Long-distance colonization, isolation by distance, and historical demography in a relictual Mexican pinyon pine (Pinus nelsonii Shaw) as revealed by paternally inherited genetic markers (cpSSRs).

Pinus nelsonii is a relictual pinyon pine distributed across a wide altitudinal range in semiarid zones in Mexico near the border between the States of Nuevo León and Tamaulipas. It also occurs in small patches in the State of San Luis Potosí. Pinus nelsonii is classified in the monotypic subsection Nelsoniae, separated from other pinyon pines (subsection Cembroides), because it possesses several distinctive characters including persistent fascicle sheaths, connate needles, and a distinctive wood anatomy. In the present study, chloroplast simple sequence repeats (cpSSRs) were used to estimate genetic variation in most known populations (nine) of P. nelsonii. The genetic variation (HT = 0.73; 27 haplotypes in 256 individuals) is moderate when compared to other pine species. Population differentiation ranged between low and moderate (FST = 0.13 and RST = 0.05), as did the Nei and Goldstein genetic distances between populations. However, this pattern varied depending on whether the infinite alleles or stepwise mutation model was used. In the former case a significant isolation by distance was found, but not in the latter. A significant association between geographical and genetic structure in one clade, through a nested clade analysis, was found, which suggested long-distance colonization between 125000 and 309000 years ago. We found weak evidence for a population expansion. A mismatch distribution suggests that P. nelsonii populations underwent an expansion 4.25 times their size between 59000 and 146000 years ago. On the other hand, the populations' star-like phylogeny and a slight parabolic relationship between coalescence times and lineage number also suggest weak population expansion. Overall, this species appears to have been in demographic stasis for a large proportion of the time detected by the markers used.