An X-linked meiotic drive allele has strong, recessive fitness costs in female Drosophila pseudoobscura

Selfish ‘meiotic drive’ alleles are transmitted to >50% of offspring, allowing them to rapidly invade populations even if they reduce the fitness of individuals carrying them. Theory predicts that drivers should either fix or go extinct, yet some drivers defy these predictions by persisting at low, stable frequencies for decades. One possible explanation for this discrepancy is that drivers are especially costly when homozygous, although empirical tests of this idea are rare and equivocal. Here, we measure the fitness of female Drosophila pseudoobscura carrying zero, one, or two copies of the X-linked driver Sex-Ratio (SR). SR had strong negative effects on female offspring production and the probability of reproductive failure, and these effects were largely similar across four genetic backgrounds. SR was especially costly when homozygous. We used our fitness measurements to parameterise a population genetic model, and found that the female fitness costs observed here can explain th...