Hybridization can substantially alter the evolutionary trajectory of a species. When species hybridize following a species introduction or anthropogenic disturbance, hybridization becomes an important conservation concern. Negative effects of hybridization on native species can include genetic or demographic swamping, potentially leading to extinction or local extirpation. Hybrids can also compete with parental species, and sometimes have higher fitness. It is difficult to predict outcomes of hybridization due to the potential for geographic variation, the poorly understood phenotypes and relative fitness of hybrids and parental species, and the potential for environmental context to influence hybridization outcomes. It is therefore essential to more precisely quantify variation in hybridization and explicitly link these outcomes to genetic and ecological mechanisms. We quantified variation in hybridization among 27 streams where Yellowstone cutthroat trout come into contact with introduced rainbow trout in the North Fork Shoshone River basin (Wyoming, USA) using genomic data. Using environmental data and fish stocking records, we modeled variation in genomic outcomes of hybridization as a function of environmental and ecological correlates.
Results/Conclusions
Hybridization occurred at all locations, but the identity of hybrids (F1, F2, backcrosses, etc.) varied substantially among locations. Variation in hybridization suggests geographic heterogeneity in persistence of native Yellowstone cutthroat trout and therefore variable prospects for conservation among locations. We also explored potential mechanisms that might explain variation in prezygotic reproductive isolation among species, including environmental correlates, history of trout stocking, and history of anthropogenic disturbance. Environmental conditions can influence hybridization outcomes by altering the degree of spatial and temporal overlap in spawning between parental species. Stocking history can alter the ratio of parental species, and therefore might affect mate choice and drive variation in hybridization outcomes. Anthropogenic disturbance can affect hybridization by both of these mechanisms, or might change the fitness landscape for hybrids. The strongest predictors of both persistence of native Yellowstone cutthroat trout and relative abundance of hybrid individuals were related to historical stocking of both native and non-native trout in the North Fork Shoshone river drainage. This finding suggests that variation in prezygotic barriers – in this case, the probability of finding a conspecific mate – might account for variation in hybridization outcomes. Our ongoing work in this system will directly quantify postzygotic selection against hybrids, and will result in better understanding of mechanisms that cause variable hybridization outcomes.