Cytogenetics of Some Diploid Species of the Solanum Nigrum Complex

A few species of the Solanum nigrum L. complex were selected to determine the relationship among them as revealed by crossability studies and chromosome-pairing behaviour of their hybrids. Cytomorphology of S. americanum Mill., Indian diploid S. nigrum L., S. nodiflorum Jacq., and S. douglasii Dun. was studied. Crosses between S. douglasii and other species were compatible if S. douglasii was used as the male parent. The isolation of S. americanum, Indian diploid S. nigrum, and S. nodiflorum from S. douglasii by hybrid breakdown is most likely due to differences between the parents in chromosome structure as well as gene content. Although the F1 hybrids were normal and fertile, the F2 plants showed meiotic breakdown and reduced fertility. It is suspected that S. nodiflorum, S. nigrum (2x), and S. americanum represent a single biological species, S. americanum. The distinction of S. douglasii as a separate species is justified. Key words: Solanum, diploid, hybrids.