The Evolution of Nonbiting Mosquitoes

by: Marissa Beachell, Stephen Kulka, Mackenzie Lachkey, & Katherine O’Malley (Stonehill College, BIO 323 Evolution, Spring 2018)

There isn’t much worse than walking in after a night spent outside in the summertime and seeing your arms and legs covered in mosquito bites! While not every species or type of mosquito needs to bite to obtain food, many species do get their nutrients from a blood meal. What if there were a way that we could never get bitten by mosquitoes again but the mosquitoes actually live… and even thrive? We spend so much time and money on mosquito repellents and tiki torches that supposedly ward off these pests, while those in some other parts of the world set up mosquito nets to avoid a bite. It’s easy to brush off a few mosquito bites in Massachusetts or another urban area of the U.S., but for those countries with preventative mosquito nets, a bite could mean life or death. This whole landscape could be changing, however, as recent scientific research has illuminated a possible solution in which humans and mosquitoes both win.

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Researchers from the University of Oregon set out to observe a single species of mosquito that lives in Florida, called Wyeomyia smithii, or the pitcher plant mosquito. What is particularly interesting about this species of mosquito is that has two unique populations or groups that inhabit Florida: one Southern group that is blood-feeding and a Northern group that does not bite. How could this be? These two populations are completely interfertile, meaning they can mate with each other, and the Northern population evolved from the Southern population. The researchers were interested in understanding how the Northern population evolved to avoid biting while their Southern ancestors were and are biting mosquitoes.

To examine this question and understand how evolution acted on this particular species of mosquito, the researchers had three main objectives. Primarily, they were interested in whether the non-biting Northern population arose from natural selection or from drift. Basically, they sought to understand if the Northern population arose because non-biting gave them some sort of advantage in their Northern habitat, or if the evolution from biting to non-biting was more of a random event, happening largely by chance. The second objective was to examine how non-biting and biting mosquitoes differ in some of the processes that happen in their bodies due to the way they obtain food. They focused on metabolic pathways, processes in the body that can help breakdown food or build up molecules through a series of steps. The researchers were determined to find differences between how these processes functioned when a mosquito needed to bite for food and when a mosquito did not. The third and final objective of researchers was to observe how we can hijack the process of the evolution of how mosquitoes obtain food and to be able to turn off that biting gene or ability in mosquitoes around the world. We thereby could abolish blood meals by mosquitoes, saving our own skin and at the same time still providing mosquitoes with alternative food sources.

In order to address these objectives, the researchers collected larvae from a population of W. smithii from Florida that seldom bit, and larvae of an obligate non-biting population from Maine. 14,000 of the Florida individuals were reared to adulthood and selected for “avid biting” for seven generations. The group selected for avid biting, as well as the original seldom-biting and obligate non-biting populations, were then synchronized in a diapause, or a state of dormancy and halted growth. After one month in diapause, four replicates of 1,200 larvae from each group were reared to adulthood in 12 separate cages. 5 days after the first adult female emerged, each of the 12 cages were then offered anesthetized female rats as a blood source three times a week for 15 minutes over 2 weeks. Each cage was observed individually, and any female that inserted her proboscis, or feeding appendage, into the rat was scored an avid biter. Once scored as an avid biter, a disinterested biter, or an obligate non-biter, the females were frozen, decapitated, and their heads were sampled and processed for further analysis.

The results of the experiment were gathered and used to compare patterns of differential gene expression among the selected avid biters, the seldom-biting population from Florida, and the obligate non-biters from Maine. Using a tool to detect gene expression (DEET), the researchers were able to identify 1,459 genes that were useful for comparing biting and non-biting mosquitoes. It was found that 95% of these genes showed a positive correlation in differential gene expression between artificial selection on biting and the evolution of non-biting in nature due to selection over time within obligate non-biting W. smithii populations. The results were then analyzed and related to the functional metabolic pathways. They showed that non-biters allocated more energy to diverse metabolic processes than blood feeders, who spent much more energy on one pathway necessary for breaking down its blood meal. This allows non-biters to give more energy to functions such as reproduction and DNA replication.

Through the researcher’s experimentation, it was concluded that the evolutionary transformation from a blood-feeding to an obligate non-biting lifestyle in W. smithii is due to selection in nature. Although there is a lot of information understood about the species, there is still ambiguity amongst the science community as to what specific genes regulate the alteration of the associated metabolic processes that control this transformation. It is without a doubt that mosquitoes fit the title of the “world’s most dangerous animal.” This is due to their ability to transmit many harmful pathogens that are responsible for the deaths of thousands of humans per year. If the researchers are able to understand the process of the evolution and are able to identify the biting gene ability in mosquitoes around the world, then there surely is the slight possibility that researchers can completely prevent, or at least better control transmission of mosquito-borne diseases, because “if there is no bite, there is no disease.”

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Citation: Bradshaw, W.E., J. Burkhart, J. K. Colbourne, R. Borowczak, J. Lopez, D. L. Denlinger, J. A. Reynolds, M. E. Pfrender, & C. M. Holzapfel. 2018. Evolution of a Nonbiting Lifestyle In a Mosquito. Proceedings of the National Academy of Sciences 115: 1009­–1014.

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