Population genetics, comparative floral morphology, and rare hybridization for two serpentine Phlox species

Hybridization between rare and widespread species can result in loss of genetic integrity for the rarer species, which can have management and conservation implications. One rare species, Phlox hirsuta, is a serpentine endemic in northern California, and it frequently co-occurs with a widespread congener, Phlox speciosa. Putative hybrids were recognized based on intermediate morphology, so the possibility of hybridization was explored using floral morphological and molecular data. Ninety-eight individuals of P. hirsuta and P. speciosa were collected from each of three sites, and floral features were measured and compared. Eleven microsatellite loci were amplified for species and putative hybrids, and inter- and intraspecific genetic diversity and relationships were investigated with multiple methods. Variation in morphological and molecular data was recovered. Floral variation was greater for P. hirsuta than P. speciosa. Putative hybrids were genetically allied with P. speciosa, but two individuals of P. hirsuta were resolved to have genetic similarity with P. speciosa. While hybridization is possible between the species, it is uncommon and appears to be primarily unidirectional. The small number of recognized hybrids may be due to ineffective interspecific pollination, hybrids being less fit than parents, and/or small sample sizes. Reinforcement does not appear to play a role in secondary contact between species. Both microsatellite loci and floral morphology varied across the small geographic range of P. hirsuta, suggesting local differentiation and adaptation are possible over short distances.