Maintaining Diversity In a Recurrent Selection Breeding Program for Southern Highbush Blueberries

The University of Florida blueberry breeding program has developed southern highbush blueberry (Vaccinium corymbosum L.) cultivars using a phenotypic recurrent selection strategy for over 50 years.  Florida-native Vaccinium species such as Vaccinium darrowii Camp and Vaccinium virgatum Aiton were used as sources for the low chilling requirement necessary for production in sub-tropical climates.  However, the reliance on relatively few sources for low chilling requirement during the early stages of the breeding program resulted in a small number of founding clones for southern highbush blueberry cultivars developed at the University of Florida.  Of the 26 cultivars released from the University of Florida blueberry breeding program with complete pedigree information, 25 descend from four highly used clones: Florida 4A (V. darrowii), Earliblue (V. corymbosum), Myers (V. virgatum), and Black Giant (V. virgatum).  Because of this potentially narrow germplasm base, previous breeders sought to include new sources of wild germplasm in crossing designs.  To assess the utility of these crosses within the context of a recurrent selection program, coefficients of coancestry were calculated from pedigree information for two groups of germplasm: cultivars released from the breeding program, and final stage selections – the genotypes selected each year for replicated trialing prior to cultivar release decisions.  The average coefficient of coancestry among cultivars released from the program (n=26) was 0.051.  In 1993, the average coefficient of coancestry among final stage selections was 0.095 (n=10), with nearly half of the selections showing a half-sib relationship or greater (> 0.125).  By 2010, the average coefficient of coancestry among final stage selections (n=24) was 0.036.