In honey bee colonies, reproduction is usually a tightly controlled affair. The queens mate within the first few days of life, before spending the next few years using the stored sperm to fertilize eggs and produce workers, who forgo reproduction for the good of the hive. But in one isolated population of western honey bees (Apis mellifera) in South Africa, the workers do something a little different: They manage to reproduce without males.
“The question of why this population of honey bees in South Africa has evolved to reproduce asexually is still a mystery,” explains Matthew Webster, who co-authored the paper published in PLOS Genetics looking into this unique subspecies, the Cape honey bee (Apis mellifera capensis). “This study will help us to understand how genes control biological processes like cell division and behavior.”
The researchers decided to turn to DNA to hunt for clues, by sequencing the entire genome of the Cape honey bee and then comparing it to those of 100 other western honey bee subspecies. What they found was not that surprising: On the whole, the genomes of all the subspecies were fairly similar. But there were some key differences when looking at a few specific regions of the genome.
They found multiple genes related to the bees' social parasitism, including ones that controlled their hormonal signaling, which may cause the worker bees' ovaries to activate, and others that may be involved in allowing two daughter cells to fuse to produce the diploid eggs they lay. Not only that, but there may be others involved in producing pheromones that mask the eggs from other bees.
Cape honey bees are a unique subspecies among honey bees. In normal honey bee colonies, the queen can lay eggs that are unfertilized, meaning they are haploid (containing only one set of chromosomes), which develop into male drones. The Cape bees, however, turn this on its head. Some of the workers in the colony, not just the queen, can lay eggs and rather than these being haploid they are diploid, meaning that they contain two sets of chromosomes as if they have been fertilized.
This rare, but not wholly unusual, form of reproduction also aids in another aspect of the Cape honey bee’s behavior. Sometimes the female worker bees will leave their original colony and seek out new hives of honey bees. Sneaking in, they will lay their own diploid eggs within the new colony, which will then hatch and feed off the honey stores. Known as “social parasitism”, the Cape bees can destroy many colonies of honey bees this way, yet how they have managed to evolve this unusual method of reproduction and behavior has so far been a complete mystery.