Unusually high recombination rate detected in the sex locus region of the honey bee (Apis mellifera)

Abstract

Sex determination in Hymenoptera is controlled by haplo- diploidy in which unfertilized eggs develop into fertile haploid males. A single sex determination locus with several complementary alleles was proposed for Hymenoptera [so-called complementary sex determination (CSD)]. Heterozygotes at the sex determination locus are normal, fertile females, whereas diploid zygotes that are homozygous develop into sterile males. This results in a strong heterozygote advantage, and the sex locus exhibits extreme polymorphism maintained by overdominant selection. We characterized the sex-determining region by genetic linkage and physical mapping analyses. Detailed linkage and physical mapping studies showed that the recombination rate is <44 kb/cM in the sex-determining region. Comparing genetic map distance along the linkage group III in three crosses revealed a large marker gap in the sex-determining region, suggesting that the recombination rate is high. We suggest that a 'hotspot' for recombination has resulted here because of selection for combining favorable genotypes, and perhaps as a result of selection against deleterious mutations. The mapping data, based on long-range restriction mapping, suggest that the Q DNA-marker is within 20,000 bp of the sex locus, which should accelerate molecular analyses.