Haplodiploid Sex-determination System

Haplodiploid Sex-determination System

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.