To self, or not to selfyA review of outcrossing and pollen-mediated gene flow in neotropical trees

Despite the typically low population densities and animal mediated pollination of tropical forest trees, outcrossing and long-distance pollen dispersal are the norm. We reviewed the genetic literature on mating systems and pollen dispersal for neotropical trees to identify the ecological and phylogenetic correlates. The 36 studies surveyed found490% outcrossed mating for 45 hermaphroditic or monoecious species. Self fertilization rates varied inversely with population density and showed phylogenetic and geographic trends. The few direct measures of pollen flow (N1/411 studies) suggest that pollen dispersal is widespread among low-density tropical trees, ranging from a mean of 200m to over 19 km for species pollinated by small insects or bats. Future research needs to examine (1) the effect of inbreeding depression on observed outcrossing rates, (2) pollen dispersal in a wide range of pollination syndromes and ecological classes, (3) and the range of variation of mating system expression at different hierarchical levels, including individual, seasonal, population, ecological, landscape and range wide.