Phylogeny of Smelowskia and related genera (Brassicaceae) based on nuclear its DNA and chloroplast trnL intron DNA sequences

Abstract

Using sequence data from the ITS region (internal transcribed spacers ITS1 and ITS2 of nuclear ribosomal DNA and the 5.8S rRNA gene) and chloroplast DNA sequence data from the trnL intron, we examined the evolutionary relationships of Old and New World Smelowskia species with its putatively related genera (Brassicaceae). ITS and trnL sequence data were obtained from all 13 Smelowskia species and from 13 taxa of the related genera Descurainia, Ermania, Gorodkovia, Hedinia, Polyctenium, Redowskia, Sinosophiopsis, and Sophiopsis. Results from maximum parsimony analyses showed a polyphyletic origin for Smelowskia, with the taxa split into two major clades. Smelowskia taxa plus Ermania, Gorodkovia, Hedinia, Redowskia, Sinosophiopsis, and Sophiopsis formed one clade (Smelowskia s.l. clade), whereas the other clade included Smelowskia holmgrenii (Holmgrenii subclade) and the genus Polyctenium (Polyctenium subclade). Descurainia formed a separate clade. ITS and trnL molecular data, along with a comparison of key morphological traits for each genus, support recognition of a single genus that would include Smelowskia, the three monotypic genera Ermania, Gorodkovia, and Redowskia, as well as the genera Hedinia, Sinosphiopsis, and Sophiopsis (although not all species were examined in these genera). The molecular data are consistent with an Asian origin for Smelowskia s. str., followed by diversification in Asia and North America, subsequent to migration into arctic-alpine regions of western North America. ITS sequence divergence estimates ranged from 0 to 4.1% within Smelowskia s. str., and up to 6.45% within the Smelowskia s.l. clade; whereas trnL sequence divergence estimates ranged from 0 to 1.85% within Smelowskia s. str., and up to 3.71% within the Smelowskia s.l. clade, indicating pleistocenic speciation within Smelowskia as a whole.