Genetic entities and hybridisation within the Acacia microbotrya species complex in Western Australia

Abstract

Species complexes are often taxonomically challenging because the evolutionary relationships among closely related taxa may not be well reflected by morphological divergence. Molecular data can be a powerful tool in resolving these ambiguities. Using nuclear restriction fragment length polymorphisms (RFLPs), we examined genetic variation among 22 populations of the Acacia microbotrya complex and assessed its congruence with five taxa previously identified using morphological criteria. Four distinct genetic clusters were consistently detected across multiple analytical approaches (Bayesian, UPGMA, maximum likelihood, and principle coordinate). These genetic clusters were strongly congruent with each of four morphologically defined taxa, Acacia amblyophylla, Acacia daphnifolia, A. microbotrya and A. microbotrya (Dandaragan variant). From this, we suggest that the Dandaragan variant warrants formal recognition at the same taxonomic level as the other three taxa. Individuals of the fifth morphologically defined species, Acacia splendens, were not consistently assigned to any one genetic cluster having affinities with A. microbotrya (Dandaragan variant) and A. amblyophylla, although at the population level it showed closest affinity to A. microbotrya (Dandaragan variant). Despite strong genetic and morphological differentiation of these taxa in allopatric regions, we detected a hybrid zone of genetic admixture between A. microbotrya and A. daphnifolia where their geographic ranges overlap. Together, these results highlight a complex evolutionary history of fragmentation, divergence and hybridisation in the A. microbotrya complex. While further work is required to resolve the phylogenetic status of A. splendens, this study provides compelling evidence for the recognition of these four genetic clusters as distinct evolutionary entities in the A. microbotrya complex that should be treated as separate units for conservation, commercial or management purposes.