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Background  Ring species, exemplified by salamanders of the  Ensatina eschscholtzii  complex, represent a special window into the speciation process because they allow the history of species formation to be traced back in time through the geographically differentiated forms connecting the two terminal forms of the ring. Of particular interest is the nature and extent of reproductive isolation between the geographically terminal forms, in this case  E. e. eschscholtzii  and  E. e. klauberi . Previous studies have documented infrequent hybridization at the end of the ring. Here, we report the first fine-scale genetic analysis of a hybrid zone between the terminal forms in southern California using individual-based Bayesian analyses of multilocus genetic data to estimate levels and direction of hybridization and maximum-likelihood analysis of linkage disequilibrium and cline shape to make inferences about migration and selection in the hybrid zone.    Results  The center of the hybrid zone has a high proportion of hybrids, about half of which were classified as F1s. Clines are narrow with respect to dispersal, and there are significant deviations from Hardy-Weinberg equilibrium as well as nonrandom associations (linkage disequilibria) between alleles characteristic of each parental type. There is cytonuclear discordance, both in terms of introgression and the geographic position of mitochondrial versus nuclear clines. Genetic disequilibrium is concentrated on the  eschscholtzii  side of the zone. Nearly all hybrids possess  klauberi  mtDNA, indicating that most hybrids are formed from female  klauberi  mating with male  eschscholtzii  or male hybrids (but not vice versa).    Conclusions  Our results are consistent with a tension zone trapped at an ecotone, with gene combinations characteristic of  klauberi  showing up on the  eschscholtzii  side of the zone due to asymmetric hybridization. We suggest that the observed asymmetry is best explained by increased discriminatory power of  eschscholtzii  females, or asymmetric postzygotic isolation. The relatively high frequency of hybrids, particularly F1s, contrasts with other contacts between the terminal forms, and with other contacts between other divergent  Ensatina  lineages, highlighting the diverse outcomes of secondary contact within a single species complex.

bmc evolutionary biology

Asymmetric reproductive isolation between terminal forms of the salamander ring species Ensatina eschscholtzii revealed by fine-scale genetic analysis of a hybrid zone