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-mediat ed gene flow within and among populations of Cecropia obtusifolia, a d ioecious, wind-pollinated, tropical pioneer tree. We used allozyme dat a collected from leaf samples of adult trees and seeds from known moth ers in an 8.64 ha study plot located in pristine forest in the Los Tux tlas range in southern Mexico. Trees within the study plot have a patc hy distribution. Four populations (including the reference population) were in natural forest areas, and these were paired with four 'acahua l' populations (populations growing on human-disturbed sites). The pai red sites were located from 1 to 43 km from the reference population, with a ninth (acahual) population 100 km away. We addressed the follow ing questions: (1) Do males from the reference population contribute d ifferentially 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 contribution s from within and outside the study population? Male reproductive cont ributions 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 less than or equal to 0.001), independent of the male-specific diff erences. The addition of the eight outpopulations to the model showed that 37% of the offspring in the reference population were probably fa thered by males from the other populations. We observed a strong isola tion by distance effect for these populations, but long-distance polle n 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 F-ST anal yses.