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Understanding the coexistence of sperm-dependent asexual species and their sexual hosts : the role of biogeography, mate choice, and relative fitness in the Phoxinus eos-neogaeus (Pisces: Cyprinidae) system Mee, Jonathan Alan


In sperm-dependent asexual reproduction, sperm is not required for its genetic contribution, but it is required for stimulating zygote development. In my dissertation, I address several questions related to the coexistence of sperm-dependent asexuals and the sexually-reproducing species on which they depend. I have focused my research on a sperm-dependent asexual fish, Phoxinus eos-neogaeus, that originated via hybridization between P. eos and P. neogaeus. Using a mathematical model of mate choice among sexuals and sperm-dependent asexuals, I showed that stable coexistence can occur when there is variation among males in the strength of preference for mating with sexual females and when males with stronger preference pay a higher cost of preference. My model also predicts that coexistence is facilitated when the asexuals suffer a fitness disadvantage relative to the sexuals. Subsequent empirical work, in which I compared the repeat swimming performance, fecundity, and growth rate of asexual and sexual Phoxinus, provided results that are consistent with this prediction: the asexuals are, at best, as fit as the sexuals. I sampled Phoxinus populations from across the species’ North American distribution and the pattern of mitochondrial DNA variation across these populations suggests that all P. eos-neogaeus have originated from hybridization events that took place in a Mississippi River glacial refugium. Also, cytoplasmic hybrids (which are P. eos with P. neogaeus mitochondrial DNA) appear to have replaced ‘pure’ P. eos in all northern populations, which may reduce the disparity in fitness between P. eos-neogaeus and their sperm donors if P. neogaeus mitochondria are adapted to northern environments. The pattern of nuclear DNA variation across P. eos-neogaeus populations suggests that those using P. eos sperm are genetically distinct from those using P. neogaeus sperm. These genetically distinct populations match their host species in size, suggesting that sperm-dependent asexuals may be adapted to be proficient at soliciting sperm from particular sexual species. Persistence of sperm-dependent asexuals depends on stable coexistence with sexual species. My work highlights the importance of the relative fitness of asexual and sexual species, and of male mate choice in maintaining this stable coexistence.

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