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Study Points to Standing Genetic Variation in Formation of Malaria Mosquito Groups

Standing genetic variation, including evolutionarily old chromosomal rearrangements, helped drive formation of different forms of the African mosquito Anopheles funestus that can transmit malaria parasites, according to a study appearing in the Proceedings of the National Academy of Sciences. Within An. funestus are two chromosomally differentiated groups that are colocalized, morphologically indistinguishable, and reported to differ both in ecology and behavior relevant to malaria transmission and control. Yet the genetic basis and environmental determinants of this diversification is unclear. To investigate, a group led by University of Notre Dame researchers performed deep whole-genome sequencing on insects from the two groups, specifically testing the hypothesis of whether the two forms of mosquitos are ecotypes differentially adapted to different breeding environments: natural swamps and irrigated rice fields. They uncover genome-wide differentiation between the groups despite extensive microsympatry, synchronicity, and ongoing hybridization. Their findings further suggest that the timing of the groups' formation closely followed the growth of domesticated rice cultivation in Africa and that, consistent with local adaptation, the genomic regions of highest differentiation were under selection during lineage splitting. And because the origin of nearly all variations implicated in adaptation — including chromosomal inversions — predates the ecotype split, the rapid adaptation was likely fueled by standing genetic variation and not de novo point or structural mutations, the study's authors write. Developing tests that can differentiate the two ecotypes, which likely play different roles in malaria transmission, may help monitor the disease's transmission, they add.