Phylogeography of Podocarpus matudae (Podocarpaceae): pre-Quaternary relicts in northern Mesoamerican cloud forests
Cloud forests of northern Mesoamerica represent the northern and southern limit of the contact zone between species otherwise characteristic of North or South America. Several phylogeographic studies featuring temperate conifer species have improved our understanding of species responses to environmental changes. In contrast, conifer species that presumably colonized northern Mesoamerica from South America are far less studied. A phylogeographic study of Podocarpus matudae (Podocarpaceae) was conducted to identify any major evolutionary divergences or disjunctions across its range and to determine if its current distribution is associated with pre-Quaternary climatic and/or long-distance dispersal events. Location
Northern Mesoamerica (Mexico and Guatemala). Methods
Sixteen populations (157 individuals) of P. matudae were screened for variation at two plastid DNA markers. The intra-specific phylogenetic relationships among haplotypes were reconstructed using Bayesian inference. Population genetic analyses were undertaken to gain insight into the evolutionary history of these populations. To test whether genetic divergence among populations occurred at different time-scales plastid DNA sequence data and fossil- and coalescent-based calibrations were integrated. Results
The combination of plastid markers yielded 11 haplotypes. Differentiation among populations based on DNA variation (GST) (0.707, SE 0.0807) indicated a clear population structure in P. matudae. Differentiation for ordered alleles (NST) (0.811, SE 0.0732) was higher than that for GST, indicating phylogeographical structure in P. matudae. Most of the total variation (81.3%, P < 0.0001) was explained by differences among populations. The estimated divergence time between the unique haplotypes from a Guatemalan population and the two most common haplotypes from the Sierra Madre Oriental in Mexico was between 10 and 20 Ma, and further haplotype divergence in the poorly resolved clade of the Sierra Madre Oriental occurred between 3 and 0.5 Ma. Main conclusions
Divergence estimations support the hypothesis that extant Podocarpus matudae populations are pre-Quaternary relicts. This finding is consistent with fossil and pollen data that support a Miocene age for temperate floristic elements in Mesoamerican cloud forests, whereas further haplotype divergence within the Sierra Madre Oriental, Chiapas and Guatemala occurred more recently, coinciding with Pleistocene cloud forest refugia.