High ploidy diversity and cytotype distribution in a biodiversity hotspot

The south-western tip of the African continent hosts a unique and exceptionally rich flora. It includes the Cape Floristic Region (CFR) and the Succulent Karoo Region, which together form the winter-rainfall region known as the Greater CFR. Both areas are recognized as important centres of plant diversity and endemism. This region is home to about 9000 plant species in an area of approx. 90 000 km2 – such species richness is remarkable and comparable with Neotropical floras. The Succulent Karoo biome is the only arid region in the world identified as a biodiversity hotspot. An intriguing feature of the Cape flora is the elevated diversification of a limited number of plant lineages; 50% of the species diversity of the CFR can be ascribed to only 33 radiation events. Molecular dating has revealed that, despite considerable variation in the ages of these 33 lineages, most modern species of the Cape flora are recent and evolved during the Plio-Pleistocene.

The genus Oxalis (Oxalidaceae) is an important component of the South African flora that has undergone major radiation in the Greater CFR. The most recent checklist recognizes 195 native species (plus nearly 50 varieties), and several novel species have been described in the last decade. All native South African taxa survive unfavourable periods as subterranean bulbs, with growth confined to periods when temperature and moisture regimes offer suitable conditions (usually during winter and spring).

Conventional chromosome number determination is time- and labour-intensive, which precludes analysis of large population samples. However, knowledge of exact chromosome number is not always necessary, as it can be substituted by the estimation of ploidy level, provided that individual DNA ploidy levels are confirmed by chromosome counts. A new study in Annals of Botany employes DNA flow cytometry complemented by conventional chromosome counts to assess ploidy variation in a widespread member of this important geophytic CFR genus (Oxalis obtusa). This study represents the most comprehensive investigation of ploidy variation in any plant species native to the Greater CFR. It finds a surprising range of ploidy levels in this widespread member of the geophytic genus Oxalis. The distribution of cytotypes was also found to be associated with vegetation types, climatic parameters and geography.

High ploidy diversity and distinct patterns of cytotype distribution in a widespread species of Oxalis in the Greater Cape Floristic Region. Ann Bot (2013) 111 (4): 641-649. doi: 10.1093/aob/mct030
Genome duplication is widely acknowledged as a major force in the evolution of angiosperms, although the incidence of polyploidy in different floras may differ dramatically. The Greater Cape Floristic Region of southern Africa is one of the world’s biodiversity hotspots and is considered depauperate in polyploids. To test this assumption, ploidy variation was assessed in a widespread member of the largest geophytic genus in the Cape flora: Oxalis obtusa.
DNA flow cytometry complemented by confirmatory chromosome counts was used to determine ploidy levels in 355 populations of O. obtusa (1014 individuals) across its entire distribution range. Ecological differentiation among cytotypes was tested by comparing sets of vegetation and climatic variables extracted for each locality.
Three majority (2x, 4x, 6x) and three minority (3x, 5x, 8x) cytotypes were detected in situ, in addition to a heptaploid individual originating from a botanical garden. While single-cytotype populations predominate, 12 mixed-ploidy populations were also found. The overall pattern of ploidy level distribution is quite complex, but some ecological segregation was observed. Hexaploids are the most common cytotype and prevail in the Fynbos biome. In contrast, tetraploids dominate in the Succulent Karoo biome. Precipitation parameters were identified as the most important climatic variables associated with cytotype distribution.
Although it would be premature to make generalizations regarding the role of genome duplication in the genesis of hyperdiversity of the Cape flora, the substantial and unexpected ploidy diversity in Oxalis obtusa is unparalleled in comparison with any other cytologically known native Cape plant species. The results suggest that ploidy variation in the Greater Cape Floristic Region may be much greater than currently assumed, which, given the documented role of polyploidy in speciation, has direct implications for radiation hypotheses in this biodiversity hotspot.