The Rel2 pathway and the alternative splicing of AgDscam has shown to be significant in Plasmodium resistance in Anopheles mosquitoes, and the carboxypeptidase (Cp) and vitellogenin (Vg) promoters play a major role in the production of such antimalarial factors. A group of researchers conducted a variety of experiments testing how anti-Plasmodium transgenes impacted essential factors such as fitness, mating preference, and longevity of resistance.

In biology, fitness is a measure of an organism’s reproductive success, which is essential when considering the introduction of GM mosquitoes to natural environments. If the impacts of genetic engineering negatively impacts fitness, then the GM mosquitoes will have less offspring, which means that GM traits are less likely to spread in a wild population. Of course, the entire point of creating transgenic mosquitoes that are more resistant to Plasmodium infection than wild mosquitoes is to reduce rates of malaria transmission, so fitness costs are not optimal. Researchers conducted an experiment to see if there were any evident fitness costs to the GM mosquitoes by crossing them with wild mosquitoes for five generations and treated them with consistent conditions to avoid confounding variables. Four of the five lines they examined showed no fitness costs, measured by survival and egg-laying and hatching rates and sex ratio, though they did find an elongation of pupation time in GM mosquitoes, which could lead to a slight disadvantage. They also tested the strength of Plasmodium resistance after several generations, culturing two GM lines, one that acts in the midgut and one that acts in the fatbody, for more than 50 generations, and both maintained their resistance to the malaria parasite.

Researchers then chose to examine the competitiveness of transgenic mosquitoes in comparison to wt (wild type) mosquitoes. They selected the two genes that were most effective at inhibiting Plasmodium colonization in the midgut tissue, CpRel215 and CpDsPfs3, along with one gene that acts in the fatbody, VgRel21, and put each variant in an environment with 50% GM and 50% wt mosquitoes. With each subsequent generation, they measured the portion of mosquitoes with the antimalarial transgene, and found that by the first generation, the percentage of GM mosquitoes was 90%, a ratio that persevered in all nine following generations. This showcases the competitiveness of the GM lines, as Hardy-Weinberg equilibrium predicts a prevalence of just 75% for the GM trait without an advantage. 

The advantage in competitiveness of the GM lines is likely due to a phenomenon in the mating preference of male mosquitoes. When CpRel215 males were given the choice between WT and GM females, they preferred to mate with WT mosquitoes, measured by the percentage of females that did or did not lay eggs when exposed to the GM males. This tendency to mate with mosquitoes that don’t already carry the transgene accelerates its spread, and the inverse mating behavior is observed in WT males, which preferred GM females to WT females, furthering the spread of the anti-Plasmodium transgene. What, then, causes this mating preference?

The reason that the CpRel215 and CpDsPfs3 transgenes were selected for these experiments is that they affect Anopheles mosquitoes’ immune pathways in a way that inhibits Plasmodium oocyst development, which is desirable for decreasing malaria transmission rates. However, they have a wider impact than just Plasmodium resistance; they also fight bacteria, and that change to the mosquito microbiota can have a wide range of effects, including alterations to mosquitoes’ mating preference. To see if the 90% GM prevalence found in the experiment that monitored the percentage of GM mosquitoes for ten generations was caused by mating changes made because of the shift in bacterial colonies in the mosquitoes, researchers crossed GM and wt mosquitoes in two separate groups, treating one group with antibiotics to remove the effect of the altered microbiota caused by the transgene. As a result, GM prevalence in the first generation was about 75%, as predicted by Hardy-Weinberg equilibrium. The following generations had increasing levels of transgenic mosquitoes in comparison to wt mosquitoes, presumably because the original microbiota reestablished itself in subsequent populations. These findings support the hypothesis that the change in microbiota created by genetic modification impacted mating preferences in a genetically advantageous way, though lab mosquitoes having different bacterial compositions than wild mosquitoes, so these experiments should be replicated in natural conditions to further validate these conclusions.

Confused about this pathway? Check out the last article!

Exploring the IMD Pathway – Education in Epidemiology 21

The article this post is based off of is almost completely responsible for my choice to cover this topic in the first place. It was mentioned in the TPWKY episode on malaria, so when I covered it earlier this year, it was planted in my mind as a fascinating feat of genetic engineering. I’m so glad I chose it for this project, because it’s incredibly interesting and has taught me a lot.

Something I’ve really been enjoying is keeping up with this particular path of genetic modification and this research team, because I’ve been able to fully understand the mechanism of defense and how it’s deployed. I feel like there’s still a lot to learn, and I’m wondering how progress is looking currently. Either way, very cool. Unfortunately, this is the last of the articles I prepared over spring break, and I’ll have to work on fitting the writing of these papers back into my schedule. Which is like… how. Anyway, wish me good luck on my exams this week, and stay tuned to learn with me!

Pike, A., Dong, Y., Dizaji, N. B., Gacita, A., Mongodin, E. F., & Dimopoulos, G. (2017). Changes in the microbiota cause genetically modified Anopheles to spread in a population. Science, 357(6358), 1396-1399. https://doi.org/10.1126/science.aak9691