Sex chromosomes have evolved independently in multiple lineages, and how they evolve has been a topic for theoretical and empirical research. Although the canonical theory of sex chromosome evolution suggests that the degeneration of the Y/W chromosome is expected to have a linear relationship with time, there is increasing evidence that this is not always the case. Some fishes in the Poecilia genus share the same origin to their sex chromosomes but differ vastly in the degeneration state of their Y chromosome. I first aimed to elucidate the evolutionary history of the sex chromosomes in five Poecilia species: P. reticulata, P. wingei, P. picta, P. parae, and P. bifurca. Using a comparative approach in combination with next-generation sequencing and bioinformatics, I found evidence for a single, recent origin to the sex chromosomes for these species, about 20 million years ago. Furthermore, I provided a reliable genomic resource for P. bifurca, one of the more understudied poeciliids. My second aim was to explore the consequences of Y chromosome degeneration. Using a sex chromosome system that is the same age and does not have complete dosage compensation, I focused on the small non-recombining region on the Y chromosome in P. reticulata that shows divergence from the X chromosome. By combining genomic and transcriptomic data, I found evidence that Y chromosome degeneration follows a stepwise process, first degenerating on the sequence level before degenerating through gene regulation and expression. The process of Y chromosome degeneration in P. reticulata differs from previous studies that find Y chromosome degeneration occurs first at the regulatory level. Together, my dissertation supports alternative theories of sex chromosome evolution and strives to better our understanding of sex chromosome evolution.
Lydia J. M. Fong (Thu,) studied this question.