Using conventional -statistics to study unconventional sex-chromosome differentiation.

Species with undifferentiated sex chromosomes emerge as key organisms to understand the astonishing diversity of sex-determination systems. Whereas new genomic methods are widening opportunities to study these systems, the difficulty to separately characterize their X and Y homologous chromosomes poses limitations. Here we demonstrate that two simple -statistics calculated from sex-linked genotypes, namely the genetic distance () between sexes and the inbreeding coefficient () in the heterogametic sex, can be used as reliable proxies to compare sex-chromosome differentiation between populations. We correlated these metrics using published microsatellite data from two frog species (and ), and show that they intimately relate to the overall amount of X-Y differentiation in populations. However, the fits for individual loci appear highly variable, suggesting that a dense genetic coverage will be needed for inferring fine-scale patterns of differentiation along sex-chromosomes. The applications of these -statistics, which implies little sampling requirement, significantly facilitate population analyses of sex-chromosomes.