Phylogeography of sex ratio and multiple mating in stalk-eyed flies from southeast Asia.

The factors maintaining sex chromosome meiotic drive, or sex ratio (SR), in natural populations remain uncertain. Coevolution between segregation distortion and modifiers should produce transient SR distortion while selection can result in a stable polymorphism. We hypothesize that if SR is maintained by selection, then phylogenetically related populations should exhibit similar SR frequency and intensity. Furthermore, when drive is present, females should mate with multiple males more often both to insure fertility and to increase the probability of producing male progeny. In this paper we report on variation in SR frequency and multiple mating among seven populations and three species of stalk-eyed flies, genus Cyrtodiopsis, from southeast Asia. Using a phylogenetic hypothesis based on 1100 bp of mtDNA sequence we find that while sex chromosome meiotic drive is present in all populations of C. whitei and C. dalmanni, the frequency and intensity of drive only differs between populations or species with greater than 4.8% sequence divergence. The frequency of females mating with multiple males is higher in populations with SR. In addition, SR males mate less often, possibly to compensate for sperm depletion. Our results suggest that sex chromosome drive is maintained by balancing selection in populations of C. whitei and C. dalmanni. Nevertheless, coevolution between drive and suppressors deserves further study.