Genetic structuring in the spotted gum complex (genus Corymbia, section Politaria)

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 on knowledge of geographic origin as well as subtle morphological or leaf-oil variation. In the present paper, we explore a classification for spotted gums, without assuming predefined geographic or taxonomic groups but 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 (minimum pairwise F_st = 0.19). Four populations were evident within the spotted gums (n = 93) but structure was weak (pairwise F_st range 0.13–0.05). Geographic distance, topography and distribution disjunction 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 from any other population (minimum pairwise F_st 0.08). Three clusters were evident within the northern taxa but alignment with taxonomic groupings was poor. C. citriodora material from north of a major disjunction in central Queensland formed a Northern population. C. citriodora, C. variegata and C. henryi material south of this disjunction but north of the Border Range, formed a Central population, whereas a Southern population comprised C. variegata and C. henryi from predominately south of the Border Range.