Cytological evidence for triploid males and females in the bumblebee, Bombus terrestris

The presence of both triploid males and females in Bombus terrestris was detected by distinct chromosome observation. These cytological features are novel among the higher Hymenopteran insects. We thus strictly applied the complementary sex determination (CSD) model previously proposed for Hymenopteran insects. Three out of 60 sibling queens that were mated with diploid males produced both triploid males and females, and founded colonies. The male to female ratio of the bees which emerged from the fertilized eggs of the queens was approximately 1 to 1. Thus we reconfirm that the sex in B. terrestris is determined by a single multi-allelic locus. The body size of the triploid males was smaller than that of the diploid and haploid males. We found hatched eggs laid by one triploid female (worker). One of these developed into a 3rd instar larva, however most of the triploid individuals were sterile.