Niche partitioning among sexual and unisexual Ambystoma salamanders

Organisms that have ecologically similar sexual and asexual forms present an evolutionary puzzle, as theory predicts that eventually one form should eliminate the other. However, both forms may persist if there is niche partitioning between them. Geographical parthenogenesis is a hypothesis that predicts that in terms of niche use the asexual form in such pairs should be more ecologically successful in marginal habitats. This model has rarely been considered for gynogenetic taxa, as they are sperm dependent and thus constrained to the geographical range of sexual "sperm donor" species. Unisexual Ambystoma salamanders present a unique opportunity to look for evidence of niche partitioning and geographical parthenogenesis in a gynogenetic lineage, as there are multiple possible sperm donor species and so the geographical distributions of unisexuals are less constrained. We used broad sampling, comparative ecological niche models, and a model selection approach to determine (1) whether niche partitioning occurs among sexual and unisexual salamanders and (2) whether unisexual lineages indeed occupy marginal habitat relative to their most similar sexual species. Nearly all unisexual and sexual types showed significant niche differentiation. Predictions consistent with geographical parthenogenesis were upheld for one sexual-unisexual pair (A. jeffersonianum and LJJ unisexuals), but not for a second pair (A. texanum and LTT unisexuals). Our study provides evidence that different biotypes within the unisexual lineage have distinct ecological interactions with sexual taxa, supporting a role for these differences as a mechanism promoting coexistence between some sexual and unisexual forms. However, geographical parthenogenesis is only a partial explanation for sexual-unisexual coexistence, suggesting that other ecological and genetic mechanisms also play important roles in mediating coexistence among these salamanders.