Genetic diversity and phylogeographic analysis of Pinus leiophylla: a post-glacial range expansion

Aim Mexico is a centre of diversity for species of the genus Pinus, most of which have restricted geographical distributions. An exception is Pinus leiophylla Schiede and Deppe, which is widely distributed throughout most of Mexico's mountainous regions. We attempt to reconstruct the phylogeographic history of this species, in order to determine if its current broad distribution is associated with major events of environmental change that occurred during the Quaternary. Location Coniferous forests in Sierra Madre Occidental, Eje Volcanico Transversal and Sierra Montanosa del Norte de Oaxaca, Mexico. Methods A total of 323 individuals of both P. leiophylla var. leiophylla and P. leiophylla var. chihuahuana sampled from 22 populations were screened for variation at six paternally inherited chloroplast DNA microsatellite markers (cpSSR). In addition to haplotypic diversity estimates and neutrality tests, the following clustering methods were employed: principal components analysis (PCA), analysis of molecular variance (AMOVA), spatial analysis of molecular variance (SAMOVA), haplotype network and a technique similar to Croizat's panbiogeographical method of individual and generalized tracks. Results The combination of mutations at the six microsatellites yielded a total of 92 different haplotypes. The percentage of shared haplotypes between varieties (P. leiophylla var. leiophylla and P. leiophylla var. chihuahuana) was only 2.2%. The average haplotypic diversity for the species was H = 0.760. PCA and SAMOVA indicate the presence of four main genetic clusters. The estimated divergence time between the two most frequent haplotypes was between 75,000 and 110,000 years. Significantly large negative F(s) values suggest that most of the sampled populations are currently expanding. Individual and generalized tracks identified three potential zones that may have harboured ancestral populations of P. leiophylla and from which the expansion of this species started, as well as two secondary contact zones between the two varieties. Main conclusions The results indicate that one of the three potential areas hypothesized to have harboured ancestral populations of P. leiophylla may be related to the origin of P. leiophylla var. chihuahuana, while the other two may be related to the origin of P. leiophylla var. leiophylla. The current broad distribution of P. leiophylla is probably associated with its strong colonization ability.