Hybridization between Cyprinodon atrorus and C. bifasciatus: History, patterns, and dynamics

This study sought to address whether environmental variation is important to the dynamics of hybridization between two pupfish species, Cyprinodon atrorus and C. bifasciatus, endemic to the Cuatro Cienegas basin, Coahuila, Mexico. To address this question, the history of hybridization between C. atrorus and C. bifasciatus was first investigated by conducting a basin level survey of nuclear gene (CK-A7, RAG-1, and TPI-B4) and mitochondrial (cyt b) variation in these two species. This analysis revealed that at the nuclear gene level, C. atrorus and C. bifasciatus are diagnostically different. Nuclear gene variation within these species suggests that the largest and oldest populations occur in the central and western basin for C. atrorus and C. bifasciatus , respectively. At the mitochondrial level, there has been complete replacement of C. bifasciatus mitochondrial genome by that of C. atrorus. Subsequent to this replacement there has been diversification of mitochondrial haplotypes along hydrogeographic regions, rather than by species. For the second facet of this project, reciprocal transplant experiments were conducted with C. atrorus and C. bifasciatus to test whether these species differ significantly in their environmental tolerances. Results from this study confirm the presumed euryplasticity of C. atrorus and stenoplasticity of C. bifasciatus, and suggest that introgression of C. bifasciatus genes into C. atrorus populations is limited at least in part by environmental tolerance differences between these species. For the final aspect of this project, spatio-temporal analyses of the relationship between environmental and genetic variation in a hybrid zone between C. atrorus and C. bifasciatus were conducted. On a spatial scale, there was a positive correlation between C. atrorus -specific allele frequency and highly variable environments, while there was a positive correlation between increased C. bifasciatus -specific allele frequency and thermally buffered environments, a pattern at least partially explained by the physiological characteristics of these species. On a temporal scale, allele frequency in the hybrid zone tracked environmental change over time, with C. bifasciatus-specific allele frequency declining in both cold and hot conditions, but increasing when thermal regimes of habitats acquired a spring-like thermal profile. Conversely, C. atrorus-specific allele frequency increased as physicochemical environments became more variable.