Leaf morphological evidence of natural hybridization between two oak species (Quercus austrocochinchinensis and Q-kerrii) and its implications for conservation management

Natural hybridization is known to be a potential threat to rare and endangered species due to the risk of extensive genetic swamping or assimilation. However, hybridization may also beneficial for rare species by increasing their genetic variation and adaptive potential. Quercus austrocochinchinensis is an endangered oak species that overlaps geographically with the common, widespread Q. kerrii. Morphological intermediates are common in contact zones between the species, suggesting interspecific hybridization. This phenomenon may lead to pure Q. austrocochinchinensis facing a larger threat. Given the importance of leaf morphology for hybrid identification in oaks, we characterized leaf morphological and anatomical features of pure populations of Q. kerrii and Q. austrocochinchinensis and a population of probable hybrids at the contact zone. The results demonstrate that (1) leaf morphological features of putative hybrids are stable and distinct from the parental taxa, and more similar to those of Q. kerrii; (2) anticlinal wall patterns of abaxial epidermal cells show the most significant differences between parental species and probable hybrids, providing an important character for species identification; and (3) putative hybrids were intermediate in leaf anatomical features between the two parental species. The leaf traits for identified Q. austrocochinchinensis, Q. kerrii and hybrids can be used to estimate the hybridization ratio at the contact zone. The morphological evidence, sympatry and phenology strongly suggest that the rare Q. austrocochinchinensis forms hybrids with the more widespread Q. kerrii. Natural hybridization might play an important future role in both migration and adaptation of the species to novel environments. Thus, the characters we identify here for diagnosing hybrids provide an important tool for conserving species and genetic diversity. We suggest that a management plan for Q. austrocochinchinensis should address the conservation of both of the pure species, investigation of gene flow dynamics in the hybrid zone and investigation of the impacts of gene flow on fitness of the two species.