Genetic diversity of the complementary sex-determiner (csd) gene in two closed breeding stocks ofVarroa-resistant honey bees

Honey bee (Apis mellifera) breeding programs that use a closed mating system are particularly vulnerable to low genetic diversity. Inadequate diversity at the complementary sex-determiner (csd) locus is problematic and potentially catastrophic in honey bee populations because it causes low brood viability. In typical commercial populations, queens are open mated andcsddiversity is fostered by high rates of introgression. In this study, we examine genetic diversity within the highly variable region (HVR) ofcsdin two stocks bred for resistance toVarroa destructor: Pol-line and Hilo, both of which use closed mating systems. We sampled 47 Pol-line colonies and 41 Hilo colonies and found 60 protein alleles that were condensed into 35 allele groupings by sequence similarity. We found that proportionately, HVR diversity levels were comparable with those in other closed breeding populations as well as open-mated populations ofA. melliferaworldwide. Distinct patterns are observed among Pol-line and Hilocsdprotein alleles in both the phylogeny and allele frequency distributions, suggesting early divergence of the two stocks. When compared with an African outgroup, both stocks shared alleles with the outgroup, suggesting ancestral lineages are present and not all diversity is due to new mutations. Periodic monitoring ofcsddiversity is recommended for closed breeding programs. Thecsddiversity data reported here are currently being used to make breeding decisions in these two mite-resistant populations of honey bees.