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Nuclear ribosomal (ITS) and plastid (trnL-trnF) DNA sequence data from representatives of most New World species of Thlaspi (Brassicaceae), including the widespread North American T. montanum,
have been studied to investigate their relationship to the Eurasian members now placed in the genus Noccaea , especially the European T. montanum (= N. montana) complex. ITS sequence data
of the New World taxa were added to a previously published data set, which includes sequences of representative taxa of 11 of the 12 genera segregated from Thlaspi, to elucidate overall relationships
within Thlaspi s.l. Sequences from Callothlaspi, Kotschyella, Masmenia, Microthlaspi, Neurotropis, Noccaea, Noccidium, Pseudosempervivum, Raparia, Thlaspi s.s., Thlaspiceras, and Vania,
as well as of the closely related genera Bivonaea, Alliaria, Peltaria, and Teesdalia, were included in the study. The results show that the New World taxa of Thlaspi form a monophyletic
group that should be assigned to Noccaea and that the North American "T.montanum" appear to be unrelated to the European plants referred to this species. Sequence distance values are of an order
of magnitude that favors Pleistocene speciation and migration of the perennial Thlaspi species into the New World. It is concluded that the North American infraspecific taxa of T. montanum
be recognized as Noccaea fendleri. The new combinations N. fendleri subsp. californica, N. fendleri subsp. glauca, N. fendleri subsp. idahoense, and N. fendleri subsp.
siskiyouense are proposed.