Student Theses and Dissertations

Nipun Basrur

Date of Award

2022

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Abstract

Only female mosquitoes of the species Aedes aegypti have evolved to hunt humans in order to feed on their blood. In striking contrast, male mosquitoes have no drive to seek hosts or drink their blood. Through the act of blood-feeding, only female mosquitoes can transmit pathogens that cause deadly diseases like dengue, chikungunya, and Zika. The genetic and neural circuit basis of this striking sex difference in behavior, one that is of critical importance to global public health, remain poorly characterized.

Across the insects, sex-specific alternative splicing is known to control sex-specific morphological and behavioral traits. We took advantage of this knowledge to conduct an unbiased comparative transcriptomic screen and identified a number of genes that are alternatively spliced between the brains of males and females across blood-feeding mosquito species. Two of these genes were specifically expressed in the nervous system. The first gene, fruitless, encodes a set of male-specific transcription factors well-known to be required for male-specific mating behavior in Drosophila and other insects. We used CRISPR-Cas9 to generate fruitless mutant Aedes aegypti mosquitoes and found that these mutants are unable to mate, consistent with the ancestral function of this gene. Surprisingly, fruitless mutant male mosquitoes also gained a strong and specific attraction to a live human subject that was specifically elicited by human odor, and not heat or other human cues. fruitless mutant males did not gain the ability to blood-feed, suggesting that it is specifically required to inhibit attraction to humans in male mosquitoes and that other genes specify host-seeking in both male and female mosquitoes. These results indicate that fruitless, a conserved gene, has gained a new and unexpected function over the course of evolution, acting to repress host-seeking behavior in male mosquitoes.

The second gene identified from this screen, the previously undescribed 11211, is sex-specifically spliced into a predicted short female and long male protein isoform, which we showed localizes to the nucleus in both sexes. With the notable exception of non-mosquito flies, 11211 orthologs are sex-specifically spliced in other insects such as bees and beetles. However, in contrast to the mosquito, all other insects encode longer female and shorter male 11211 protein isoforms, suggesting that sex-specific splicing of this gene has evolved a new role in the mosquito. In mosquitoes, but not in other insects, 11211 is enriched in neurons in a part of the brain known to control feeding behavior. We found that these neurons show sexual dimorphism in their inputs, as female 11211-expressing neurons receive input from sensory neurons that detect the taste of blood. When we silenced the activity of these 11211-expressing neurons, females were able to feed on nectar and retain an attraction to humans, but strikingly lost the ability to initiate blood feeding. These results suggest that a rapidly-evolving novel gene marks a population of neurons that are specifically required for blood-feeding behavior.

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