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Hoai Trang Nguyen - Botany 2010

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Uploaded on Aug 10, 2010

Evolutionary relationships of the 'sky island' pines (Pinus subsection Ponderosae) based on nuclear and plastid microsatellite loci. Co-authors: Nicole Segear, David A. Gernandt and Ann Willyard

Recent evidence from two low-copy nuclear gene trees suggests that pines (Ponderosae: Pinaceae) growing on a high-altitude 'sky island' (Mt. Lemmon, Arizona) do not belong to either of the taxa with which they have been classified: Pinus ponderosa Douglas ex. P.&C. Lawson or P. scopulorum Lemmon. Rather, all three exemplars resolve as sister to several Ponderosae from south central Mexico. Here, we present the first molecular profiles for morphologically similar pines growing on other sky islands in southeastern Arizona, testing three alternative hypotheses: 1) the sky island pines fit within an extensive southern Rockies group (i.e. P. brachyptera Engelm.); 2) the sky island pines form a taxon distinct from P. brachyptera; 3) reticulate evolution created the distinctive genotype observed on Mt. Lemmon. Introgressive hybridization between sky island pines and P. arizonica Engelm. and/or P. engelmannii Carriere is possible, both of which have affinity to Mexican Ponderosae. Species-level Pinus phylogenies have been confounded by incomplete coalescence in nuclear gene trees and by the sharing of plastid lineages. To overcome these obstacles, we used ca. 250 samples from 15 populations: sky island pines representing four different mountain ranges in Arizona; P. brachyptera from eastern New Mexico; P. arizonica and P. engelmannii from Arizona; P. ponderosa from California, Idaho, and Oregon; and P. scopulorum from northern Utah. Seven nuclear and fifteen plastid microsatellite loci (SSRs) were genotyped. Allele frequencies were used in separate genetic admixture models for nuclear and plastid genomes to infer the extent of inter-specific hybridization. We visualized clustering of individuals within their putative species using evidence from each genome based on genetic distance models with differing assumptions: the stepwise mutation model and the geometric chord distance. Our discussion places these population-level results into the phylogenetic perspective provided by low-copy nuclear gene trees.

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