Nature of sterility and mechanism of the evolution of higher ploidy in Solanum section Solanum (Maurella)

Cytomorphology of solanum nigrum L. (2n = 6x = 72) and S. americanum Mill. (2n = 2x = 24) was studied. Crosses were made between these two species, but crosses were successful only when the higher ploidy form was used as the maternal parent. The F1 hybrid was a tetraploid (2n = 4x = 48) and showed highly irregular meiosis. It did not set fruit either on selfing or on open pollination. A fertile octoploid branch spontaneously grew from the sterile hybrid. The octoploid had regular meiosis and produced several fruits with viable seeds. Pollen stainability was 82%. From cytological studies of the tetraploid and its amphiploid, it is concluded that hybridization and polyploidy have played a significant role in the origin and evolution of higher chromosomal forms of Solanum section Solanum (Maurella).Key words: Solanum, polyploid, sterility.