Systematics of Solanum sect. Solanum (Solanaceae) in North America

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Numerical taxonomic studies show that eleven species of Solanum sect. Solanum occur in North America. Five diploid (n = 12) species, S. americanum, S. douglasii, S. interius, S. pseudogracile, and S. ptycanthum are apparently native. A sixth diploid species, S. sarrachoides, is an introduction from South America. Three polyploid species, S. furcatum, S. nigrum, and S. villosum, occur infrequently. Two other polyploid species, S. scabrum and S. retrofiexum, are sometimes cultivated. A notable nomenclatural change, necessitated by a previously selected lectotype for S. americanum, is the use of the name S. americanum for the species previously known as S. nodfiorum. The name S. ptycanthum is now used for the species of the north- eastern United States formerly called S. americanum. Solanum sect. Solanum (also known as the S. nigrum complex) is a cos- mopolitan group of about 30 annual or short-lived perennial, herbaceous species of weeds. A variety of historical and biological factors combine to make the taxonomy of this section difficult (Edmonds 1979a). The use of experimental taxonomic techniques in several regional studies has clarified the taxonomy of this section in much of its range (Edmonds 1972, 1979a; Henderson 1974; Heiser et al. 1979). The present study covers the North American species of the section. Sterility of interspecific hybrids and ease of obtaining hybrids in sect. Solanum make it tempting to attempt to define species on the basis of genetic isolating mechanisms. However, the high frequency of intraspe- cific hybrid sterility and its lack of correlation with morphological diver- gence indicate that hybrid sterility is not useful in delimiting species in this section (Schilling and Heiser 1979). Because the breeding system of most species of sect. Solanum is primarily autogamous (Schilling 1978), there is probably little gene flow between populations with the result that biological species (Mayr 1963) are not present in this section. Hence, although data from crossing studies have been incorporated into this study as phenetic characters, they are not accorded any special signifi- cance as evidence of an isolating mechanism that separates species. To classify species in this section, an operational approach using techniques of numerical taxonomy to discover discontinuities by which species may be recognized has been used.