SEXES AND CASTES OF HONEY BEES

Sexes and Castes

Honey bees have three castes: drones, workers, and queens. Drones are male, while workers and queens are female.

Drones

Drones are typically haploid, having only one set of chromosomes, and primarily exist for the purpose of reproduction. They are produced by the queen if she chooses not to fertilize an egg or by an unfertilized laying worker. There are rare instances of diploid drone larvae. This phenomenon usually arises when there is more than two generations of brother-sister mating. Sex determination in honey bees is initially due to a single locus, called the complementary sex determiner (csd) gene. In developing bees, if the conditions are that the individual is heterozygous for the csd gene, they will develop into females. If the conditions are so that the individual is hemizygous or homozygous for the csd gene, they will develop into males. The instances where the individual is homozygous at this gene are the instances of diploid males. Drones take 24 days to develop, and may be produced from summer through to autumn, numbering as many as 500 per hive. They are expelled from the hive during the winter months when the hive's primary focus is warmth and food conservation. Drones have large eyes used to locate queens during mating flights. They do not defend the hive or kill intruders, and do not have a stinger.

Workers

Workers have two sets of chromosomes. They are produced from an egg that the queen has selectively fertilized from stored sperm. Workers typically develop in 21 days. A typical colony may contain as many as 60,000 worker bees. Workers exhibit a wider range of behaviors than either queens or drones. Their duties change upon the age of the bee in the following order (beginning with cleaning out their own cell after eating through their capped brood cell): feed brood, receive nectar, clean hive, guard duty, and foraging. Some workers engage in other specialized behaviors, such as "undertaking" (removing corpses of their nestmates from inside the hive).

Workers have morphological specializations, including the pollen basket (corbicula), abdominal glands that produce beeswax, brood-feeding glands, and barbs on the sting. Under certain conditions (for example, if the colony becomes queenless), a worker may develop ovaries.

Worker honey bees perform different behavioural tasks that cause them to be exposed to different local environments. The gut microbi composition of workers varies according to the landscape and plant species they forage, such as differences in rapeseed crops, and with different hive tasks, such as nursing or food processing.

Queens

Queen honey bees are created when worker bees feed a single female larvae an exclusive diet of a food called "royal jelly". Queens are produced in oversized cells and develop in only 16 days; they differ in physiology, morphology, and behavior from worker bees. In addition to the greater size of the queen, she has a functional set of ovaries, and a spermatheca, which stores and maintains sperm after she has mated. Apis queens practice polyandry, with one female mating with multiple males. The highest documented mating frequency for an Apis queen is in Apis nigrocincta, where queens mate with an extremely high number of males with observed numbers of different matings ranging from 42 to 69 drones per queen. The sting of queens is not barbed like a worker's sting, and queens lack the glands that produce beeswax. Once mated, queens may lay up to 2,000 eggs per day. They produce a variety of pheromones that regulate behavior of workers, and helps swarms track the queen's location during the swarming.

Queen-worker conflict

When a fertile female worker produces drones, a conflict arises between her interests and those of the queen. The worker shares half her genes with the drone and one-quarter with her brothers, favouring her offspring over those of the queen. The queen shares half her genes with her sons and one-quarter with the sons of fertile female workers. This pits the worker against the queen and other workers, who try to maximize their reproductive fitness by rearing the offspring most related to them. This relationship leads to a phenomenon known as "worker policing". In these rare situations, other worker bees in the hive who are genetically more related to the queen's sons than those of the fertile workers will patrol the hive and remove worker-laid eggs. Another form of worker-based policing is aggression toward fertile females. Some studies have suggested a queen pheromone which may help workers distinguish worker- and queen-laid eggs, but others indicate egg viability as the key factor in eliciting the behavior. Worker policing is an example of forced altruism, where the benefits of worker reproduction are minimized and that of rearing the queen's offspring maximized.

In very rare instances workers subvert the policing mechanisms of the hive, laying eggs which are removed at a lower rate by other workers; this is known as anarchic syndrome. Anarchic workers can activate their ovaries at a higher rate and contribute a greater proportion of males to the hive. Although an increase in the number of drones would decrease the overall productivity of the hive, the reproductive fitness of the drones' mother would increase. Anarchic syndrome is an example of selection working in opposite directions at the individual and group levels for the stability of the hive.

Under ordinary circumstances the death (or removal) of a queen increases reproduction in workers, and a significant proportion of workers will have active ovaries in the absence of a queen. The workers of the hive produce a last batch of drones before the hive eventually collapses. Although during this period worker policing is usually absent, in certain groups of bees it continues.

According to the strategy of kin selection, worker policing is not favored if a queen does not mate multiple times. Workers would be related by three-quarters of their genes, and the difference in relationship between sons of the queen and those of the other workers would decrease. The benefit of policing is negated, and policing is less favored. Experiments confirming this hypothesis have shown a correlation between higher mating rates and increased rates of worker policing in many species of social hymenoptera.