Haplodiploidy

Haplodiploidy determines the sex in all members of the insect orders Hymenoptera (bees, ants, wasps and sawflies) and Thysanoptera (thrips). The system also occurs sporadically in some spider mites, Hemiptera, 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. Additionally, if a eusocial-insect colony has only one queen, and she has only mated once, then the relatedness between workers (diploid females) in a hive or nest will be on average 3⁄4. This means the workers in such monogamous single-queen colonies are significantly more closely related than in other sex determination systems where the relatedness of siblings is usually no more than 1⁄2. It is this point which drives the kin selection theory of how eusociality evolved. Whether haplodiploidy did in fact pave the way for the evolution of eusociality is still a matter of debate.

A feature of the haplodiploidy system is that recessive lethal and deleterious alleles are removed from the population rapidly because they are automatically expressed in the males: dominant lethal and deleterious alleles are removed from the population, as they kill or harm every individual they arise in.

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, each fertile hymenopteran female stores 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 her oviduct, the egg is fertilized. In solitary hymenopterans, the females lay unfertilized male eggs on poorer food sources while laying the fertilized female eggs on better food sources, possibly because the fitness of females will be more adversely affected by shortages in their early life. Sex ratio manipulation is practiced by haplodiploid ambrosia beetles, which lay more male eggs when the opportunity for males to disperse and mate with females in different sites is greater.

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 male bees' genetic makeup is therefore entirely derived from the mother, while 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". It is also possible for a laying worker bee to lay an unfertilised egg, which is always a male.

There are rare instances of diploid drone larvae. This usually occurs when there is more than two generations of brother-sister mating. Sex determination in honey bees is initially due to a single locus, 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 such that the individual is hemizygous or homozygous for the csd gene, they develop into males. The instances where the individual is homozygous at this gene are the instances of diploid males. Diploid males do not survive to adulthood, as the nurse worker bees will cannibalize the diploid males upon hatching.

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 helps 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. However, in many other species of bees, including bumblebees, the queen mates with only one drone. This means that sisters are almost always more related to one another than they would be to their own offspring, thus eliminating the conflict of variable relatedness present in honeybees.

Sex determination in chalcidoid wasps

In chalcidoid wasps of the genus Nasonia, the method of sex determination does not involve the complementary sex determiner (CSD) mechanism. The mechanism, Maternal Effect Genomic Imprinting Sex Determination (MEGISD), involves a masculinizing/virilizing maternal effect gene that "imprints upon" the oocyte cytoplasm, and an "unimprinted" paternal contribution (in female offspring) provides a counter effect to virilization, allowing female development to occur. All diploid eggs become female, unlike in CSD where diploid eggs can become males if they are homozygous or hemizygous.