Sex Chromosome Heteromorphism and the Fast-X Effect in Poeciliids

Abstract

AbstractAn accelerated rate of sequence evolution on the X chromosome compared to autosomes, known as Fast-X evolution, has been observed in a range of heteromorphic sex chromosomes. However, it remains unclear how early in the process of sex chromosome differentiation the Fast-X effect becomes detectible. Recently, we uncovered an extreme variation in sex chromosome heteromorphism across Poeciliid fish species. The common guppy, Poecilia reticulata, Endler’s guppy, P. wingei, and the swamp guppy, P. picta, appear to share the same XY system and exhibit a remarkable range of heteromorphism. The sex chromosome system is absent in recent outgroups, including P. latipinna and Gambusia holbrooki. We combined analyses of sequence divergence and polymorphism data across Poeciliids to investigate X chromosome evolution as a function of hemizygosity and reveal the causes for Fast-X effects. Consistent with the extent of Y degeneration in each species, we detect higher rates of divergence on the X relative to autosomes and a strong Fast-X effect in P. picta, while no change in the rate of evolution of X-linked relative to autosomal genes in P. reticulata. In P. wingei, the species with intermediate sex chromosome differentiation, we see an increase in the rate of nonsynonymous substitutions on the older stratum of divergence only. We also use our comparative approach to test different models for the origin of the sex chromosomes in this clade. Taken together, our study reveals an important role of hemizygosity in Fast-X and suggests a single, recent origin of the sex chromosome system in this clade.