as potential effector molecules in transgenic mosquitoes to block transmission of vector-borne diseases. In contrast, dsRNA knock-out of the Defensin gene in An. gambiae had no effect on the development of Plasmodium(Blandin et al., 2002). In our preliminary tests, two independent transgenic Ae. aegypti strains overexpressing Defensin A exhibited 65-70% inhibition of P. gallinaceum oocyst growth (V. A. Kokoza, M. Shahabuddin, A. Ahmed and A. S. Raikhel, unpublished results). Further studies are required to implicate AMPs in anti-Plasmodium activity.

Development of dominant negative knock-out for the IMD/Relish pathway in Ae. Aegypti

In Drosophila, three types of Rel regulatory molecules can affect the expression of numerous immune genes, including AMP genes(Hoffmann et al., 1996 Hultmark, 2003). They are involved in two distinct pathways: the Toll pathway, which activates primarily anti-fungal and anti-Gram-positive responses and is mediated by Dorsal-related Immunity Factor (Dif) and Dorsal and the Imd pathway, which is regulated by Relish and predominantly directed against Gram-negative bacteria.

We have characterized the Ae. aegypti Relish gene(Shin et al., 2002). The primary structure of the Aedes Relish gene exhibited three unique features compared with Drosophila Relish: (1) the mosquito Relish gene encodes three alternatively spliced transcripts that give rise to different proteins, (2) a `Death Domain' is present at the extreme C terminus, and (3) a short His/Gln-rich stretch followed by a long S-rich region is present at the putative N-terminal transactivation domain. Aedes Relish transcripts were induced by bacterial injection, and their product bound to κB motifs located within the promoters of insect AMP genes (Shin et al.,2002).

We generated genetically immune-deficient transgenic mosquitoes by overexpression of a dominantly negative construct of Aedes Relish(Shin et al., 2003). Relish-mediated immune deficiency (RMID) phenotype was created by transforming an Ae. aegypti with the ΔRel driven by the Vg promoter using the pBac[3xP3-EGFP afm] vector (Fig. 3A). A stable, transformed strain had a single copy of the Vg-ΔRel transgene, the expression of which was highly activated by blood feeding. These transgenic mosquitoes were extremely susceptible to infection by Gram-negative bacteria(Fig. 3B)(Shin et al., 2003). In order to establish whether or not RMID was dominant-negative, we crossed the wild-type (UGAL strain) mosquitoes with the RMID transgenic mosquitoes. The heterozygous RMID/UGAL mosquitoes exhibited the same susceptibility to the E. cloacae or E. coli infection as the RMID transgenic mosquitoes (Fig. 4A). These experiments clearly showed that the RMID phenotype originated from the Vg-ΔRel transgene and that it was genetically dominant. A hypothetical model of how dominant-negative ΔRel is expressed is presented in Fig. 5.

Structure of the transformation vector for transgenic alteration of the IMD/Relish pathway and its expression in transgenic mosquitoes. (A) Schematic diagram of the pBac[3xP3-EGFP afm, Vg-ΔRel]transformation vector that was transformed into the Ae. aegypti germ line. Three alternative spliced transcripts of Aedes Relish andΔRel structure are shown. Q/H-rich, glutamine/histidine-rich domainS-rich, serine-rich domain RHD, Rel homology region 1 IPT, Ig-like plexin transcription factor domain NLS, nuclear localization signal ANK, ankyrin domain DD, death domain. (B) Marked susceptibility to bacterial infection of the transgenic mosquitoes. Survival rates (%) of the parental wild-type (UGAL)and transgenic blood-fed mosquitoes (RMID) at 24 h after the injection of E. cloacae are shown. None of the transgenic mosquitoes at 24 h post-blood meal (PBM) survived more than 24 h, presenting the immune-deficient phenotype. (C) Resistance recovery of immune-compromised mosquitoes overexpressing the Defensin gene. Female RMID transgenic mosquitoes were mated to male Vg-DefA transgenic mosquitoes, and their progeny were subjected to survival test with E. cloacae. (Reproduced from Shin et al., 2003, with permission.)

Structure of the transformation vector for transgenic alteration of the IMD/Relish pathway and its expression in transgenic mosquitoes. (A) Schematic diagram of the pBac[3xP3-EGFP afm, Vg-ΔRel]transformation vector that was transformed into the Ae. aegypti germ line. Three alternative spliced transcripts of Aedes Relish andΔRel structure are shown. Q/H-rich, glutamine/histidine-rich domainS-rich, serine-rich domain RHD, Rel homology region 1 IPT, Ig-like plexin transcription factor domain NLS, nuclear localization signal ANK, ankyrin domain DD, death domain. (B) Marked susceptibility to bacterial infection of the transgenic mosquitoes. Survival rates (%) of the parental wild-type (UGAL)and transgenic blood-fed mosquitoes (RMID) at 24 h after the injection of E. cloacae are shown. None of the transgenic mosquitoes at 24 h post-blood meal (PBM) survived more than 24 h, presenting the immune-deficient phenotype. (C) Resistance recovery of immune-compromised mosquitoes overexpressing the Defensin gene. Female RMID transgenic mosquitoes were mated to male Vg-DefA transgenic mosquitoes, and their progeny were subjected to survival test with E. cloacae. (Reproduced from Shin et al., 2003, with permission.)

Genetically dominant phenotype of the Vg-ΔReltransgene. Female Vg-ΔRel transgenic (RMID) mosquitoes were mated to male wild-type (UGAL) mosquitoes, and their progeny were challenged with bacteria. These hybrid mosquitoes showed a marked susceptibility to live bacteria, E. cloacae and E. coli. Heat inactivation of E. cloacae was performed by incubating the bacterial suspension at 95°C for 30 min. (Reproduced from Shin et al., 2003, with permission.)

Genetically dominant phenotype of the Vg-ΔReltransgene. Female Vg-ΔRel transgenic (RMID) mosquitoes were mated to male wild-type (UGAL) mosquitoes, and their progeny were challenged with bacteria. These hybrid mosquitoes showed a marked susceptibility to live bacteria, E. cloacae and E. coli. Heat inactivation of E. cloacae was performed by incubating the bacterial suspension at 95°C for 30 min. (Reproduced from Shin et al., 2003, with permission.)


Generally, WNV is transferred between mosquitoes and birds. However, scientists noticed that between late summer and early fall the transfer of WNV from mosquitoes to humans became more common than between mosquitoes and birds.

The American robin is the primary food source of the mosquito during early summer. Image by Dakota Lynch.

Scientists knew that the transfer of WNV became more common from humans to mosquitoes, but they didn’t know why. They thought that perhaps this change was due to mosquitoes shifting their feeding behavior from birds to mammals, which included humans. If mosquitoes fed mainly on birds in the summer and then switched to humans in the fall, it could explain the increase in transmission of WNV to humans during that time of the year.


What a beheading feels like: The science, the gruesome spectacle -- and why we can't look away

By Frances Larson
Published February 3, 2015 12:00PM (EST)

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There are lots of good physiological reasons why people find heads fascinating, and powerful, and tempting to remove. The human head is a biological powerhouse and a visual delight. It accommodates four of our five senses: sight, smell, hearing and taste all take place in the head. It encases the brain, the core of our nervous system. It draws in the air we breathe and delivers the words we speak. As the evolution­ary biologist Daniel Lieberman has written, ‘Almost every particle entering your body, either to nourish you or to provide information about the world, enters via your head, and almost every activity involves something going on in your head.’

A huge number of different components are packed into our heads. The human head contains more than 20 bones, up to 32 teeth, a large brain, of course, and several sensory organs, as well as dozens of mus­cles, and numerous glands, nerves, veins, arteries and ligaments. They are all tightly configured and intensely integrated within a small space. And people’s heads look good too. The human head boasts one of the most expressive set of muscles known to life. It is adorned with vari­ous features that lend themselves to ornamentation: hair, ears, nose and lips. Thanks to an impressive concentration of nerve endings and an unrivalled ability for expressive movement, our heads connect our inner selves to the outer world more intensely than any other part of our body.

This extraordinary engine room – distinctive, dynamic and densely packed – is set on high for all to see. Our bipedal posture means that we show off our relatively round, short and wide heads on top of slim, almost vertical necks. The necks of most other animals are broader, more squat and more muscular, because they have to hold the head out in front of the body, in a forward position. The human head, because it sits on top of the spinal column, requires less musculature at the back of the neck. There is so little muscle in our necks that you can quite easily feel the main blood vessels, the lymph nodes and the ver­tebrae through the skin. In short, it is much easier to decapitate a human than a deer, or a lion, or any of the other animals that are more usually associated with hunting trophies.

Which is not to say that it is easy. Human necks may be, compared to other mammals, quite flimsy, but separating heads from bodies is still hard to do. Countless stories of botched beheadings on the scaf­fold attest to this, particularly in countries like Britain, where beheadings were relatively rare and executioners were inexperienced. The swift decapitation of a living person requires a powerful, accurate action, and a sharp, heavy blade. No wonder the severed head is the ultimate warrior’s trophy. Even when the assassin is experienced and his victim is bound, it can take many blows to cut off a person’s head. When the Comte de Lally knelt, still and blindfolded, for his execution in France in 1766, the executioner’s axe failed to sever his head. He toppled forward and had to be repositioned, and even then it took four or five blows to decapitate him. It famously took three strikes to sever the head of Mary, Queen of Scots in 1587. The first hit the back of her head, while the second left a small sinew which had to be sawn through with the axe blade. It was hard even when the victim was dead. When Oliver Cromwell’s corpse was decapitated at Tyburn, it took the axeman eight blows to cut through the layers of cerecloth that wrapped his body and finish the job.

For all its unpredictability, when it is skillfully performed on a com­pliant victim, beheading is a quick way to go, although it is impossible to be sure how quick since no one has retained consciousness long enough to provide an answer. Some experts think consciousness is lost within two seconds due to the rapid loss of blood pressure in the brain. Others suggest that consciousness evaporates as the brain uses up all the available oxygen in the blood, which probably takes around seven seconds in humans, and seven seconds is seven seconds too long if you are a recently severed head. Decapitation may be one of the least tor­turous ways to die, but nonetheless it is thought to be painful. Many scientists believe that, however swiftly it is performed, decapitation must cause acute pain for a second or two.

Decapitation in one single motion draws its cultural power from its sheer velocity, and the force of the physical feat challenges that elusive moment of death, because death is presented as instantaneous even though beheadings are still largely inscrutable to science. The his­torian Daniel Arasse has described how the guillotine, which transformed beheading into a model of efficiency, ‘sets before our eyes the invisibility of death at the very instant of its occurrence, exact and indistinguishable’. It is surprisingly easy to forget, when con­templating the mysteries of death, that decapitation is anything but invisible. Beheading is an extremely bloody business, which is one of the reasons it is no longer used for state executions in the West, even though it is one of the most humane techniques available. Decapitation is faster and more predictable than death by hanging, lethal injection, electric shock or gassing, but the spectacle is too grim for our sensibilities.

Decapitation is a contradiction in terms because it is both brutal and effective. A beheading is a vicious and defiant act of savagery, and while there may be good biological reasons why people’s heads make an attractive prize, a beheading draws part of its power from our inability to turn away. Even in a democratic, urbanized society, there will always be people who want to watch the show. Similarly, severed heads themselves often bring people together, galvanizing them in intensely emotional situations, rather than – or as well as – repelling them. Decapitation is the ultimate tyranny but it is also an act of creation, because, for all its cruelty, it produces an extra­ordinarily potent artefact that compels our attention whether we like it or not.

Even the relationship between the perpetrator and the victim can bring surprises, because there is sometimes a strange intimacy to the interaction, occasionally laced with humour, as well as sheer brutal­ity. Each different encounter with a severed head – whether it be in the context of warfare, crime, medicine or religion – can change our understanding of the act itself. People have developed countless ways to justify the fearsome appeal of the severed head. The power that it exerts over the living may well be universal. For all their gruesome nature, severed heads are also inspirational: they move people to study, to pray, to joke, to write and to draw, to turn away or to look a little closer, and to reflect on the limits of their humanity. The irre­sistible nature of the severed head may be easily exploited, but it is also dangerous to ignore. This book tells a shocking story, but it is our story nonetheless.

The scaffold is the ultimate stage, where, for centuries, life and death were acted out for real. In the mid-eighteenth century, Edmund Burke observed that theatregoers enjoying a royal tragedy would have raced to the exit at the news that a head of state was about to be executed in a nearby public square. Our fascination with real misfortune, he pointed out, is far more compelling than our interest in hardships that are merely staged. He might have said the same today, but in the digital age, the internet mediates our view of grisly executions, simultaneously keeping us at a distance and giving us front-row seats. Today, severed heads are held up for the camera and the spectators can watch at home. During the Iraq War, the extraordinary allure of beheading videos was proved for the first time, and in no uncertain terms.

As the American and British ‘war on terror’ moved across Afghanistan and into Iraq in the years following the September 11th terrorist attacks, a new mode of killing took the media by surprise: Europeans and Americans were taken hostage by Islamic militant groups, held for ransom and then beheaded, on camera. Throughout history, criminals have been decapitated for their crimes now, the crim­inals were decapitating civilians in terrifying circumstances, and graphic videos of their deaths were circulated online for anyone to see.

The first American victim was Wall Street Journal reporter Daniel Pearl, who was kidnapped in Pakistan in January 2002. His captors demanded the release of Taliban fighters in Afghanistan, in what was to become a typically unrealistic ultimatum. They beheaded Pearl on 1 February. A few weeks later the video of Pearl’s death emerged. It started to circulate online in March, and in June the Boston Phoenix newspaper provided a link to it from their website, a move which proved extremely unpopular with commentators in the United States who scorned the paper’s ‘callous disregard for human decency’, but the Boston Phoenix site nonetheless spawned a wave of further links to the video, and discussions about the rights and wrongs of viewing Pearl’s brutal death proliferated online.

The second American to be killed in this way, and the first to be beheaded in Iraq, was Nick Berg, an engineer who was kidnapped on 9 April 2004 and killed in early May. This time, two years after Pearl’s death, Reuters made the unedited video available within days, arguing that it was not within its remit to make editorial decisions on behalf of its clients. In contrast to the video of Pearl’s execution, which was only shown on CBS as a thirty-second clip, all the major US television news networks showed clips of the Berg video, although they stopped short of actually broadcasting the beheading itself. The traditional news media refrained from showing the footage in full, but by now television producers were following the crowd rather than breaking the story it was internet users who, in the privacy of their own homes, dared to watch Berg’s beheading.

Nick Berg’s execution video quickly became one of the most searched-for items on the web. The al-Qaeda-linked site that first posted the video was closed down by the Malaysian company that hosted it two days after Berg’s execution because of the overwhelming traffic to the site. Alfred Lim, senior officer of the company, said it had been closed down ‘because it had attracted a sudden surge of massive traffic that is taking up too much bandwidth and causing inconven­ience to our other clients’. Within a day, the Berg video was the top search term across search engines like Google, Lycos and Yahoo. On 13 May, the top ten search terms in the United States were:

nick berg video
nick berg
berg beheading
beheading video
nick berg beheading video
nick berg beheading
berg video
berg beheading video
‘nick berg’
video nick berg

The Berg beheading footage remained the most popular internet search in the United States for a week, and the second most popular throughout the month of May, runner up only to ‘American Idol.’

Berg’s death triggered a spate of similar beheadings, by a number of militant Islamic groups in Iraq, that were filmed and circulated online. There were 64 documented beheadings in Iraq in 2004, seventeen of the victims were foreigners, and 28 decapitations were filmed. The fol­lowing year there were five videotaped beheadings in Iraq, and the numbers have dwindled since. In 2004, those that received the most press attention proved particularly popular with the public. In June, an American helicopter engineer, Paul Johnson, was kidnapped and beheaded on camera in Saudi Arabia, and in the weeks after his death the most popular search term on Google was ‘Paul Johnson’. When the British engineer Kenneth Bigley was kidnapped in Iraq in September 2004 and beheaded by his captors the following month, one American organization reported that the video of his death had been downloaded from its site more than one million times. A Dutch web-site owner said that his daily viewing numbers rose from 300,000 to 750,000 when a beheading in Iraq was shown.

High school teachers in Texas, California and Washington were placed on administrative leave for showing Nick Berg’s beheading to their pupils in class. When the Dallas Morning News printed a still image of one of Berg’s assailants holding his severed head, with his face blocked out, it said that its decision had been inspired by interest generated in the blogosphere. The paper’s editorial pointed out that ‘[o]ur letters page today is filled with nothing but Berg-related letters, most of them demanding that the DMN show more photos of the Berg execution. Not one of the 87 letters we received on the topic yes­terday called for these images not to be printed.’

It is, of course, impossible to know how many people actually watched the videos after downloading them, but a significant number of Americans wanted to see them and discuss them, particularly the video of Berg, who was the first American to be beheaded in Iraq, and whose execution was the first to be recorded on camera since Pearl’s, two years earlier. Berg was killed just as public support for the war in Iraq was beginning to decline, and the popularity of the video underlined the extent to which the internet had eclipsed more traditional news media when it came to creating a story. Television news producers may have edited their clips of the video, but it did not matter because people were watching the footage online. The internet allowed people to protest against the perceived ‘censorship’ of the mainstream media, or else simply circumvent the media altogether when the mood took them. Whether people thought it ‘important’ to see Berg’s execution for themselves, or simply watched out of curiosity, there can be little doubt that ‘the crowd’ was taking control, or was out of control, depending on your perspective.

One survey, conducted five months after Berg’s death, found that between May and June, 30 million people, or 24 per cent of all adult internet users in the United States, had seen images from the war in Iraq that were deemed too gruesome and graphic to be shown on tele­vision. This was a particularly turbulent time during the war that saw not only Berg’s beheading, but also the release of photographs show­ing the abuse of prisoners at Abu Ghraib by American military personnel, and images showing the mutilated bodies of four American contract workers who had been killed by insurgents in Fallujah, dragged through the streets and hung from a bridge over the Euphrates. Nonetheless, Americans were seeking these images out: 28 per cent of those who had seen graphic content online actively went looking for it. The survey found that half of those who had seen graphic content thought they had made a ‘good decision’ by watching.

The decision to view Berg’s beheading became politicized online. Bloggers claimed it was no coincidence that the liberal news media dwelt on the harrowing images from Abu Ghraib, which undermined the Bush administration’s credibility in Iraq, while – as they saw it – sidestepping the Berg story by giving it fewer column inches and refus­ing to show the full extent of the atrocity. ‘One day the media was telling us we had to see the pictures from Abu Ghraib so we could understand the horrors of war,’ Evan Malony wrote. ‘But with Berg’s beheading, we’re told we can’t handle the truth . . . The media that had – rightfully, in my opinion – showed us the ugly reality of Abu Ghraib prison refused to do the same with Berg’s murder.’ Professor Jay Rosen was more explicit: ‘They aren’t showing us everything: the knife, the throat, the screams, the struggle, and the head held up for the camera. But the sickening photos from Abu Ghraib keep showing up.’

Other viewers admitted to watching execution videos simply out of curiosity, with no ‘higher’ purpose. One anonymous internet user said, ‘You almost can’t believe that a group of people could be so pitiless as to carry out something so cruel and bestial, and you need to have it confirmed . . . Watching them evokes a mixture of emotions – mainly distress at the obvious fear and suffering of the victim, but also revul­sion at the gore, and anger against the perpetrators.’ Meanwhile, website editors expressed a similar range of attitudes towards showing the content. They made the videos available either because they were dedicated to the fight against terror (people should see) or because they were opposed to the ‘censorship’ of the mainstream news media (people should be able to see), while ‘shock sites’ posted the footage purely as macabre entertainment alongside the other violent and provocative videos that drew their clients (watch this!).

Decapitation videos draw viewers who watch unapologetically and viewers who watch despite their own deep misgivings, and the internet offers everyone anonymity. The camera promises spectators a degree of detachment, but the action is only a click away, and this combina­tion gives the videos far greater reach. As the military analyst Ronald Jones put it, with little more than a camcorder and internet access, a militant group can create an ‘international media event . . . that has tremendous strategic impact’. Indeed, as terrorist attacks go, decapi­tating your victim on camera is an extremely efficient and effective strategy. It requires little money, training, equipment, weaponry or explosives: beyond the initial kidnapping, it does not rely on compli­cated coordination or technology that might fail, and the results are easy to disseminate. According to Martin Harrow, another analyst, it is a strategy that ‘has maximum visibility, maximum resonance and incites maximum fear’.

No wonder, then, that the Iraq hostage beheadings were ‘made for TV’. Other terrorist activities, like suicide attacks or bombings, are hard to capture on camera because they are necessarily clandestine, unpredictable and frenetic events, but the decapitation of a hostage can be carefully stage-managed, choreographed and rehearsed while still remaining brutally authentic. The footage is clear and close up. The murderers are offering their viewers a front-row seat at their show and what they want to show is their strength, their organization, their com­mitment to the cause, their complete control and domination of their victim. When one Italian hostage, a security officer named Fabrizio Quattrocchi, jumped up at the moment he was about to be shot by his captors on film and tried to remove his hood, shouting, ‘Now I’ll show you how an Italian dies!’, Al Jazeera withheld the resulting video because it was ‘too gruesome’. Was this a small victory for Quattrocchi in the face of certain death? No one saw the footage of his murder online, either for entertainment or for education, and his captors could not capitalize on his death in the way that they had planned.

During these carefully staged execution rituals, everyone, even the victim, must play their part. The whole procedure is a piece of theatre designed to create power and cause fear, just as with state executions stretching back to the thirteenth century, except, as John Esposito, a professor at Georgetown University, pointed out, when it comes to exe­cutions like Berg’s, ‘it’s not so much the punishing of the individual as the using of the individual’. Even when the victim is an innocent hostage, the power that comes from killing is exerted over a wider com­munity. The crowd is compliant too. By turning up to see the show, or by searching Google for the latest execution video, the people watching also have their part to play.

‘The point of terrorism is to strike fear and cause havoc – and that doesn’t happen unless you have media to support that action and show it to as many people as you can,’ said one analyst interviewed by the Los Angeles Times shortly after Nick Berg’s execution. These mur­derers post their videos on the internet because they know that the news media will be forced to follow the crowd. Television news pro­grammes either become redundant by refusing to air videos that are freely available online, or else they do exactly what the murderers want and show the footage to a wider audience. Meanwhile, the internet pro­vides a ‘void of accountability’, in the words of Barbie Zelizer, where it is unclear who took the images, who distributed them and who saw them. The whole experience is lost in the crowd.

Adapted from "Severed: A History of Heads Lost and Heads Found" by Frances Larson. Copyright © 2014 by Frances Larson. With permission of the publisher, Liveright Publishing Corp. All rights reserved.

Frances Larson

Frances Larson is an honorary research fellow in anthropology at Durham University. She is the author of a biography of Henry Wellcome, "An Infinity of Things," published to considerable critical acclaim and subsequently shortlisted for the MJA Awards and chosen as a Sunday Times Book of the Year as well as a New Scientist Best Book of 2009. She is also the co-author of "Knowing Things," a book on the history of the Pitt Rivers Museum in Oxford. Larson lives in Durham, England.


Spreading the seeds of million-murdering death * : metamorphoses of malaria in the mosquito

Plasmodium spp. undergo a complex obligate developmental cycle within their invertebrate vectors that enables transmission between vertebrate hosts. This developmental cycle involves sexual reproduction and then asexual multiplication, separated by phases of invasion and colonization of distinct vector tissues. As with other stages in the Plasmodium life cycle, there is exquisite adaptation of the malaria parasite to its changing environment as it transforms within the blood of its vertebrate host, through the different tissues of its mosquito vector and onwards to infect a new vertebrate host. Despite the intricacies inherent in these successive transformations, malaria parasites remain staggeringly successful at disseminating through their vertebrate host populations.

This title and some subheadings are taken from lines in Ronald Ross' poem In Exile, Reply – What Ails the Solitude, written on 21 August 1897, the day after he made his Nobel-Prize-winning discovery of parasite stages in the mosquito. ‘This day relenting God hath placed within my hand a wondrous thing and God be praised. At His command, seeking His secret deeds with tears and toiling breath I find thy cunning seeds, O million-murdering Death. I know this little thing a myriad men will save. O Death, where is thy sting, thy victory, O Grave!’


The 'Gene Drive' That Builds a Malaria-Proof Mosquito

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On Monday, scientists announced they could cheat the laws of evolution: They had devised a way to force a gene that kills malaria parasites to spread through a whole population of mosquitoes that normally carry the parasite---at least in a lab. No malaria in mosquitoes means, hypothetically, no malaria in people, either. All this is possible thanks to a controversial new technology known as a gene drive. Or Crispr. Or maybe both?

It's worth taking a minute to explain which is which, and what the difference is. To do it will require a trip into barely-charted gene-editing territory.

Let's start with gene drives. Scientists have been toying with this idea, theoretically at least, for decades. A "gene drive" is a generic term for anything that makes a gene spread rapidly through a population. A typical gene---say, one that gives mosquitoes red eyes, has a 50/50 chance being passed from parent to offspring. A mama mosquito has two copies of the eye color gene---let's say red and not-red---one on each of a pair of chromosomes. (A chromosome is the packaged version of DNA at the heart of a cell.) Mama passes one copy of each chromosome to each baby mosquito. Hence, 50/50.

So if you're a clever and overachieving gene, you want copy yourself onto the other chromosome, too. You want mama to be red/red. Or not-red/not-red. Doesn't matter which. In naturally occurring gene drives---there are those---genes code for enzymes that cut the other chromosome in the gene's analogous location. Under certain conditions, when chromosomes get damaged, the cell's natural DNA-repair mechanism uses the undamaged chromosome as a template for repair. And hey, look! The undamaged chromosome happens to have, our gene of interest in the right spot! That's a funny coincidence! So the gene drive copies it into the right spot on the other chromosome, and now that gene is sitting pretty on both.

Intrigued by natural gene drives, the geneticist Austin Burt proposed in 2003 making a synthetic one to take a gene that kills the malaria parasite (mice have one) and spread it into mosquitoes. But the proposal remained theoretical because scientists weren't sure how to get a self-copying gene to cut a chromosome and reliably copy itself as well as a long malaria parasite-killing gene next to it.

Then in 2012 Crispr/Cas9, a much hyped genome-editing tool, came along. Crispr/Cas9 is essentially a pair of highly targetable DNA scissors. (This is useful for lots of other forms of genome-editing, too.) Define the cut for a gene drive, and you've also defined the region to copy, which can be thousands of letters of DNA or multiple genes long. Earlier this year, scientists published a way to make gene drives that uses Crispr/Cas9's cutting enzyme in fruit flies. And yesterday, scientists reported they could use a similar gene drive in the Anopheles stephensi mosquito---the one that carries the malaria parasite and allows it to infect human beings.

The gene drive part is crucial. It allows the anti-malaria gene to keep spreading through procreation after procreation and generation after generation, even if its effects carry no reproductive benefit for the mosquito. (Typically that would be the rule evolution would use to determine which genes get passed down.) Another gene drive idea is to change the balance of sexes for mosquitoes---more of one than another rather than roughly even, making reproductive options as limited as a Sunday night at a single-sex boarding school. That obviously carries huge downsides for the mosquito but plenty of potential upsides for humans.

Still, malaria's not beat yet. The gene drive's creators are proceeding cautiously, and have no plans to released gene drive systems into the wild. As you can imagine, messing with evolution and the genomes of entire populations makes scientists worry about unpredictable outcomes. In fact, a different research group recently announced that theyɽ built gene drives that erase edits from another gene drive as a safety switch. But they haven't released their anti-gene-drive gene drives into the wild yet, either.


Trends

Mosquitoes are natural vectors that allow systematic and persistent arbovirus infection. The infection in mosquitoes is usually associated with few fitness costs, allowing the mosquitoes to transmit the virus efficiently. Mosquitoes have evolved systemic and tissue-specific antiviral mechanisms to limit viral propagation to nonpathogenic levels.

Mosquitoes ingest an arbovirus-infected blood meal into the midgut. The virus subsequently infects the midgut epithelial cells and spreads systematically through the hemolymph to other tissues.

RNAi and several conserved innate immune pathways play systemic roles against arbovirus infection of mosquitoes.

Specific antiviral strategies are armed in the mosquito midgut, hemolymph, salivary glands, and neural tissues for the control of arboviral propagation.

Mosquito-borne viral diseases are a major concern of global health and result in significant economic losses in many countries. As natural vectors, mosquitoes are very permissive to and allow systemic and persistent arbovirus infection. Intriguingly, persistent viral propagation in mosquito tissues neither results in dramatic pathological sequelae nor impairs the vectorial behavior or lifespan, indicating that mosquitoes have evolved mechanisms to tolerate persistent infection and developed efficient antiviral strategies to restrict viral replication to nonpathogenic levels. Here we provide an overview of recent progress in understanding mosquito antiviral immunity and advances in the strategies by which mosquitoes control viral infection in specific tissues.


“Maleness” Gene Found in Malaria Mosquito

Anna Azvolinsky
Jun 30, 2016

Male Anopheles gambiae ANT, SINKINS Researchers have identified a gene that kickstarts the male-specific genetic program in the African malaria mosquito Anopheles gambiae. When expressed in genetically female mosquito embryos, the gene, called Yob, is lethal. The results, published today (June 30) in Science, highlight a way toward genetic approaches to propagate male-only mosquitoes that could help kill malaria parasite&ndashcarrying females in the wild.

&ldquoThis is a breakthrough in the field and potentially very useful for control of the malaria-transmitting mosquito,&rdquo said Steven Sinkins, who studies mosquito-borne diseases at Lancaster University in the U.K. &ldquoThere is also an interesting evolutionary story that could be revealed as a result of this work,&rdquo added Sinkins, who penned an accompanying editorial but was not involved in the work.

&ldquoThis is an important step forward in understanding the biology of this malaria vector and also has potential for applications to control malaria,&rdquo.

Insect species employ a variety of different genetic mechanisms to confer either the female or male sex. While the downstream components are evolutionarily well conserved, the initiating gene that instructs male-specific development of the embryo has been elusive in most species. Among the insects with a Y chromosome, only the maleness gene of A. aegypti had previously been determined.

To try to find the maleness gene in A. gambiae, Jaroslaw Krzywinski of the Pirbright Institute in the U.K. and colleagues isolated the minute amount of messenger RNA (mRNA) in early male and female mosquito embryos and performed transcriptome analyses to identify those transcripts found in male—but not female—embryos. The researchers found transcripts that mapped to the Yob gene on the Y chromosome, which overlapped with a previously described YG2 gene, a candidate male-determining gene.

Krzywinski’s team indeed found that this gene had the characteristics of a male-determining gene: the transcripts were present from two hours after eggs were laid and continued for the duration of a mosquito’s life. Yob also controlled the synthesis of male-specific gene products in the sex determination pathway, the researchers found.

“There are very small time windows [mosquito embryo development] and very small amounts of RNA involved,” noted Sinkins. “Which makes this a technical tour de force, to be able to sex early embryos and to separate their mRNA pools and make the comparison that allows gene identification. It’s very impressive work.”

When the researchers injected Yob mRNA into early-stage embryos along with a green fluorescent protein (GFP) marker, the surviving mosquitos were all fertile males. A group of control embryos injected with just the GFP marker developed into an equal distribution of males and females. This demonstrated that the delivery of the mRNA was lethal to the developing female embryos.

Because the converse experiment—knocking down Yob expression—yielded predominantly females, the team concluded that Yob affects dosage of the X chromosome-linked gene products. In XX female embryos, Yob expression results in death because of overexpression from the X chromosome while insufficient expression from the X chromosome in XY males also leads to lethality.

Researchers have long sought to use genetic methods to control malaria vectors, but “we needed a large-scale method to segregate males from females and release only males which increases the efficiency of such control approaches and is a strict prerequisite when targeting pathogen-transmitting mosquitoes,” Krzywinski explained. “We now have a fantastic tool to make male-only transgenic strains of major African vectors of malaria.”

Krzywinski and colleagues are now working to develop strains that could conditionally kill only female mosquito embryos. (Female mosquitos bite and transmit the malaria-causing Plasmodium parasite.)

“I hope in less than 10 years, we have genetic methods effectively controlling Anopheles mosquitoes,” Krzywinski told The Scientist.

One approach, according to Sinkins, is to conditionally express Yob from an autosome, creating, then releasing, male-only populations. “But a system that could spread itself, requiring only seeding releases would be much more effective,” said Sinkins.

One such way is to drive preferential propagation of the Y chromosome in embryos and producing predominantly male offspring. (See “CRISPR-Powered Malaria Mosquito Gene Drive,” The Scientist, November 24, 2015 “Fewer Female Mosquitoes, Less Malaria?” The Scientist, June 11, 2014.)

Tu is a proponent of so-called gene drive to increase the ratio of male to female mosquitos. However, he said, “we need to see whether there is near 100 percent penetrance of the transgene.”

Another factor to consider, according to Tu, is whether the transgenic insects will be as competitive as their wild counterparts.


Watch the video: Mechanism of a Mosquito Bite (May 2022).