Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/5/2008
Publication Date: 4/20/2008
Citation: Brevis, P.A., Bassil, N.V., Ballington, J., Hancock, J. 2008. Impact of Wide Hybridization in Southern Highbush Blueberry Breeding. Journal of the American Society for Horticultural Science. 133(3):427-437. Interpretive Summary: The use of a number of wild blueberry species in breeding for new cultivars of highbush blueberry has resulted in incorporation of new traits from wild blueberries and expansion of the geographic limits of blueberry production. The objectives of this study were: 1) to estimate the impact of using these wild species on the genetic diversity of the cultivated highbush blueberry, 2) to establish the usefulness of DNA-based microsatellite or simple sequence repeat (SSR) markers in assessing genetic relationships among southern highbush blueberries (SHB), and 3) to report the genetic contribution of wild blueberries to SHB cultivars. Comparison of pedigree-based diversity estimates and SSR-based genetic diversity measures across 68 genotypes indicated that the effects of using various species on genetic diversity of cultivated blueberry were lower than previously thought. Furthermore, pedigree-based genetic distances and SSR-based genetic distances were significantly correlated, indicating that microsatellite markers are a reliable tool for assessing the genetic relationships among SHB cultivars. Finally, seven blueberry species constitute the current genetic make-up of cultivated SHB and we report their contributions to 38 SHB cultivars.
Technical Abstract: The utilization of inter-specific hybridization in blueberry breeding has contributed to incorporate novel traits from wild germplasm and expand the geographic limits of highbush blueberry production. The objectives of this study were: 1) to estimate the impact of wide hybridization on inbreeding, heterozygosity and coancestry of the cultivated tetraploid highbush blueberry, 2) to establish the usefulness of microsatellite markers in assessing genetic relationships among southern highbush blueberries (SHB), and 3) to report the genetic contribution of wild Vaccinium clones to SHB cultivars. Pedigrees of 107 highbush cultivars were used to calculate tetrasomic inbreeding coefficients (F), pedigree-based genetic distances, and genetic contributions of wild clones to SHB cultivars. Genotyping data from 21 single-locus microsatellite markers screened across 68 genotypes were used to calculate heterozygosity and proportion of shared alleles distances (Dsa). The results indicated that the effects of wide hybridization on genetic diversity of cultivated blueberry were lower than previously thought. First, SHB exhibited similar levels of molecular coancestry than historical northern highbush blueberry (NHB) cultivars (median Dsa between cultivars equals to 0.19 in both cultivar groups), despite the broader genetic base of SHB cultivars. Second, levels of heterozygosity in modern NHB cultivars were not statistically different than those of SHB cultivars ('2=4.0; df=3; P>0.05), despite the considerably higher levels of inbreeding in the former cultivar group (median F equals to 0.0035 and 0.0013, respectively). We hypothesize that strong selection pressure targeted on many loci has limited the introgression of rare alleles from non-cultivated Vaccinium L. species into SHB cultivars. Furthermore, pedigree-based genetic distances and Dsa were significantly correlated (r=0.57, P<0.0001), indicating that microsatellite markers are a reliable tool to assess the genetic relationships among SHB cultivars. Finally, seven Vaccinium species constitute the current genetic base of cultivated SHB. In this paper, we report the genetic contributions of V. angustifolium Ait. V. constablaei Gray, V. corymbosum L., V. darrowii Camp, V. elliottii Chapman, V. tenellum Ait., and V. virgatum Ait. (syn. V. ashei Reade) clones to 38 SHB cultivars.