Geographical and historical determinants of microsatellite variation in Eucalyptus pilularis

Geographically distributed genetic variation is expected in species like Eucalyptus pilularis that have wide latitudinal and habitat ranges. 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 at microsatellite markers (n=12) divided a range-wide sample of 424 E. pilularis trees into two zones, the region to the south of Sydney forming one zone distinct from regions to the north. 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. Structuring into regions (PhiRT = 3%), localities within regions (PhiPT = 2%), between coastal and inland provenances (PhiPT = 2%), or due to isolation by distance, was subtle. These factors, along with the lack of evidence for bottlenecks, suggested cohesion within zones due to gene flow and large population sizes historically. Recent colonisation and adaptation better explained the low levels of diversity and poor growth performance of the Fraser Island ecotype rather than genetic isolation as there was no evidence of inbreeding.