UC Riverside

There is remarkable diversity in the form and function of vertebrate reproductive mode, and adaptive explanations for the vast differences among species have fallen short. Instead, parent-offspring conflicts provide a parsimonious framework that describes why evolutionary transitions occur from one mode to another, and how the differences among species change the nature of sexual selection and speciation. My dissertation examines the effect of reproductive mode on vertebrate evolution by examining two topics – the evolution of matrotrophy following a transition from oviparity to viviparity and the effect differences in reproductive mode have the evolution of reproductive isolation and the rate of speciation. Cyprinodontiformes, an order of small mostly freshwater fish, are notable for exhibiting a wide range of reproductive phenotypes. I make use of the repeated transitions from oviparity to viviparity and from lecithotrophy to matrotrophy in Cyprinodontiformes to test hypotheses that parent-offspring conflicts have driven the evolution of reproductive mode.

In chapter one I demonstrate eggs from three oviparous species from Cyprinodontiformes and one from Atherinomorpha are capable of acquiring molecules from their surrounding environment via pinocytosis, a property that predisposes them to the evolution of matrotrophy following the transition to viviparity. In chapter two I find evidence that post-zygotic reproductive incompatibilities are evolving faster among populations within the placental species Poeciliopsis prolifica than within two closely related non-placental species of Poeciliopsis. In the placental species, offspring size decreased significantly as a function of increasing interpopulation distance, but offspring from non-placental species suffered no such fitness loss. In chapter three I demonstrate that interspecific post-zygotic reproductive isolation evolves at an accelerated rate among viviparous species relative to oviparous species, and that estimated levels of post-zygotic isolation are higher among matrotrophic species than among lecithotrophic species at all genetic distances. Similarly, I find diversification rates estimated from molecular phylogenies to be significantly higher for viviparous taxa than oviparous taxa, but marginally higher for lecithotrophic species than for matrotrophic species. As a whole, the results of this dissertation are consistent with hypotheses that parent-offspring conflicts have played a part in the evolution of vertebrate reproductive mode, and that variation in the nature of conflicts among taxa influence speciation.