Fitness effects of X chromosome drive in the stalk-eyed fly, Cyrtodiopsis dalmanni

Sex-ratio (SR) males produce predominantly female progeny because most Y chromosome sperm are rendered nonfunctional. The resulting transmission advantage of XSR chromosomes should eventually cause population extinction unless segregation distortion is masked by suppressors or balanced by selection. By screening male stalk-eyed flies, Cyrtodiopsis dalmanni, for brood sex ratio we found unique SR alleles at three X-linked microsatellite loci and used them to determine if SR persists as a balanced polymorphism. We found that XSR/XST females produced more offspring than other genotypes and that SR males had lower sperm precedence and exhibited lower fertility when mating eight females in 24 h. Adult survival was independent of SR genotype but positively correlated with eye span. We infer that the SR polymorphism is likely maintained by a combination of weak overdominance for female fecundity and frequency dependent selection acting on male fertility. Our discovery of two SR haplotypes in the same population in a 10-year period further suggests that this SR polymorphism may be evolving rapidly.