Sex chromosomes have evolved independently countless times throughout the eukaryotes. As such, sex chromosomes represent one of the most pervasive examples of convergent evolution, as analogous yet unrelated sex chromosomes share many unique features that distinguish them from the rest of the genome. Although models for sex chromosome evolution have been proposed, they have been difficult to empirically test, largely because most model systems are at a terminal phase of sex chromosome divergence. Most work on sex chromosomes has therefore focused on the consequences of sex chromosome evolution, rather than the causes. In order to understand the forces catalyzing sex chromosome evolution, we require a study system at earlier stages of sex chromosome divergence, ideally one where there is still extensive polymorphism among populations and closely related species in the degree and region of recombination suppression, and with easily identified sexually antagonistic traits. These traits are all exhibited by the Poeciliid fishes, the focus of this project, which will use chromium sequencing in guppies to assess Y chromosome haplotype diversity and structural variation. The results will offer a unique window onto the early evolution of sex chromosomes.