Marilyn Z. Oracion
The somatic chromosome numbers (2n) in seven sweetpotato cultivars, four diploid (2x) Ipomoea species, tetraploid (4x) and hexaploid (6x) I. trifida strains, and twenty selected hybrids between sweetpotato and 6x l. Trifida were determined using a Fealgen-acetocarmine squash technique designed for Ipomoea chromosomes. The data revealed that euploid counts of 2n=30, 60 and 90 were the modes for the 2x, 4x, and 6x Ipomoea, respectively. These counts coexisted with aneuploidy cells having either less (hypoploid) or more (hyperploid) chromosomes that the euploid or modal counts. The hypoploid cells were majority among the aneuploidy variants. The percentage frequency ratio of euploid to aneuploid cells were 60%:40% among diploids, 45%:55% among tetraploids, and 30%:70% and 34%:66% amon the 6x I. trifida and sweetpotatoes, respectively. The wide hybrids exhibited extreme aneuploidy with 16%:84% euploid to aneuploidy cell ratio. Two hybrids did not show cells with 2n=90. In general, the hybrids resembled the sweetpotato parents in having chromosome numbers closer to 6x or 90 whereas the 6x I. trifida and the 4x strains exhibited somatic counts encompassing the 2x, 4x and 6x counts. The results suggest that the prevailing cytogenetic system among the Ipomoea is that of somatic euploidy-polyploidy combined with aneuploidy. Further, the amount of aneuploidy cells appeared higher with taxa of higher ploidy and highest among the hybrids. The findings are discussed in relation to sweetpotato breeding, germplasm conservation and evolution of new phenotypes through vegetative propagation.
Keywords: Aneuploidy. Cytogenetics. Euploidy. Evolution. Ipomoea. Somatic cells. Sweetpotato breeding. Wide hybrids. 2n.
Annals of Tropical Research 17 Nos. 1-4:(1995)