Genetic erosion of Platanus racemosa by hybridization

Synopsis: The California native sycamore, Platanus racemosa, is an important component of riparian communities along the Sacramento River, but faces a recent invasive competitor, the widely planted ornamental sycamore Platanus x hispanica. The ornamental species, often referred to as the London Plane Tree, is itself a hybrid of the common sycamore of the Eastern US (P. occidentalis) and the European plane tree (P. orientalis). When planted in the Eastern US, the London Plane has considerable advantages over the native species, including increased resistance to fungal diseases. In California, previous work demonstrated hybridization may be occurring between the planted ornamentals and the native P. racemosa. If this hybridization is extensive, it may result in a lack of genetic distinctiveness for P. racemosa if P. x hispanica escapes from ornamental planting habitats, and if hybrids outcompete the native species, threatening its continued existence.

We designed and used microsatellite markers to investigate the extent of hybridization between the ornamental and native species. Using principle coordinate analysis and admixture analysis in Structure, we demonstrated that two clear genetic clusters could be observed in 434 trees sampled along the northern Sacramento River. These two clusters correspond to P. racemosa and P. x hispanica, but also indicate a large number of individuals with mixed ancestry. We also collected tree size data (diameter) which we used as a proxy for age, and showed that admixed individuals tended to be younger, and are beginning to outnumber “pure” P. racemosa individuals among the youngest age classes. If the current situation continues, it may result in a loss of genetic distinctiveness for P. racemosa in California. We recommended that restoration efforts use our genetic data to select individuals with larger proportions of P. racemosa and conservation efforts be directed at reducing ornamental planting of P. x hispanica.

My role: I realize that this work does not particularly fall under #mossmatters, but I was excited to be a part of this project. I was invited, based on my experience on previous manuscripts, to analyze the microsatellite data and present the results by the main PIs– Kristina Scheirenbeck and Paul Manos. This was my first experience conducting a genetic analysis on a species with conservation implications, and the first time that something I’d written may have implications for restoration efforts of a native species. I also enjoyed a brief respite from the unending crush of genomic data to re-visit my old friend: Structure plots. I also realized our figure showing that hybrids are slowly accumulating based on the principal coordinate analysis and age classes would make for an interesting animated GIF. I figured out how to use ImageMagick to turn a multi-page PDF generated in R into a GIF (click the image to animate):


The full manuscript in Conservation Genetics is available online now.

M.G. Johnson, K. Lang, P. Manos, G.H. Golet, and K.A. Schierenbeck. 2016. “Evidence for genetic pollution of a California native tree, Platanus racemosa, via recent, ongoing introgressive hybridization with an introduced ornamental species.” Conservation Genetics. First online: 07 January 2016. doi:10.1007/s10592- 015-0808-z.

This work was funded by the Nature Conservancy and California State University at Chico.

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  1. Pingback: Unraveling the Mystery of the Western Sycamores that Weren’t - OVL Web

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