Haplodiploidy
Haplodiploidy determines the sex in all members of the insect orders Hymenoptera (bees, ants, and wasps)[2] and Thysanoptera ('thrips').
[3] The system also occurs sporadically in some spider mites, Hemiptera, Coleoptera (bark beetles), and rotifers.
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 is 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.
[5][6] Another feature of the haplodiploidy system is that recessive lethal and deleterious alleles will be removed from the population rapidly because they will automatically be expressed in the males (dominant lethal and deleterious alleles are removed from the population every time they arise, as they kill any individual they arise in).
After mating, each fertile hymenopteran female stores sperm in an internal sac called the spermatheca.
[7] Social bees, wasps, and ants can modify sex ratios within colonies which maximizes relatedness among members and generates a workforce appropriate to surrounding conditions.
[15] While workers can lay unfertilized eggs that become their sons, haplodiploid sex-determination system increases the individual's fitness due to indirect selection.
The most recent accepted model for this non-CSD system is called Maternal Effect Genomic Imprinting Sex Determination (MEGISD).
[20] The haplodiploidy hypothesis proposes that the unusual 3⁄4 relatedness coefficient amongst full haplodiploid sisters is responsible for the frequency of evolution of eusocial behavior in hymenopterans.
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.
But when the father is haploid all the sperms are identical (except for a small number where gene mutations have taken place in the germ line).
These ratios apply, for example, throughout a bee hive, unless some laying workers produce offspring, which will all be males from unfertilised eggs: in that case, average relatedness will be lower than shown.
