Pinyon pine hybridization is widely acknowledged, but the frequency of and contributors to such interspecific mating remain largely unstudied. Pinus quadrifolia has three to four needles per fascicle, suggesting it is a result of hybridization between the five-needled P. juarezensis and the single-needled P. monophyla. In this study Ryan Buck and colleagues address the taxonomic validity of P. juarezensis, the hybrid origin of P. quadrifolia, and the presence of hybridization and intermediate morphology as a result of interspecific hybridization in this complex.
The authors found that the five-needled pinyons did not differ genetically from the four-needled P. quadrifolia, reducing P. juarezensis’ status to P. quadrifolia. They also found no evidence that P. quadrifolia is of hybrid origin from P. juarezensis x P. monophyla but is instead a genetically distinct species with natural needle number variation that has yet to be explained. Hybridization does occur in this complex, but mostly between P. quadrifolia and P. californiarum, and less commonly between P. quadrifolia and P. monophyla. Interestingly, some hybrid derivatives were detected between both single-needled taxa, P. monophyla and P. californiarum, a hybrid combination that has not yet been proposed. Hybrids have intermediate morphology when they have similar genetic contributions from both parental species, however, when one parent contributes more, hybrid derivatives resemble the parent with higher genetic contribution, resulting in cryptic introgression.
“This study provides an insight into the morphological and genetic outcomes of species undergoing gene flow,” write Buck and colleagues. “We plan to further explore these outcomes and patterns of hybridization in pinyon pines, especially in our newly discovered admixture combination of P. monophyla and P. californiarum. While our study addresses long-standing taxonomic issues within a small group of pinyon pines, the NGS methods and genetics first approach we used can be applied to a variety of taxa with controversial standings and cryptic introgression.”