A cytotaxonomic study of Australian Cardamine

Abstract

A cytogenetic study of Cardamine, based on material collected in southern New South Wales and the Australian Capital Territory, indicated the presence of at least six distinct species in this area. Two species, the 16-chromosome C. hirsuta and the 32- chromosome C. flexuosa, were introduced with white settlement. The other four species are native to the area, occupying a variety of habitats between sea-level and 7200 ft. One species, with a somatic chromosome number of 64, is confined to altitudes below 4000 ft, whereas the other three species (2n = 48) are found only above 4000 ft. Of the 48-chromosome species, one comprises a single population found at Mt. Franklin and the remaining two are confined to particular altitudinal ranges, one between 4000 and 5400 ft and the other between 5200 and 7200 ft. All four native species are completely isolated from one another by the inviability of the F₁ hybrids. Within species, however, all populations were fully interfertile and there was no evidence of any other form of reproductive isolation. Despite the magnitude of reproductive isolation there were no distinct morphological differences between the native species except in the case of the 48-chromosome form from Mt. Franklin, which had entire rather than pinnate leaves. The other three species were in fact more comparable to ecotypes in respect of their altitudinal distribution and the degree of morphological differentiation. Speciation mechanisms involved in the evolution of the native species of Cardamine are discussed.