HYBRIDIZATION AND SPECIATION IN SWORDTAIL FISH (POECILIIDAE: XIPHOPHORUS)

Abstract

Understanding what makes a species is fundamental to understanding evolutionary biology, yet there is so much about this process that still puzzles us. Until recently, mating between different species, or hybridization, was thought to be a rare event. With the advent of genome sequencing, biologists began to realize that hybridization is remarkably common throughout the tree of life. Despite this realization that hybridization is common, there is much we do not know about its evolutionary effects. In my dissertation work, I use a combination of genomic and computational techniques to ask how selection on hybrids shapes the genomes and evolution of hybrids and parental species. As a model system for this work, I focus on a group of freshwater fish, the swordtails (Poeciliidae: Xiphophorus). In my first chapter, I examine the genome of a proposed hybrid species, the swordtail fish Xiphophorus clemenciae. In my second chapter, I return to the question of hybrid speciation using theoretical and simulation approaches. I use models that describe how gene combinations that are under selection in hybrids (or “hybrid incompatibilities”) to predict how these regions of the genome will evolve in hybrids, and potentially contribute to reproductive isolation between hybrids and parent species. In my third chapter, I study the genomes of recent, natural hybrids between the sister species X. birchmanni and X. malinche. I identify on the order of two hundred pairs of hybrid incompatibilities and find evidence that these loci may actually play a role in limiting gene flow between species at functionally important genomic regions. In my fourth chapter, I study an ancient hybridization event between the swordtail fish X. nezahualcoyotl and X. cortezi. I find that at least two thousand generations have passed since initial hybridization in these species, allow time for selection to act on hybrid ancestry in the genome. I find that selection has constrained what regions of the genome are hybridization-derived to less functionally essential genomic regions. Overall, my dissertation work yields new insights into the importance of hybridization in speciation and how selection acts on hybridization-derived regions to shape hybrid ancestry in the genome.