XO sex-determination system

This system determines the sex of offspring among:

• Most arachnids with the exception of mites where a small majority are haplodiploid
• Almost all apterygote and Paleopteran insects (e.g., dragonflies, silverfish)
• Most exopterygote insects (e.g., grasshoppers, crickets, cockroaches)
• Some nematodes, crustaceans, gastropod molluscs, and bony fish, notably in the genus Ancistrus
• Several mammals, including:
  • A few species of bat, including the hammer-headed bat, Buettikofer's epauletted fruit bat, Franquet's epauletted fruit bat, Peters's epauletted fruit bat, and Gambian epauletted fruit bat
  • The Ryukyu spiny rat and Tokunoshima spiny rat

In a variant of this system, most individuals have two sex chromosomes (XX) and are hermaphroditic, producing both eggs and sperm with which they can fertilize themselves, while rare individuals are male and have only one sex chromosome (XO). The model organism Caenorhabditis elegans—a nematode frequently used in biological research—is one such organism.

Most spiders have a variation of the XO system in which males have two different X chromosomes (X₁X₂O), while females have a pair of X₁ chromosomes and a pair of X₂ chromosomes (X₁X₁X₂X₂). Some spiders have more complex systems involving as many as 13 different X chromosomes.

Some Drosophila species have XO males. These are thought to arise via the loss of the Y chromosome.

In humans the XO designation attaches to individuals with Turner syndrome.

Evolution

XO sex determination can evolve from XY sex determination within about 2 million years. It typically evolves due to Y-chromosome degeneration. As the Y-chromosome is not paired (though see pseudoautosomal region), it is susceptible to decay by Muller's ratchet.

Similarly, the W chromosome in a ZW sex-determination system is susceptible to decay, resulting in a ZZ/ZO system.