Abstract
When non-native, genetically diverse species are introduced, hybridization with native congeners may erode the genetic composition of local species, perhaps even resulting in extinction. While such events may lead to adverse consequences at the community and ecosystem level, few studies exist on ecologically important tree species. In the genus Platanus, introgressive hybridization is widespread, and one common ornamental species, introduced to California during the late 19th century, is itself a hybrid. Our microsatellite analysis of more than 400 Platanus trees from north-central California reveals a complex pattern of invasion and hybridization in an age-structured population. By using size as a proxy for age, we have demonstrated that the Platanus population of north-central California has recently gained genetic diversity and effective population size. Principal coordinate analysis (PCoA) and genetic admixture analysis (STRUCTURE) both reveal a strong differentiation of genotypes into two main genetic clusters, with a large number of admixed genotypes. One of the genetic clusters identified is heavily biased towards younger trees, including samples from locations with relatively recently planted ornamental trees likely to be P. × hispanica (formerly known as P. × acerifolia). We conclude that the two genetic clusters correspond to the native P. racemosa and the introduced invasive hybrid species P. × hispanica. Additional hybridization between the invasive ornamental and the native species has occurred in California, and recent hybrid trees are more likely to be younger than trees without admixture. Our findings suggest that the observed increase in genetic diversity among California Platanus is due to rampant ongoing introgression, which may be threatening the continued genetic distinctiveness of the native species. This is cause for concern from a conservation standpoint, due to a direct loss of genetic distinctiveness, and a potential reduction in habitat value of associated species.
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Acknowledgments
This project was funded by a grant from The Nature Conservancy to KAS. We greatly thank Evan Padgett, Celia Eckert, Clara Buchholtz, and Camellia Boutros for assistance with the collection of the plant material and DNA extractions, and Shannon Still for help creating maps in R. We also thank two anonymous reviewers for their comments on an earlier version of the manuscript. The entire dataset for this study, including microsatellite alleles, GPS coordinates, PCoA results, and admixture analysis, can be found in Supplementary Online Material.
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Johnson, M.G., Lang, K., Manos, P. et al. Evidence for genetic erosion of a California native tree, Platanus racemosa, via recent, ongoing introgressive hybridization with an introduced ornamental species. Conserv Genet 17, 593–602 (2016). https://doi.org/10.1007/s10592-015-0808-z
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DOI: https://doi.org/10.1007/s10592-015-0808-z