Tuesday, November 14, 2006

Differences in Interspecific Crossability Among Tetraploid and Diploid Cotton Species Affect Introgression Efficiency.

Erik Sacks, USDA-ARS, USDA-ARS, PO Box 345, Stoneville, MS 38776

Upland cotton, Gossypium hirsutum, is a tetraploid (AADD) that originated in Mesoamerica and has become the predominant species of commercial fiber production throughout the world. The diploid (AA) species, G. arboreum and G. herbaceum, were independently domesticated in Asia and Africa, and represent a large germplasm pool that can potentially be used to improve upland cotton. However, introgression is hindered by post-zygotic breeding barriers as well as the difference in chromosome number. Crosses between upland cotton and the A-genome diploids typically result in fruit abscission within a week of pollination due to endosperm abortion and embryo degeneration. In-vitro ovule rescue can be used to obtain triploid interspecific progeny but there is little information about the relative crossability of different parental genotypes. In this study, ten upland cotton genotypes were crossed as females in a factorial design with one G. herbaceum and four G. arboreum genotypes as males. The female parents included two pairs of lines near-isogenic for glanded/glandless. Fruit were harvested four days after pollination. The ovules of each fruit were removed aseptically and placed on a Petri dish containing 25 ml of modified Murashige and Skoog media. Ovules were incubated at 30 C with 12 hr of fluorescent light per day. For each parental combination, ovule cultures were established successfully for two to ten fruit. The number of germinated embryos per fruit was recorded. Variation for average crossability was greater among the upland cotton genotypes (0.4 3.1 seedlings/fruit) than among the A-genome diploids (1.1 2.0 seedlings/fruit). The glandless trait was not advantageous. Two of the three most crossable upland cotton parents were previously reported to be good genotypes for obtaining somatic embryos. Thus for cotton, somatic embryogensis and interspecific ovule rescue have features in common, and methods developed for one may be applicable to the other.