Multiple nuclear loci reveal the distinctiveness of the threatened, neotropical Pinus chiapensis

Pinus chiapensis is a threatened species of pine from southern Mexico and Guatemala. It was first described as a disjunct variety of P. strobus from the eastern United States and Canada. Subsequent morphological work indicates that P. chiapensis is a distinct species, but this interpretation is controversial. To explore the distinctiveness of this taxon, we sequenced three low-copy, unlinked nuclear loci in multiple accessions of P. chiapensis and its three most probable progenitors (P. ayacahuite, P. monticola, and P. strobus). Pinus chiapensis had the lowest combined nucleotide diversity of the four species (0.0031), and had only a single allele rangewide at one locus. Pinus chiapensis does not share alleles with any of the possible progenitors and all of its alleles are monophyletic at two of the three loci. At the third locus, allelic nonmonophyly is statistically indistinguishable from monophyly. While our results show that P. chiapensis is at least as distinct as the remaining three widely accepted species, determination of the most recent common ancestor is complicated by lack of allelic monophyly within potential progenitors and interlocus variability. Based on our sample of individuals and loci, P. ayacahuite appears to be the least likely progenitor, but there is no clear resolution of whether P. chiapensis is more closely related to P. monticola or P. strobus.