Morphology, phylogeny, and taxonomy of Microthlaspi (Brassicaceae: Coluteocarpeae) and related genera

The genus Thlaspi has been variously subdivided since its description by Linnaeus in 1753, but due to similarities in fruit shape several segregates have still not gained broad recognition, despite the fact that they are not directly related to Thlaspi. This applies especially to segregates now considered to belong to the tribe Coluteocarpeae, which includes several well-studied taxa, e.g., Noccaea caerulescens (syn. Thlaspi caerulescens), and the widespread Microthlaspi perfoliatum (syn. Thlaspi perfoliatum). The taxonomy of this tribe is still debated, as a series of detailed monographs on Coluteocarpeae was not published in English and a lack of phylogenetic resolution within this tribe was found in previous studies. The current study presents detailed phylogenetic investigations and a critical review of morphological features, with focus on taxa previously placed in Microthlaspi. Based on one nuclear (ITS) and two chloroplast (matK, trnL-F) loci, four strongly supported major groups were recovered among the Coluteocarpeae genera included, corresponding to Ihsanalshehbazia gen. nov., Friedrichkarlmeyeria gen. nov., Microthlaspi s.str., and Noccaea s.l. In addition, two new species of Microthlaspi, M. sylvarum-cedri sp. nov. and M. mediterraneo-orientale sp. nov., were discovered, which are well supported by both morphological and molecular data. Furthermore, M. erraticum comb. nov. (diploid) and M. perfoliatum s.str. (polyploid) were shown to be distinct species, phylogenetically widely separate, but with some overlap in several morphological characters. Detailed descriptions, notes on taxonomy, geographical distribution, and line drawings for the new species and each species previously included in Microthlaspi are provided. In addition, the current taxonomic state of the tribe Coluteocarpeae is briefly discussed and it is concluded that while several annual taxa are clearly distinct from Noccaea, many perennial taxa, after thorough phylogenetic and morphological investigations, may have to be merged with this genus.