Haplodiploidy



Haplodiploidy is a sex-determination system in which males develop from unfertilized eggs and are haploid, and females develop from fertilized eggs and are diploid. Haplodiploidy is sometimes called arrhenotoky.

Haplodiploidy determines the sex in all members of the insect order Hymenoptera (bees, ants, and wasps), p408 Coccidae, and the Thysanoptera ('thrips'). The system also occurs sporadically in some spider mites, Homoptera, Coleoptera (bark beetles), and rotifers.

In this system, sex is determined by the number of sets of chromosomes an individual receives. An offspring formed from the union of a sperm and an egg develops as a female, and an unfertilized egg develops as a male. This means that the males have half the number of chromosomes that a female has, and are haploid.

The haplodiploid sex-determination system has a number of peculiarities; for example, a male has no father and cannot have sons, but he has a grandfather and can have grandsons. And, more to the point, the relatedness between worker bees (diploid females) in a hive or nest is 0.75. This means the workers are significantly more closely related than siblings in other sex determination systems. It is this point which drives the kin selection theory of how eusociality evolved. p465 Whether haplodiploidy did in fact pave the way for the evolution of eusociality is still a matter of debate.

Another feature of the haplodiploidy system is that lethal and deleterious alleles will be removed from the population rapidly because they will automatically be expressed in the males.

Mechanisms
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 locus, 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 would be infertile, as their cells would not undergo meiosis to form sperm. Therefore the sperm would be diploid, which means that their offspring would be triploid. Since hymenopteran mother and sons share the same genes, they 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 diploid queen has 32 chromosomes and the haploid drones have 16 chromosomes. Drones produce sperm cells that contain their entire genome, so the sperm are all genetically identical except for mutations. The genetic makeup of the female worker bees is half derived from the mother, and half from the father, but the male bees' genetic makeup is entirely derived from the mother. 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 increases the individual's fitness due to indirect selection. Since the worker is more related to the queen's daughters (her sisters) than to her own offspring, helping the queen's offspring to survive aids the spread of the same genes that the worker possesses more efficiently than direct reproduction. 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 more closely related than a specific batch of brood laid at a later date.

Relatedness ratios in haplodiploidy
Relatedness is used to calculate the strength of kin selection (via Hamilton's rule). The haplodiploidy hypothesis states that the unusual 3/4 relatedness coefficient amongst full haplodiploid sisters is responsible for the frequency of evolution of eusocial behavior in hymenoptera.

In normal sexual reproduction the father has two sets of chromosomes, and crossing over takes place between the chromatids of each pair during the meiosis which produces the sperm. Therefore, the sperms are not identical, because in each chromosome of a pair there will be different alleles at many of the loci. But when the father is haploid all the sperm will be identical (except for a small number where gene mutations have taken place in the germ line). So, as long as all the female queen mates only with one male, all the female offspring will inherit the male's chromosome 100% intact. In fact in hymenoptera, the males almost all produce enough sperm to last the female for a whole lifetime.

Relatedness coefficients in haplodiploid organisms are as follows.

Controversy
The haplodiploidy hypothesis for the evolution of eusociality in hymenoptera is often given as a clear example of kin selection.

Since full hymenopteran sisters share more genes than a parent shares with its offspring, it follows that helping to rear sisters should be favored over having children as an evolutionary strategy. However, this argument neglects that eusocial workers help raise their brothers in addition to their sisters, which, given the typical sex ratio of 1:1, results in raising siblings with an average relatedness coefficient of 1/2, which is no better than that of young. Ratios can be even lower in colonies where the queen has mated with multiple males, which is common in some species. This, combined with the discovery of multiple diploid eusocial organisms and at least one haplodiploid eusocial species with a male nonreproductive caste, has been referred to by one of the founding figures of sociobiology, E.O. Wilson, as the "collapse of the haplodiploid hypothesis". However, not all researchers believe the hypothesis has been disproven, and evidence continues to be collected on both sides. Statistical analysis suggests that in each of the independent evolutions of eusociality, queens mated with only one male, which suggests having highly related offspring was important. Furthermore, often the female bees give more food to the females than to the males, and in times of dearth will even kill the males.