The South American Dictyophragmus (Brassicaceae)

Abstract

Two of the approximately 350 South American species of Brassicaceae (Cruciferae) have broadly flattened, linear to linear-lanceolate fruits, broadly winged seeds, and accumbent cotyledons. The first is a Peruvian species described by Muschler (1908) as Streptanthus englerianus Muschler and was later transferred by Schulz (1933) to the monotypic Dictyophragmus 0. E. Schulz. The second is a recently discovered Argentinian species described by Romanczuk (1982) as Sisymbrium punense Romanczuk. Except for the superficial resemblance in fruits and seeds, the two species have little in common with the exclusively North American Streptanthus Nuttall. Because of the common occurrence in unrelated genera of the Brassicaceae of broadly flattened fruits, broadly winged seeds, and accumbent cotyledons, it is safe to conclude that this character combination evolved independently several times within the family. Sisymbrium punense and Dictyophragmus englerianus (Muschler) 0. E. Schulz are glabrous annuals with auriculate to amplexicaul cauline leaves, small, white flowers, confluent nectar glands, entire stigmas, broadly flattened fruits, accumbent cotyledons, and broadly winged seeds. Despite the significant morphological differences and geographical disjunction, the two species are more closely related to each other than to any South American Brassicaceae. In my opinion, their relationship is better understood by placing them in one genus than by recognizing two monotypic genera, of which one should be segregated as new. Perhaps the nearest relative of Dictyophragmus is the South American Eremodraba 0. E. Schulz, a genus of two species, one of which is narrowly endemic to southern Peru and the other restricted to adjacent northern Chile (Al-Shehbaz, 1990). Species of these genera are glabrous annuals with amplexicaul cauline leaves, small flowers, confluent nectar glands, entire stigmas, and flattened fruits. On the basis of differences in the cotyledonary position, Schulz (1936) placed Dictyophragmus in the tribe Arabideae and Eremodraba in the Sisymbrieae. Students of the Brassicaceae now place much less emphasis on the cotyledonary position, and evidently the boundaries between the Arabideae and Sisymbrieae are highly artificial. It is even doubtful that these tribes are distinct or natural. In my opinion, one should be cautious about the rigid use of characters such as the cotyledonary position in assessing relationships at higher taxonomic levels within the family. In addition to the differences in cotyledonary position, Eremodraba has yellow flowers and wingless, plump, oblong seeds. In contrast, Dictyophragmus has white flowers and broadly winged, flattened, orbicular to broadly ovate seeds.