Fertility of Aneuploids between the 5x and 6x Levels in Blueberry: The Potential for Gene Transfer from 4x to 6x Levels

Vaccinium ashei (6x) /V. corymbosum (4x) pentaploid hybrids backcrossed to V. ashei yield aneuploid progeny ranging in chromosome number from 5x to 6x levels. Six backcross aneuploids having chromosome numbers of 2n = 61, 62, 64, 66, 68, and 70 were selected from this backcross and crossed in a complete diallel mating design and backcrossed (as female parents) to two V. ashei cultivars and an interspecific hexaploid hybrid. Fertility variables measured were percent fruit set, total seed per berry, developed seed per berry, percent developed seed per berry, percent seed germination, developed seed per pollination, and seedlings per pollination. A significant linear and positive relationship was found between chromosome number and all seven fertility variables. However, regression accounted for 30% or less of the variation among crosses. Diallel analysis revealed that general combining ability was the major contributing effect for all seven variables, followed by reciprocal effects. Specific combining ability was not significant. The second backcross to the hexaploid level suggested significant effects due to both the BC 1 aneuploid and hexaploid genotypes and to a significant genotype × genotype interaction for three of the variables. All six aneuploids were either fully or partially self-sterile. The findings of this study substantiate earlier suggestions that pentaploids in blueberry can be used to facilitate bilateral transfer of characteristi cs between the tetraploid and hexaploid levels in blueberry. In blueberry, genetic improvement has been directed at two ploidy levels that generally are considered reproductively iso- lated from one another. These are the tetraploid (2n = 4x = 48) and hexaploid (2n = 6x = 72) chromosome levels. Genetic improvement at the tetraploid level has largely involved Vac- cinium corymbosum L. ancestry, the highbush blueberry, while at the hexaploid level, it has largely involved V. ashei Reade ancestry, the rabbiteye blueberry. The possibility of bilateral gene transfer between these two species and the two ploidy levels these species represent would significantly broaden the germplasm base for genetic improvement programs in blue- berry. The potential for direct gene transfer from the hexaploid to the tetraploid species by way of the pentaploid hybrid "bridge" between these two species has already been established (Vorsa et al., 1987b). Genetic improvement efforts in rabbiteye blue- berry could benefit from the considerable germplasm that exists at the tetraploid level, which includes two domesticated species of blueberry, V. corymbosum and V. angustifolium Aiton. In particular, V. corymbosum could offer improved fruit quality, earlier fruit ripening (Galletta, 1975), and greater self-fertility (El-Agamy et al., 1981). Transmission of the extra genome chromosomes was found to be enhanced, relative to what would be expected with random transmission, when the pentaploid hybrids were backcrossed to the hexaploid species (Vorsa, 1988). Chromosome numbers in BC1 seedlings ranged from hypopentaploid (2n = 4x + 11 = 59) to hexaploid, with means of 2n = 66.2 and 2n = 68.0 for the 5x × 6x and 6x × 5x progenies, respectively. More than 95% of the progeny were aneuploid, with <2% being hexa-