Influence of climatic niche suitability and geographical overlap on hybridization patterns among southern Californian oaks

Aim: Information on the association between geographical patterns of hybridization and the climatic niche requirements and co-occurrence of the species involved can provide important insights that further our understanding of the factors promoting the formation of hybrid zones. Here, we test whether climatic niche suitability explains patterns of hybridization beyond spatial overlap in distributions of the geographically restricted Engelmann oak (Quercus engelmannii) and the widespread scrub oak species complex (Quercus spp.).

Location: Southern California, USA.

Methods: We sampled Engelmann and scrub oaks across 31 localities and genotyped 343 individuals at nine microsatellite loci. We used climatic niche modelling to assess the suitability of habitats for each parental species and hybrids and to analyse whether climatic niche suitability in addition to co-occurrence are associated with observed spatial patterns of hybridization.

Results: Our data indicated that hybrid zones are located in areas that are geographically constrained by the presence of scattered patches of Engelmann oak within a matrix broadly occupied by scrub oaks. However, the climatic niche of hybrids differed from that occupied by both Engelmann and scrub oaks, suggesting the importance of climate on their establishment and persistence beyond the simple role of geographical overlap of parental species distributions. We also found that even moderate levels of introgression contributed to increased genetic diversity in Engelmann oak but not in scrub oaks.

Main conclusions: These results show that hybrid zones follow a mosaic pattern constrained by the presence of scattered patches occupied by Engelmann oak and climatically suitable areas that favour the establishment of hybrids. The low rates of ongoing hybridization and the bimodal distribution of parental genotypes found here suggest that hybridization is not threatening Engelmann oak and this phenomenon could instead be promoting the acquisition of favourable alleles that may increase the adaptive potential of this vulnerable species.