The extent and pattern of polyploidy in the tuber-bearing Solanums varies among the many taxonomic series that have been identified in this subsection of Solanum . While several series appear to be entirely diploid, others exhibit a range of ploidy levels from 2x to 6x . and some contain only polyploid species .In many diploid . triploid and tetraploid species 2n gametes (gametes or gametophytes with the sporophytic chromosome number) have been detected . Both 2n eggs and 2n pollen occur . 2n gametes provide the opportunity for unilateral and bilateral sexual polyploidization . The genetic determination and consequences of sexual polyploidization strongly suggest that 2n gametes have been the major instrument in the polyploid evolution of the tuber-bearing Solanums . Somatic doubling of species and interspecific hybrids appears to be of very limited importance .New evidence for the occurrence of 2n eggs and 2n pollen in many species is reported, and data from the literature are added to illustrate the widespread distribution of 2n gametes throughout the subsection . A very high correlation is found between polyploidy and 2n gametes, and its significance is discussed . Proof is presented for the occurrence of alleles governing 2n pollen production in the cultivated tetraploids, providing additional evidence for the hypothesis that 2n gametes have been involved in their origin .Multiple unilateral and bilateral sexual polyploidizations are proposed for the origin of the cultivated tetraploids ; this accounts for the large variability encountered in this group, which closely resembles that of the related diploids . Similar evolutionary pathways are hypothesized for the other polyploid complexes in the subsection .A scheme is proposed in which participation of both 2n and n gametes link together all ploidy levels in the tuber-bearing Solanums, thus overcoming the ploidy barriers and providing for gene flow throughout the sympatric species of the subsection .
