Haplo-diploid sex-determination system

The Haplodiploid sex-determination system determines the sex of the offspring of many Hymenopterans (bees, ants, and wasps), and coleopterans (bark beetles). It may help to explain the evolution of eusociality in these species.

Several models have been proposed for the genetic mechanisms of haplodiploid sex-determination. The model most commonly referred to is the complementary allele model. According to this model, if an individual is heterozygous for a certain allele, it develops into a female, whereas hemizygous and homozygous individuals develop into males. In other words, diploid offspring develop from fertilized eggs, and are normally female, while haploid offspring develop into males from unfertilized eggs. Diploid males are infertile, as their sperm do not undergo meiosis which means that their offspring would be triploid. This also means that Hymenopterans may be especially sensitive to inbreeding: Inbreeding reduces the number of different sex alleles present in a population, hence increasing the occurrence of diploid males.

After mating, fertile Hymenopteran females store the sperm in an internal sac called the spermatheca. The mated female controls the release of stored sperm from within the organ: If she releases sperm as an egg passes down the oviduct, the egg is fertilized. Social bees, wasps, and ants can modify sex ratios within colonies to maximize relatedness among members, and to generate a workforce appropriate to surrounding conditions.

Sex-determination in honey bees
In honeybees the drones (males) are entirely derived from the queen, their mother. The queen has 32 chromosomes and the drones have 16 chromosomes. Drones produce genetically identical sperm. Males do not contribute to males - therefore males have no fathers or sons. The genetic makeup of the female worker bees is half derived from the mother, and half from the father. Thus, if a queen bee mates with only one drone, any two of her daughters will share, on average, 3/4 of their genes. The diploid queen's genome is recombined for her daughters, but the haploid father's genome is inherited by his daughters "as is".

While workers can lay unfertilized eggs that become their sons, haplodiploid sex-determination system is beneficial to the individual due to indirect selection. The worker is more related to the queen's daughters (her sisters) than to the workers' sons (her nephews). Helping the queen's offspring to survive is aiding the spread of the same genes that the worker possesses Batches of worker bees are short lived and are constantly being replaced by the next batch, so this kin selection is possibly a strategy to ensure the proper working of the hive. However, since queens usually mate with a dozen drones or more, not all workers are full sisters. Due to the separate storage of drone sperm, a specific batch of brood may be closer related than a specific batch of brood laid at a later date. Kin selection may explain the evolution of these eusocial colonies.

Shared gene proportions in haplo-diploid sex-determination system relationships