Cytological and breeding behavior of pentaploids derived from 3x x 4x crosses in potato

Cytology and breeding behavior of Solanum commersonii-S.tuberosum hybrids derived from 3x x 4x crosses was examined. The chromosome number of hybrids ranged from hypo-pentaploid (2n=5x-8=52), to hyper-pentaploid (2n=5x+7=67), with the euploid pentaploid 2n=5x=60 class predominant. The high variability in chromosome number of the 3x x 4x hybrids was attributed to the fact that meiotic restitution during megasporogenesis of the 3x female may have involved poles with various chromosome numbers, resulting in 2n eggs with 24-48 chromosomes. Microsporogenesis analyses provided evidence that chromosome pairing between S. commersonii and S. tuberosum genomes occurred. In addition, chromosome distribution at anaphase I and anaphase II revealed an average chromosome number of 29.5 and 29.1 per pole, respectively. To further study the extent of transmission of extra genome chromosomes from pentaploids, 5x x 4x and 4x x 5x crosses were performed, and the chromosome number of resulting progeny was determined. Ploidy ranged from 2n=4x=48 to 2n=5x=60 following 5x x 4x crosses, and from 2n=4x+1=49 to 2n=5x=60 following 4x x 5x crosses. These results provided indirect evidence that the pentaploid hybrids produced viable aneuploid gametes with a chromosome number ranging from 24 to 36. They also demonstrated that gametes with large numbers of extra chromosomes can be functional, resulting in sporophytes between the 4x and 5x ploidy level. Fertility parameters of crosses involving various (aneuploid) pentaploid genotypes were not influenced by chromosome number, suggesting a buffering effect of polyploidy on aneuploidy. The possibility of successfully using (aneuploid) pentaploid genotypes for further breeding efforts is discussed.