XVI International Botanical Congess
Variation in chromosome base number, size, and configuration has been proposed as a causal mechanism in speciation events. I sequenced the internal transcribed spacers (ITS) in cosmopolitan and Intermountain region endemic Draba species with the goal of understanding chromosome evolution in the genus. Phylogenetic analyses of ITS data revealed that N. American Draba cluster into two distinct clades. One clade contains the Intermountain endemic Draba species with variable base chromosome numbers including x=15,14,13,12,10,9 and rarely x=8. The sister clade contains cosmopolitan Draba whose base chromosome numbers are restricted to x=8 or polyploid derivatives of x=8. Chromosome numbers based on x=8 appear to be plesiomorphic within Draba. Potential mechanisms responsible for chromosome number variation and their reproductive implications are discussed.