Vagile but inbred: patterns of inbreeding and the genetic structure within populations of the monsoon rain forest tree Syzygium nervosum (Myrtaceae) in northern Australia

Patterns of gene flow within 21 northern Australian populations of Syzygium nervosum a dominant, mass-flowering, monsoon rain forest canopy tree were investigated using 10 isozyme loci. S. nervosum was found to have relatively high genetic diversity within populations (He = 0.307, AP = 3.7, P = 65) but also to have significantly lower frequencies of heterozygotes than expected (Ho = 0.126) and high allelic fixation (F = 0.512). Heterozygosity and allelic fixation were not correlated with measures of genetic diversity within populations, nor were they correlated with rain forest patch size, plant size or population isolation. Within populations, trees, of the same genotype (at each loci tested) were significantly clumped at short distances (c. 20 m), whereas trees of unlike genotypes were negatively associated. S. nervosum trees however, were not clonal in origin and had unique multilocus genotypes. The results suggest that the high levels of homozygosity recorded are the result of restricted pollination, primarily among flowers within individual trees or among closely related neighbouring trees, rather than rectricted seed dispersal. High homozygosity, the large fruit crop produced by trees of this species and the lack of association between heterozygosity and plant size, indicate that, S. nervosum is self-compatible, its fecundity does not appear to be impaired by inbreeding depression.