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 mountain 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.

Phylogenetic analysis of the Australian Salicornioideae (Chenopodiaceae) based on morphology and nuclear DNA

“…The use of AFLP to detect sexual and clonal plants within populations would confirm whether H. leptoclada and other Halosarcia species are facultative apomicts. Apomixis has been reported in other taxonomically complex groups present in the Australian arid zone such as Cassia (Randell 1970) and Mulga (Acacia aneura) (Miller et al 2002). Carine and Scotland (2002) stated that a successful classification requires the identification of monophyletic clades that are also easily diagnosable within a taxonomic framework.…”

“…However, asexual reproduction via apomixis can maintain highly heterozygous genotypes that are adaptive without alteration through recombination (Grant 1971). Furthermore, clonal species may also spread quickly in highly stressful environments that have few competitors (Randell 1970) and only a single coloniser is required to establish new populations. Triploids are uncommon in the Chenopodiaceae.…”

Polyploidy and apomixis in accessions of Senna rugosa (G.Don) H.S.Irwin & Barneby

“…When both sexual and asexual embryos are formed, the development of the fertilized ovule starts later than that of the adventitious embryo (Koltunow, 1993). Randell (1970) described a time lapse between the development of the apomictic and the sexual embryos in pseudogamic apomictic Cassia species in the Australian arid zone. This was observed in all the evaluated accessions, since multiple states of maturation were observed in the seedlings from the same seed (Figure 3a-3f).…”

“…This was observed in all the evaluated accessions, since multiple states of maturation were observed in the seedlings from the same seed (Figure 3a-3f). According to Randell (1970), this reproductive system in polyploids may represent an adaptation to floating environmental conditions, as is the case of the Australian arid zone, since sexual reproduction and hybridizations enhance the level of recombination and produce new genotypes for varying environmental conditions; asexual reproduction preserves the adapted biotypes produced.…”

The Stigmatic Orifice in Cassia, Senna, and Chamaecrista (Caesalpiniaceae): Morphological Variation, Function During Pollination, and Possible Adaptive Significance

“…Table 1 provides a list of the species studied and the origin of material examined. We employ the nomenclature of Irwin and Bameby (1982) and Randell (1989). Collections were made in the field, in the Botanical Gar-1390 TABLE 1.…”