Pollen-mediated gene flow and differential male reproductive success in a tropical pioneer tree, Cecropia obtusifolia Bertol. (Moraceae): a paternity analysis

We used a likelihood-based paternity analysis to examine pollen-mediated gene flow within and among populations of Cecropia obtusifolia, a dioecious, wind-pollinated, tropical pioneer tree. We used allozyme data collected from leaf samples of adult trees and seeds from known mothers in an 8.64 ha study plot located in pristine forest in the Los Tuxtlas range in southern Mexico. Trees within the study plot have a patchy distribution. Four populations (including the reference population) were in natural forest areas, and these were paired with four ‘acahual’ populations (populations growing on human-disturbed sites). The paired sites were located from 1 to 43 km from the reference population, with a ninth (acahual) population 100 km away. We addressed the following questions: (1) Do males from the reference population contribute differentially to reproduction within the stand?; (2) How do intermate distance and patch affiliation affect pollination within the reference population?; (3) What are the relative male reproductive contributions from within and outside the study population? Male reproductive contributions were significantly uneven within the reference population (P<0.001). We demonstrated modest isolation by distance and an effect of patch affiliation for mating pairs within the reference population (P≤0.001), independent of the male-specific differences. The addition of the eight outpopulations to the model showed that 37% of the offspring in the reference population were probably fathered by males from the other populations. We observed a strong isolation by distance effect for these populations, but long-distance pollen flow is enough to have a strong homogenizing effect on the regional gene pool. None of the acahual populations appears to have contributed pollen. Paternity analysis yields a more detailed view of the effects of pollen-mediated gene flow than had emerged from previous FST analyses.