Adaptations in the genetic system of Australian arid zone Cassia species (Leguminosae, Caesalpinioideae)

Abstract

Cassia is a large genus primarily of tropical and subtropical lowlands. However, one species group, series Subverrucosae of section Psilorhegma, has undergone a major development in the Australian arid zone, where it is a conspicuous element in the woody shrub flora. This species group is characterized by (1) widespread hybridization, involving most of the species of the complex over most of its area, (2) polyploidy, which occurs as an infraspecific and infravarietal phenomenon, with the highest frequency and at the highest levels recorded in the genus, and (3) apomictic reproduction through adventitious embryony. The occurrence of adventitious polyembryony in Cassia appears to be confined to series Subverrucosae and almost restricted to the polyploid forms. It is suggested that the balance between fitness and flexibility provided by these three features is an adaptation for the widely fluctuating environment of the arid zone. The diploid forms, which are parental to the complex, have relic occurrences in old moun- tain areas, and appear to be strongly reproductively isolated. Under conditions of fluctuating selection pressures, infraspecific polyploidy has had selective advantage in limiting the genetic effects of outbreeding. Polyploidy has also weakened isolation mechanisms and allowed extensive hybridization in populations colonizing the intervening plains. Adaptive biotypes have been maintained partly by the conservative effects of polyploidy but especially by the establishment of facultative apomixis, which allows continuous recombination concurrently with the fixation of successful genotypes. The equilibrium between recombination and conservation may shift in accordance with changes in habitat stability.