MALE-FERTILITY AND CYTOLOGY OF TRIPLOID HYBRIDS BETWEEN TETRAPLOID SOLANUM-COMMERSONII (2N=4X=48, 2EBN) AND PHUREJA-TUBEROSUM HAPLOID HYBRIDS (2N=2X=24, 2EBN)

Solanum commersonii Dun. is a diploid (2n = 2x = 24, 1EBN) wild species of potential value for potato breeding. It is a reproductively isolated species and cannot be crossed with Tuberosum haploids (2n = 2x = 24, 2EBN) or other diploid 2EBN Solanum species. In order to overcome the EBN barriers, triploid hybrids were produced between Phureja-Tuberosum haploid hybrids, which form 2n pollen grains by parallel spindles, and tetraploid S. commersonii. Microsporogenesis analysis of the triploids indicated a trend towards low values of chromosome distribution at Anaphase I; lagging chromosomes were often observed as well. Despite these abnormalities, the percentage of stainable pollen was very high, ranging from 5.0% to 74.3%. A high variation in pollen grain diameter was also evident. Parallel and tripolar orientation of spindles at Metaphase II of microsporogenesis was a common feature of all the triploids analyzed, but dyads and triads were observed at a very low frequency. Therefore, also the frequency of 2n pollen was very low; the different size of stainable pollen appears to represent the ploidy levels which are possible according to the distribution of chromosomes in Anaphase I. The results obtained also suggest that S. commersonii could have minor genes acting at the end of meiosis in such a way that, despite the presence of parallel/tripolar spindles, dyads/triads are not formed.