Genetic diversity within a honey bee colony
Someone asked me why their honey bee workers were different sizes. I can’t give a concrete answer to this question, but it brings up the subject of honey bee genetics.
Honey bees, like many other insects in the order Hymenoptera, have a sex determination system known as haplodiploidy. In this system, the sex of an individual is determined by the number of chromosomes the individual has. Fertilized eggs have two sets of chromosomes and become female. Unfertilized eggs have only one set of chromosomes and become male.
In the case of honey bees, the sex of each offspring is determined by the queen. On her mating flights, which occur shortly after she matures, the queen mates with multiple drones—perhaps as many as 12 to 15. This is called polyandry, which means “many males.” The sperm from these males is stored together in the spermatheca where it can remain viable for several years. As an egg passes through the oviduct, it may or may not be fertilized depending on the queen’s wishes.
The point I want to make here is that even though we refer to worker bees as “sisters,” for the most part they are only “half-sisters.” If the queen has mated with 10 drones, for example, the bees in the colony have one mother and 10 fathers. This can result in distinctly different genetics between individuals and could, perhaps, be the reason that bees of different sizes are found in one colony.
Multiple mating of this type insures genetic variability within the colony—a phenomenon which increases the colony’s chance of survival. In our example, even if the genes of one “father” are prone to disease—and the daughters of that father do not survive—the other 90% may be strong enough to pull the colony through.
On the downside, all the offspring of one queen and one drone (true sisters) are nearly identical. This happens because all the sperm cells produced by one drone are genetically identical to each other—unlike the mixing that occurs when an individual has two sets of chromosomes. So in our case, the 10% of each colony that are true sisters are going to be susceptible to the same sort of environmental stressors and pathogens.
The drones in a colony—having only one set of chromosomes from the queen—are more similar to each other than the workers are to each other. For the most genetic variability, a queen needs to mate with drones from many different colonies.