The genus Sisymbrium as currently circumscribed includes about 94 species disjunctly distributed in the Old (41 spp.) and the New World (53 spp.). Sisymbrium has been variously delimited, with several segregate genera proposed (subtribe Sisymbriinae) primarily for the new World taxa, including Schoenocrambe, Coelophragmus, and Mostacillastrum. Using sequence data from the internal transcribed spacers of nuclear ribosomal DNA and the 5.8S rRNA gene (collectively, ITS region), we examined the evolutionary relationships of Old and New World Sisymbrium species with its segregate genera and the validity of O.E. Schulz's classical sectional treatment of Sisymbrium. Sequence data were obtained from 33 Sisymbrium species, representing all 14 sections and two Sisymbrium species formerly assigned to segregate genera Coelophragmus and Mostacillastrum (subtribe Sisymbriinae), and two putative Sisymbrium species currently assigned to Neotorularia. Sequence data were also obtained from 26 taxa from segregate or related genera includingSchoenocrambe, Werdermannia (subtribe Sisymbriinae), eight genera in the Thelypodieae, Sibara (tribe Arabideae) and Pringlea (tribe Pringleeae), four members of the tribe Brassiceae, and three other Neotorularia species. Results from maximum parsimony analysis showed a polyphyletic origin for Sisymbrium and did not correspond well to Schulz's sectional classification. Sisymbrium species were split into three major clades: Old World Sisymbrium (including Neotorularia aculeolata, Neotorularia afghanica, and the type species of Schoenocrambe, Schoenocrambe linifolia, the sole New World member of this Old World clade); New World Sisymbrium (along with the remaining New World taxa) and designated as the New World Thelypodieae alliance; and the tribe Brassiceae ( including Sisymbrium supinum and Sisymbrium thellungii).Key words: Sisymbrium, Schoenocrambe, ITS, Thelypodieae, taxonomy, Brassicaceae.
