Molecular and morphological variation in the Senna artemisioides complex

The Senna artemisioides species complex is characterised by extremely variable foliar morphology. This paper presents the results of an integrative study on reproductive biology, morphology and genetic variation to explore the evolutionary and systematic relationships within this species complex at Idalia National Park in central Queensland. Senna artemisioides showed discontinuous variation at both genetic and phenotypic level. At three sites, foliar morphology was discontinuous, being represented as discrete clusters. Analysis of isozyme variation indicated that many of the discrete morphological groups are distinguished by fixed genetic differences. Morphotypes were also characterised by widespread monomorphism. Most of the genetic variation was between morphotypes rather than within morphotypes. These fixed genetic differences indicate a lack of gene flow between morphotypes and therefore an absence of hybridisation. The two most distinct morphological groups (pyllodinous, pinnate) had high genetic distances of 0.5, a value beyond the usual distance between congeneric species. Forty-two percent of pairwise comparisons between other morphotypes yielded genetic distance values that were relatively high (0.21-0.42). Some phenotypically distinct morphotypes, however, showed little or no isozyme divergence. The combined evidence suggests that genetically distinct morphotypes are independent lineages originating from an ancestral sexual reproductive event that is perpetuated by apomictic reproduction.