Derivatives of Vaccinium arboreum x Vaccinium section Cyanococcus. II. Fertility and fertility parameters

Fertility of F1 hybrids and their open-pollinated progeny was studied for the intersectional cross Vaccinium darrowi Camp x V. arboreum Marsh as part of a project to determine the feasibility of using V. arboreum to breed vigorous, drought-tolerant southern highbush blueberry cultivars. The 16 F1 hybrids that were studied were vigorous but very low in fertility. Second generation hybrids [MIKs (mother is known) obtained by open-pollination of the F1s] and MIK derivatives were extremely variable in vigor and fertility, but averaged far higher in fertility than the F1s as evidenced by pollen stainability and amount of pollen produced. F1s produced an average of 0.4 seedlings per 100 pollinated flowers when hand-pollinated in a greenhouse with pollen from V. darrowi, 0.2 when pollinated by V. arboreum and 3.4 when pollinated by cultivated highbush. Some MIKs that were crossed with other MIKs and with cultivated southern highbush were very high in male and female fertility. Female fertility was estimated in greenhouse crosses from fruit set, berry weight, number and weight of seeds, number of plump seeds per berry, and number of seedlings obtained. Male fertility was estimated by pollen stainability with acetocarmine and amount of pollen shed. Chromosome counts showed that three F1s were diploid and that four fertile MIKs were tetraploid. One MIK appeared to be aneuploid. Aneuploidy may explain much of the low fertility found in MIK populations. These results indicate that good progress is being made in returning the hybrid plants to cultivar quality in only a few generations of backcrossing