Geographical and historical determinants of microsatellite variation in Eucalyptus pilularis

Geographically distributed genetic variation is expected in species that have wide latitudinal and habitat ranges, like Eucalyptus pilularis Sm. Coastal and inland ecotypes of this tall forest tree have been distinguished in genecological studies, but patterns of regionally distributed quantitative variation are weak. At the coarsest level, variation of 12 microsatellite markers divided a rangewide sample of 424 E. pilularis trees into two zones: the region to the south of Sydney forming one zone and regions to the north forming another. Genetic structuring did not correspond with ecotypes but rather with a biogeographic division, suggesting an imprint of historical isolation. Typical and uniform levels of genetic diversity (He = 0.78 +/- 0.02 (mean +/- SE)) were found across 10 geographic regions. Genetic structuring by regions (PhiRT = 3%), by localities within regions (PhiPT = 2%), between coastal and inland provenances (PhiPT = 2%), or due to isolation by distance was subtle. These observations, along with the lack of evidence for bottlenecks, suggested genetic cohesion within zones due to gene flow and historically large population sizes. The low levels of diversity and poor growth performance of the Fraser Island ecotype were better explained by recent colonization and adaptation than by genetic isolation, since there was no evidence of inbreeding