Molecular and morphological variation in the Senna artemisioides complex

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