Ancestry and divergence of subtropical montane forest isolates:: molecular biogeography of the genus Abies (Pinaceae) in southern Mexico and Guatemala

The genus Abies has a complex history in southern Mexico and Guatemala. In this region, four closely related species, Abies flinckii, A. guatemalensis, A. hickelii, and A. religiosa, are distributed in fragmented and isolated montane populations. Range-wide genetic variation was investigated across species using cytoplasmic DNA markers with contrasted inheritance. Variation at two maternally inherited mitochondrial DNA markers was low. All species shared two of the nine mitotypes detected, while the remaining seven mitochondrial DNA types were restricted to a few isolated stands. Mitochondrial genetic differentiation across taxa was high (G(ST) = 0.933), it was not related to the taxonomic identity (AMOVA; P > 0.05) of the populations, and it was not phylogeographically structured (G(ST) approximate to N-ST). In contrast, variation at three paternally inherited chloroplast DNA microsatellites was high. Chloroplast genetic differentiation was lower (G(ST) = 0.402; R-ST = 0.547) than for mitochondrial DNA, but it was significantly related to taxonomy (AMOVA; P < 0.001), and exhibited a significant phylogeographical structure (G(ST) < R-ST). Different analyses of population structure indicated that A. flinckii was the most divergent taxon, while the remaining three species formed a relatively homogeneous group. However, a small number of the populations of these three taxa, all located at the limits of their respective ranges or in the Transverse Volcanic Belt, diverged from this main cluster. These trends suggest that the Mesoamerican Abies share a recent common ancestor and that their divergence and speciation is mainly driven by genetic drift and isolation during the warm interglacial periods.