The effects of tropical deforestation and forest fragmentation on the population dynamics and the consequent genetic conservation value of these remnant tree stands is a subject of debate. This work describes the effect of fragmentation on the levels of diversity and the patterns of gene flow between 4 remnant stands of Swietenia humilis (Zucc.): one of the 3 economically important species commonly known as mahogany. Native to the Pacific coast of Central America, the once continuous dry forest is now highly disturbed and fragmented by human and agricultural encroachment. The levels of diversity and the dynamics of pollen flow were determined using simple sequence repeats (SSRs) as a marker system. These highly variable markers detected very high levels of allelic polymorphism with a mean heterozygosity of 0.52 (range 0.04-0.87). The historical population structure pre-fragmentation, assessed by an analysis of molecular variance (AMOVA), showed that 98.3% of variation is found within fragments indicating low population differentiation (Rst=0.02). Bulked progeny arrays of selected trees from each fragment were genotyped at 4 SSR loci and a paternity exclusion analysis was performed on the resulting multi-locus combinations of alleles. It is evident that the immediate effects of fragmentation on the patterns of pollen flow is dependent on fragment size and structure. The largest fragment in this study (n=98) is a good example of 'continuous' forest and the patterns of pollen flow show a favouring for near neighbour interactions. But the other smaller fragments of lower tree density show that there is extensive pollen flow between stands with distances of up to 6 km. Thus indicating that habitat fragmentation of S. humilis over this scale does not cause the reproductive isolation of the trees and that extensive pollen flow provides connectivity between the patches of remnant forests.

Key words: paternity analysis, pollen flow, simple sequence repeats, Swietenia humilis , tropical dry forest fragmentation