Abstract
Chromosomal inversions are thought to confer a selective advantage in alternative habitats by protecting co-adapted alleles from recombination. The frequencies of two inversions (2La and 2Rb) of the afro-tropical malaria mosquito Anopheles gambiae change gradually along geographical clines, increasing in frequency with degree of aridity. Such clines can result from gene flow and local selection acting upon alternative karyotypes along the cline, suggesting that these inversions may be associated with tolerance to xeric conditions. Since water loss represents a major challenge in xeric habitats, it can be supposed that genes inside these inversions are involved in water homeostasis. To test this hypothesis, we compared the desiccation resistance of alternative karyotypes from a colonised 2Rb/2La polymorphic population of A. gambiae from Cameroon. The strain included only the molecular form S, one of the genetic units marking incipient speciation in this taxon. Day-old mosquitoes of both sexes were assayed individually for time to death in a dry environment and the karyotype of each was determined post-mortem using molecular diagnostic assays for each inversion. In agreement with expectations based on their eco-geographical distribution, we found that 2La homokaryotypes survived significantly longer (1.3 hours) than the other karyotypes. However, there was weak support for the effect of 2Rb on desiccation resistance. Larger mosquitoes survived longer than smaller ones. Median survival of females was greater than males, but the effect of sex on desiccation resistance was weakly supported, indicating that differential survival was correlated to differences between sexes in average size. We found weak evidence for a heterotic effect of 2La karyotype on size in females. These results support the notion that genes located inside the 2La inversion are involved in water balance, contributing towards local adaptation of A. gambiae to xeric habitats, beyond the adaptive value conferred by a larger body size.
Highlights
Population characteristics including morphology, phenology, and stress resistance commonly feature gradual changes in time or space associated with gradients in environmental variables such as temperature, rainfall, altitude, or insulation [1,2]
The present study demonstrates a significant association between the 2La chromosomal inversion and resistance to desiccation in the major malaria vector A. gambiae
These results support a role of this inversion in water homeostasis, which can explain the clinal pattern of 2La across the range of eco-climatic conditions encountered in Western and Central Africa
Summary
Population characteristics including morphology, phenology, and stress resistance commonly feature gradual changes in time or space associated with gradients in environmental variables such as temperature, rainfall, altitude, or insulation [1,2]. Such clines have been extensively studied in plants and animals, and are known to occur at different geographical or ecological scales. Investigations of natural populations of Drosophila in particular have demonstrated the existence of latitudinal clines in inversion frequency associated with traits as body size, developmental time, and heat or cold tolerance [8,9,10,11]
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