The Pinyon Pine Syngameon: Modeling the Past to Understand the Present and Predict the Future
Syngameons are complex, multispecies hybridization networks made up of three or more species. The consequences of gene flow within a syngameon are largely unstudied, but participation may confer synergistic benefits that surpass those experienced by hybrid pairs. Climate change and habitat perturbation will likely increase the amount of interspecies interactions and could result in more hybridization events. The pinyon pines that occur throughout the southwestern United States and Baja California, Mexico are thought to hybridize due to their overlapping distributions, long-distance wind-mediated pollen dispersal mechanism, similar pollen dispersal times, observed intermediate morphology, and demonstrated lack of genetic incompatibility barriers. While the taxonomy of some species remains unresolved, drought tolerant traits have been observed within at least three taxa. If hybridization occurs in this complex, these traits could be introgressed across species barriers, which could be essential to the future survival of these species as high pinyon mortality has been observed after prolonged periods of drought. Using morphology, chloroplast haplotypes, next-generation sequencing, and climatic data, we address the presence of species barriers and hybridization, exploring how hybridization influenced the creation of the syngameon and how climate change may impact its future. Five taxa were genetically confirmed to be in this syngameon (Pinus edulis, P. monophylla, P. quadrifolia, P. californiarum, and P. x fallax), with extensive admixture among species. Species identities appear to be maintained at range cores with introgression only occurring where species distributions overlap, making this system a range-edge syngameon. Notably, sequential hybridization was detected where two species (P. edulis and either P. monophylla or P. quadrifolia) hybridized to create P. californiarum, which is currently hybridizing with P. edulis to produce the P. x fallax lineage. Populations composed exclusively of or mostly of admixed individuals were found in areas of tri-species sympatry, suggesting that genetic swamping could be occurring in these areas. Future niche models predict the loss of suitable habitat in all climate scenarios modeled, with minor expansions northwards and up in elevation. Populations of P. californiarum showed low structure and genetic diversity, with high levels of inbreeding, indicating that this rarely recognized species should be of conservation concern.