Supplementary material from "Role-reversed polyandry is associated with faster fast-Z in shorebirds"

In birds, males are homogametic and carry two copies of the Z chromosome (‘ZZ’), whilst females are heterogametic and exhibit a ‘ZW’ genotype. The Z chromosome evolves at a faster rate than similarly sized autosomes, a phenomenon termed ‘fast-Z evolution’. This is thought to be caused by two independent processes – greater Z chromosome genetic drift due to a reduced effective population size, and stronger Z chromosome positive selection due to the exposure of partially recessive alleles to selection. Here we investigate the relative contributions of these processes, by considering the effect of role-reversed polyandry on fast-Z in shorebirds, a paraphyletic group of wading birds that exhibit unusually diverse mating systems. We find stronger fast-Z effects under role-reversed polyandry, consistent with particularly strong selection on polyandrous females driving the fixation of recessive beneficial alleles. This result contrasts with previous research in birds, which has tended to implicate a primary role of genetic drift in driving fast-Z variation. We suggest that this discrepancy can be interpreted in two ways – stronger sexual selection acting on polyandrous females overwhelms an otherwise central role of genetic drift, and/or sexual antagonism is also contributing significantly to fast-Z and is exacerbated in sexually dimorphic species.