The phylogenetics and systematics of Physaria kingii (Brassicaceae)

The subspecies of Physaria kingii (S. Watson) O’Kane and Al-Shehbaz (Brassicaceae) have historically been a difficult group to delimit taxonomically based on morphology, geography, and ecology. The taxa have been moved between genera as well among varieties, subspecies, and full species many times over. This study addressed the systematics relationships of the subspecies of P. kingii using a combination of molecular (both nuclear and chloroplast DNA sequences), morphological, geographical, and ecological data. Three non-coding DNA regions were chosen: the internal transcribed spacer (ITS) region of nuclear ribosomal DNA and the chloroplast rps intron and the chloroplast ndhC-trnV intergenic spacer. Eighty-seven aligned sequences in total were selected and networks were constructed using SplitsTree for exploratory data analyses to identify any genealogical discordance for each of the regions in addition to a combined chloroplast region. With the prior knowledge of possible hybridization among P. k. subsp. kaibabensis, P. k. subsp. wardii (formerly included within P. k. subsp. latifolia), along with other taxa in the genus found on the Kaibab Plateau, P. arizonica, and P. purpurea, three networks for the ITS region were included to attempt to identify if hybridization is involved and, if so, to determine the maternal taxa. Additionally, Bayesian analyses were completed using each of the regions in addition to combining the chloroplast sequences. The nuclear and chloroplast regions produced statistically different results, so were not combined. The subspecies were not monophyletic and will remain as subspecies of P. kingii. There was a single taxonomic change, separating P. wardii from P. kingii subsp. latifolia and moving P. wardii to a subsp. of P. kingii. Additionally, this dataset identified a possible relationship between P. kingii and P. occidentalis that suggested the P. occidentalis subspecies should be included under P. kingii. A more inclusive study will need to be done before determining this