| The species in the genus Solanum, to which both wild and cultivated potatoes belong, form polyploid series with 2x, 3x, 4x, 5x and 6x somatic chromosomes, (x = 12). Several taxonomic series within this genus stand out because of extensive polyploidization. With new explorations and more detailed studies the number of polyploid species is increasing very rapidly.;It was predicted that if genetically determined 2n gametes had been instrumental in polyploid evolution of the tuber-bearing Solanums, the allele ps had to be found in polyploids as well as in their related diploids. Therefore, plant introductions of 4x grl and 6x opl, species closely related to the diploids ifd, spg, and grl, were screened for 2n pollen. A relatively high proportion of plants of these species were found to produce 2n pollen via parallel spindles. The recessive genes controlling 2n pollen formation in both polyploids were found to be alleles. So far, this is the only mechanism of 2n pollen formation that has been found in this group of wild species.;The genetic determination of 2n gamete formation plus the advantages of sexual vs. asexual polyploidization suggest that genetically determined 2n gametes have played a very important role in polyploid evolution of the tuber-bearing Solanums. Somatic chromosome doubling, in contrast, appears to be an isolated event of limited evolutionary potential.;The ps allele is, so far, the only genetic marker reported for 4x grl and 6x opl. A rough estimate of the rate of outcrossing of both polyploid species in the experimental field was obtained from progeny testing of plants nulliplex for ps. This value was then used in the calculation of the equilibrium frequency of zygotes of 4x grl nulliplex for the marker gene.;A group of wild species from northwest Argentina, belonging to the series Tuberosa, S. spegazzini Bitt. (spg, 2n = 2x = 24), S. gourlayi Hawkes (grl, 2n = 2x = 24 and 2n = 4x = 48) and S. oplocense Hawkes (opl, 2n = 6x = 72), and Cuneolata, S. infundibuliforme Phil (ifd, 2n = 2x = 24), was used to investigate the mode of origin of polyploids in the genus Solanum. 2n gametes were detected in the diploid species ifd and spg and in a diploid race of grl, using cytological and breeding approaches. The production of 2n pollen in these species was found to be under control of the meiotic mutant ps, which effects the parallel orientation of the Anaphase II spindles during microsporogenesis.;Little information is available on the meiotic and breeding behavior of 4x grl and 6x opl. It is also obscure how 6x opl evolved from ancestors of lower ploidy level. Morphological and breeding approaches were used to obtain some information on species relationships. Results suggest that 6x opl might have originated by interspecific sexual polyploidization; Argentine and Peruvian diploid species being likely ancestors.;Taxonomists have pointed out that in undisturbed areas wild species of Solanum are reproductively isolated by ecological and other external barriers, and that closely related species are usually cross-fertile. However, it was observed that the closely related, self-compatible polyploids 4x grl and 6x opl were difficult to hybridize. Hybrids with the expected pentaploid number of chromosomes were occasionally obtained. These pentaploid hybrids, in contrast to the parents, were self-incompatible. They could be backcrossed to the original populations to produce tetraploid and pentaploid progeny, respectively. A testable hypothesis based on interactions between specific complementary genes in pollen grains and cross-incompatibility (CI) genes in pistils is postulated to explain the genetic base of this system. |
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