Impact of Wide Hybridization on Highbush Blueberry Breeding

Abstract

The use of interspecific hybridization in blueberry (Vaccinium L. section Cyanococcus Gray) breeding has resulted in the incorporation of novel traits from wild germplasm and the expansion of the geographic limits of highbush blueberry (V. ×corymbosum L.) 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 blueberry [SHB (V. ×corymbosum)] cultivars; and 3) to report on the expected genetic contribution of wild Vaccinium clones to SHB cultivars. Pedigrees of 107 highbush blueberry cultivars were used to calculate tetrasomic inbreeding coefficients (F), pedigree-based genetic distances, and expected genetic contributions of wild clones. 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 cultivars exhibited similar levels of molecular relatedness as historical northern highbush blueberry [NHB (V. ×corymbosum)] cultivars (median Dsa between pairs of cultivars equals 0.19 in both cultivar groups), despite the broader genetic base and larger pedigree distances in the former cultivar group. Second, levels of heterozygosity in modern NHB cultivars were not statistically different from those of SHB (χ2 = 4.0; df = 3; P > 0.05), despite the considerably higher levels of inbreeding in the former cultivar group (median F equal to 0.0035 and 0.0013, respectively). Furthermore, pedigree-based genetic distances were significantly correlated with Dsa (r = 0.57, P ≤ 0.0001), indicating that microsatellite markers are reliable tools in most cases to assess the genetic relationships among SHB cultivars. Finally, seven Vaccinium species constitute the current genetic base of cultivated SHB. In this article, we report on the expected genetic contributions of V. angustifolium Ait., V. constablaei Gray, V. corymbosum, V. darrowii Camp, V. elliottii Chapman, V. tenellum Ait., and V. virgatum Ait. (syn. V. ashei Reade) clones to 38 SHB cultivars.