The Evolution of Reproductive Mode and Associated Familial Conflict in Livebearing Fishes
Parent-offspring conflict arises because of the unequal patterns of relatedness among parents, offspring, and siblings. From an evolutionary perspective, parents optimize investment in each offspring to maximize their lifetime reproductive success. Females are equally related to all their offspring, so natural selection favors them allocating resources equally to each offspring. In contrast, natural selection will favor those offspring that acquire more resources, even at the expense of their mother and siblings. Recent thinking expands the potential influence of parent-offspring conflict from post-natal familial interactions to other facets of the organism’s biology. The evolution of a livebearing reproductive mode, particularly involving a placenta, for example, is predicted to cause pre-natal mother-offspring conflict. In the livebearing fish family Poeciliidae, a placenta-like organ has evolved independently at least nine times. In my dissertation, I examine the potential for reproductive barriers in placental (Heterandria formosa and Poeciliopsis prolifica) and non-placental (P. infans) poeciliid species. I perform both natural and artificial insemination crosses between populations within three separate species of fish in the family Poeciliidae. In my first chapter, I find that both placental and non-placental species exhibit reproductive incompatibility, but the location of the incompatibility is different. In my second and third chapters, I use artificial insemination to further investigate the effects of inter-population crosses on offspring size and number in two placental species, H. formosa and P. prolifica. In these species, natural differences in offspring size between populations of H. formosa are the source of conflict in offspring size. However, when females are mated to both their own and nonresident males, they produce intermediate offspring, discriminating against nonresident male’s embryos. In P. prolifica, while there are no natural offspring size differences in the two populations, we find a similar pattern of incompatibility present in crosses that is mitigated when females are provided with sperm from her own, and a genetically distinct population. These results provide a new path for studying reproductive incompatibility and conflict in placental species, as well as evidence for cryptic female choice and discrimination against genetically different males.