Sexual selection and the population genetics of a selfish gene

The segregation distorter allele (SD) found in Drosophila melanogaster
distorts Medelian inheritance in heterozygous males by causing
developmental failure of non-SD spermatids, such that >90% of the
surviving sperm carry SD. This within-individual advantage should cause SD
to rapidly fix, and yet SD is typically rare in wild populations. Here, we
explore whether this paradox can be resolved by sexual selection, by
testing if males carrying three different variants of SD suffer reduced
pre or postcopulatory reproductive success. We find that males carrying
the SD allele are just as successful at securing matings as control males,
but that one SD variant (SD-5) reduces sperm competitive ability and
increases the likelihood of female remating. We then used these results to
inform a theoretical model; we found that sexual selection could limit SD
to the frequencies observed in nature when sperm competitive ability and
female remating rate equalled the values observed for SD-5. However,
sexual selection was unable to explain natural frequencies of the SD
allele when the model was parameterized with the values found for two
other SD variants, indicating that sexual selection alone is unlikely to
explain the rarity of SD.