Genetic differentiation among and within three red mahoganies, Eucalyptus pellita, E. resinifera and E. scias

The red mahogany group (Genus Eucalyptus Subgenus Symphyomyrtus Series Annulares) contains several species of importance to forestry in the tropics and subtropics because of their high productivity and tolerance to pathogens and pests in plantations. Despite recent revisions, and a number of studies to examine genetic and quantitative traits (including morphological and oil composition) variation within and among taxa, the taxonomy of the group remains problematic, and there is no consensus on membership at the species level or below. The taxonomy of three species, E. pellita, E. resinifera and E. scias, has been particularly challenging because of wide spread (in the case of E. pellita and E. resinifera) but disjunct, overlapping distributions and the high degree of genetic and/or morphological variation leading to the recognition of subspecies or geographic races in each taxon. Using microsatellite markers (n=13), genetic differentiation, among and within these three species, was characterised in order to revisit questions of taxonomy and the influence of geographic factors in determining within-taxon genetic structuring. Despite close geographical proximity and records of natural hybridisation in North Queensland, E. resinifera (total of 77 individuals from 8 locations) and E. pellita (total of 85 individuals from 12 locations) remain genetically distinct as taxa. Within E. pellita, two genetic groups were clearly resolved: one from New Guinea (includes all Indonesia and Papua New Guinea provenances) and one from Queensland (Cape York Peninsula populations were not sampled). Geographic structuring was also evident in E. resinifera, with North Queensland populations separating from those from Fraser Island southwards. Ecological factors and species disjunctions were implicated in the genetic substructuring of these two taxa because patterns of geographic variation aligned with biogeographical regions. Eucalyptus scias (19 individuals from 5 locations) was indistinguishable from southern E. resinifera and its two or three subspecies could not be resolved. At the taxon level, the three species exhibit levels of genetic diversity within the range of other eucalypts with similar distribution types, widespread in the case of E. pellita (He= 0.81) and E. resinifera (He= 0.87), and regional with disjunctions in the case of E. scias (He= 0.88). A previous trend of increasing genetic diversity with latitude in E. pellita evident in isozymes could not be confirmed; however, a strong inverse cline was evident in E. resinifera which may indicate a southern centre of origin for this species. Several New Guinea populations of E. pellita exhibited high inbreeding coefficients (Goe and Serisa provenances mean inbreeding coefficients of 0.233 and 0.173, respectively) that were thought to have biological origins i.e. either due to inbreeding amongst relatives or selfing.