Gene flow interruption in a recently human-modified landscape: The value of isolated trees for the maintenance of genetic diversity in a Mexican endemic red oak

Gene flow within and among populations is an important factor to maintain genetic cohesiveness and diversity across landscapes. Nowadays, human land use has led to a large forest conversion, creating many fragmented areas where remnant trees play an important role in conserving biodiversity. In this study, we analyzed the effects of a recent anthropogenic forest fragmentation on the genetic diversity and genetic heterogeneity of pollen pools accepted by individuals of the red oak Quercus castanea growing in forest patches and as isolated trees in central Mexico. Pollen movement was also evaluated by the analysis of outcrossing rates using seven nuclear microsatellites. We assumed that adult trees are remnants of the populations that existed previous to the forest fragmentation, while progenies of these trees are the result of recent reproductive events occurring after the fragmentation. We found high genetic diversity in both adult trees and progenies, even though progenies of isolated trees showed a significant reduction in heterozygosity as compared to their mother trees. However, the results of TWOGENER and mating system analyses indicated similar numbers of pollen donors in the progenies of mother trees from fragments and in isolated trees. Overall, our results suggest that gene flow is still extensive among forest fragments and isolated trees, conferring them a great value for the conservation of genetic diversity and connectivity.