The ZW sex-determination system is a system that determines the sex of offspring in birds, some fish and crustaceans such as the giant river prawn, some insects (including butterflies and moths), and some reptiles, including komodo dragons. In the ZW system, the ovum determines the sex of the offspring, in contrast to the XY sex-determination system and the X0 sex-determination system, wherein the sperm determines the sex. The letters Z and W are used to distinguish this system from the XY system. Males are the homogametic sex (ZZ), while females are the heterogametic sex (ZW). The Z chromosome is larger and has more genes, like the X chromosome in the XY system.
As well as the ZW and XY sex-determination systems, there is also a temperature-dependent sex determination system found in some reptiles.
On November 3, 2010, scientists announced the discovery of a female Boa constrictor that can produce offspring without mating and, through such asexual reproduction, produced 22 female offspring, all with a WW chromosome genetic makeup. Although this result has been achieved in laboratory settings previously, never before has it been proven under natural circumstances. It is not clear as to whether the all-female snake babies will eventually mate with a male, or reproduce asexually, or do both as does their mother. However, because of their WW chromosomes, any offspring they produce will be female.
It is unknown whether the presence of the W chromosome induces female features or the duplication of the Z chromosome induces male ones; unlike mammals, no birds with a double W chromosome (ZWW) or a single Z (Z0) have been discovered. It appears possible that either condition could cause embryonic death, or that both chromosomes could be responsible for sex selection.
In Lepidoptera (moths and butterflies), examples of Z0, ZZW, and ZZWW females can be found. This suggests that the W chromosome is essential in female determination in some species (ZZW), but not in others (Z0). In Bombyx mori (the commercial silkworm), the W chromosome carries the female-determining genes.
No genes are shared between the avian ZW and mammal XY chromosomes, and, from a comparison between chicken and human, the Z chromosome appeared similar to the autosomal chromosome 9 in human, rather than X or Y, leading researchers to believe that the ZW and XY sex determination systems do not share an origin, but the sex chromosomes are derived from autosomal chromosomes of the common ancestor. A paper from 2004 compared the chicken Z chromosome with platypus X chromosomes and suggested that the two systems are related. The platypus has a ten-chromosome–based system, where the chromosomes form a multivalent chain in male meiosis, segregating into XXXXX-sperm and YYYYY-sperm, with XY-equivalent chromosomes at one end of this chain and the ZW-equivalent chromosomes at the other end.
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