Species richness and polyploid patterns in the genus Draba (Brassicaceae): a first global perspective

Abstract

Background : The genus Draba (Brassicaceae) has an arctic-alpine and montane distribution, and contains 355 accepted species that encompass a broad range of ploidy levels. Aims : We aimed (1) to explore the relationship between ploidy level and species richness throughout the geographical distribution of Draba, and (2) to estimate rates of speciation and polyploidisation in the genus. Methods : Species and their ploidy levels, obtained from Floras, herbaria and checklists, were mapped and analysed with ArcView 9.1. Speciation and polyploidisation rates were calculated. Results : We found that: (1) areas of high species richness have a high number of polyploid species; (2) diploid and polyploid distributions overlap; (3) aneuploids, and uneven and lower polyploid levels (≤ 6x) are strongly associated with the North American Cordillera; (4) species with high ploidy levels (> 10x) occur mainly in the Circum-North Region; and (5) Draba has high rates of speciation and polyploidisation compared to other herbaceous plants. Conclusions : Draba is a young polyploid complex as indicated from the geographical overlap of diploids and polyploids. The North American Cordillera Region is characterised by recent speciation and radiation events, while the Circum-North Region is characterised by both high levels of species migration and high ploidy levels. Regions of high species richness and polyploidy are those most affected by past glaciation (i.e. mountainous and arctic regions). High rates of speciation and polyploidy support the hypothesis that speciation in the genus has accelerated, and it is hypothesised that frequent glaciation cycles during the Pleistocene were a cause of this acceleration.