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Breeding system, crossability relationships and isolation mechanisms have been studied in the Solanum nigrum L. complex, using 16 accessions including seven diploids, five tetraploids and four hexaploids. Of these six are Indian forms and special attention has been paid to them. It was found that the diploids could be crossed with each other and also with natural tetraploids fairly easily, but never with the hexaploids. Crosses between two tetraploids or two hexaploids readily produced hybrids. In general, in crosses involving two ploidy levels, a cross succeeds when the higher ploidy plant was crossed as the seed parent; but exceptions were found wherein the lower ploidy plant could be successfully used as the seed parent. In this respect all diploid races are not isolated from the tetraploid races to the same extent; some races were divergent enough to be crossed only as male parents while others could be crossed in either direction. Compared to this diver-gence in crossability relationships hybrids sterility seems to be the major hurdle for gene exchange between the diploids and tetraploids. The Indian diploid and tetraploid races are isolated to a lesser extent as compared to the European diploid and tetraploid races. Autopolyploidy reverses crossability relationships. The diploid S. nigrum of India as seed parent crosses with natural tetraploid S. nigrum of India but fails to cross with natural hexaploid S. nigrum of India. However, the autotetraploid of diploid S. nigrum crosses with natural hexaploid and fails to hybridize with natural tetraploid. In addition to self-pollination and geographic isolation as factors restricting gene exchange between the different forms, hybrid sterility, hybrid breakdown and hybrid inviability were found to be operative in the complex. Diploid and hexaploid races of S. nigrum of India are isolated by hybrid inviability, while the diploid and tetraploid races of S. nigrum of India are isolated by hybrid sterility.

Breeding System, Crossability Relationships and Isolating Mechanisms in the <i>Solanum nigrum</i> Complex