Abstract
Streptanthus carinatus and S. arizonicus (Cruciferae) are mainly allopatric in southwestern United States; they are sympatric in south-central New Mexico and west Texas (the El Paso-Las Cruces area). Field study of natural hybrids, their normal meiosis and pollen, their chemical (glucosinolate) similarities, and the reconstitution of fertile hybrids in the greenhouse support the notion that the two taxa are conspecific. They are here considered subspecific variants: Streptanthus carinatus subsp. carinatus and Streptanthus carinatus subsp. arizonicus. This is the first known occurrence of natural hybridization in the genus. Two species of Streptanthus (Cruciferae), S. carinatus and S. arizonicus, are wide-ranging in southwestern United States and Mexico. Streptanthus arizonicus occurs from west Texas to Arizona, Baja California, and Chihuahua; S. carinatus is found mainly in west Texas and adjacent southcentral New Mexico, with an outlier in southern Arizona. The two taxa coexist in the desert mountains along the border country of south-central New Mexico and west Texas, and possibly in southern Arizona. One of us (RDW) has found hybrid populations of the two in several places in the Franklin Mountains of Dofia Ana County, New Mexico, and El Paso County, Texas (the Las Cruces-El Paso area). This is the first recorded instance of natural hybrids in Streptanthus, a genus of nearly 40 species indigenous to western North America. Ecological, morphological, phytochemical, and cytological evidence from plants of the hybridizing populations and from greenhouse studies support the view that the two taxa are conspecific. The idea is not new. Kearney and Peebles (1951, p. 332) wrote: It is questionable that S. arizonicus is more than varietally distinct from S. carinatus. We propose that S. arizonicus be given subspecific rank within S. carinatus and that S. arizonicus var. luteus Kearney and Peebles not be recognized taxonomically at this time. SALIENT FEATURES OF THE TAXA Streptanthus arizonicus and S. carinatus are similar in vegetative and reproductive features. Their leaves are mostly glabrous and glaucous,
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