Sex in honeybees is determined by a gene named the complementary sex determiner (csd). Unfertilised eggs emerge to become drones, whilst fertilised eggs become queen and worker bees. Like other genes, csd has numerous alleles. Alleles are different variations of the same gene.
A queen bee has two chromosomes, each carrying a different csd allele. As drones emerge from unfertilised eggs, they only carry one csd allele from their queen mother. Worker bees will carry one of their queen mother’s allele and a different allele from their drone father.
With this system, it is possible that some fertilised eggs end up with the same csd allele from both their parents, becoming abnormal drones. Experimental studies from the 1950s have shown that these abnormal drones are killed during development and as such, if a large amount of fertilised eggs develop into abnormal drones, it can severely decrease population growth and result in unproductive colonies.
It is important that all honeybee populations maintain a high number of csd alleles to decrease the chance of a queen meeting a drone with the same allele copy as itself. A decrease in csd alleles can happen for several reasons, for instance in a closed breeding population if the queens are produced from a small number of colonies. Another reason is the rapid decrease of colony population size in a short period of time due to diseases or bushfires.
csd diversity in honeybee populations have been investigated around the world, but this had yet to be done in Australia until recently through the Plan Bee program.
csd in Australia
The diversity of csd has been tested in two Australian honeybee populations, 56 colonies from Better Bees WA (BBWA) and 41 colonies from a NSW queen bee breeder. Our results show that BBWA had 28 different csd alleles, while NSW had 32. In total, we found 51 alleles, 12 that had not been previously reported worldwide. 9 alleles were found in both populations.