Honey Bee Genetics

Honey bees and other social insects have an unusual reproductive genetic system, different to us and most other insects – it is called haplodiploidy. In this system the females, which develop from fertilised eggs (egg and sperm) have two sets of chromosomes, and are called diploid, and the males, which develop from unfertilised eggs, (just egg) have only one set of chromosomes, are called haploid. In essence, drones do not have fathers and so it follows that they cannot have sons. But they do have grandfathers and can have grandsons!

Why this system, which is predominantly found in of social insects, has evolved is a point of much debate but one explanation lies in the fact that this results in higher degree of relatedness between the siblings in the colony. Because drones come with genetic material just from their mother they pass on all of that material in their sperm which means that all drone sperm is identical. Thus, all females fathered by the same drone are called super-sisters as they share about 75% of their genetic material (25% from mother and 50% from the father) and studies have shown that this encourages altruistic behaviour in the colony, putting the colony’s interests above that of an individual. Half-sisters, females that have same mother and different fathers share about 25% of their genetic material.

It is important to note here that queens typically mate with 10-20 drones (multiple mating or Polyandry) so normally in a colony there are 10-20 different super-sister lines. As a beekeeper you will have probably noticed this, differences in the worker population of the same hive, most obvious being when some workers are black and others yellow (see photo below).  The reason for queens promiscuity is explained by the advantage that if confers to the family to have a wider genetic diversity of its offspring groups so that it has greater plasticity to deal with unfavourable conditions.

Dark queen

Diploid drones (and cannibalism)
While genetic relatedness is important for social cohesion, the genetic diversity is essential for the fitness of the colony. Queens fly very far from the hive (to drone congregation areas) in order to avoid mating with related males, which would result in in-breeding. In an extreme case of in-breeding when a queen mates with her brothers or cousins the resulting offspring can be diploid drones (remember from above, all drones are haploid). These individuals arise from fertilised eggs (as females do) but because they carry the same genes at the specific sex-determination spot they develop into males rather than females. However, these individuals do not live, they are normally cannibalised by the workers as soon as they hatch; bees are not wasteful, unviable brood is a good source of protein after all!

Workers Laying Drones (And swtich from altruism to selfishness)
Finally, another interesting situation that involves drones arises when worker bees become layers. Workers are born with rudimentary ovaries which are not functional as their development is suppressed by the presence of the queen and brood pheromones. However, occasionally in a colony that is queenless, whether following swarming or due to failing queen, some worker bees start laying eggs. As these eggs are not fertilised they inevitably develop into drones, i.e. they are haploid. These drones tend to be smaller as they are often raised in worker cells (see photo below).

Worker laying drone

If not remedied the colony is destined to fail but the question remains as to what the advantage of this behaviour is. The answer again lies in the genetics and relatedness between the individuals. From a Darwinian point of view an individual’s success is measured in its ability to pass on as many copies of its genes. As described above the workers are more related to their super-sisters and sisters than they would be to their offspring which also contributes to the fact that they do not reproduce. However, when the colony’s queen is replaced or missing, worker’s offspring is the last chance to pass on the genetic material. In this case the whole idea of social altruism is replaced by an individual’s drive to pass on the genetic material. For the more scientifically inclined here is an interesting study that goes into detail of this behaviour.