The X0 sex-determination system is a system that hymenopterans, grasshoppers, crickets, cockroaches, and some other insects use to determine the sex of their offspring. In this system, there is only one sex chromosome, referred to as X. Males only have one X chromosome (X0), while females have two (XX). The zero (sometimes, the letter O) signifies the lack of a second X. Maternal gametes always contain an X chromosome, so the sex of the animals' offspring is decided by the male. Its sperm normally contain either one X chromosome or no sex chromosomes at all.
In a variant of this system, certain animals are hermaphroditic with two sex chromosomes (XX) and male with only one (X0). The model organism Caenorhabditis elegans — a nematode frequently used in biological research — is one such organism.
Some Drosophila species have X0 males[1]. These are thought to arise via the loss of the Y chromosome.
In humans, having only a single X chromosome is known as Turner syndrome. However, such individuals generally present as female, as humans have an XY sex-determination system.