Developing a genetic classification for gene pool management of spotted gums
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Spotted gums (Genus Corymbia Section Politaria) occur as a species replacement series along the eastern seaboard of Australia, their distributions marked by regions of disjunction and sympatry. Their taxonomy remains controversial, with species assignment often challenging and reliant upon knowledge of geographic origin as well as subtle morphological or leaf oil variation. In this paper we explore a classification for spotted gums without assuming predefined geographic or taxonomic groups, instead using genetic structure at microsatellite marker loci (n=9) and a Bayesian model-based clustering approach implemented in STRUCTURE software. The C. torelliana outgroup (n=21; Section Cadagaria) formed a well resolved cluster (min. pairwise Fst = 0.19). Four populations were evident within the spotted gums (n=93) but structure was weak (pairwise Fst range 0.13 -0.05). Geography, both distance and topography were major determinants of structure, with migration among populations approximating a linear stepping-stone model. Corymbia maculata was resolved as a taxon and had the greatest genetic distance to any other population (min pairwise Fst 0.08). Three clusters were evident within the northern taxa but alignment with taxonomic groupings was poor. Corymbia citriodora material from north of a major disjunction in Central Queensland formed a Northern population. Corymbia citriodora, C. variegata and C. henryi material from below this disjunction but north of the Border Ranges, formed a Central population, whereas a Southern population was comprised of C. variegata and C. henryi from predominately south of the Border Ranges. Fewer ambiguous assignments occurred using genetic rather than taxonomic groups for self classification of the spotted gum reference population.