ZW sex-determination system

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ZW sex determination in birds (as exemplified with chickens)
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The ZW sex-determination system is a

chromosomes
.

In contrast to the XY sex-determination system and the

homogametic sex (ZZ), while females are the heterogametic sex
(ZW). The Z chromosome is larger and has more genes, similarly to the X chromosome in the XY system.

Significance of the ZW and XY systems

No genes are shared between the avian ZW and mammalian XY chromosomes,

common ancestor. These autosomes are thought to have evolved sex-determining loci that eventually developed into the respective sex chromosomes once the recombination between the chromosomes (X and Y or Z and W) was suppressed.[3]

The

therian mammals is now considered a mistake.[6]

Bird and snake ZW are unrelated, having evolved from different autosomes.[7] However, the bird-like chromosomes of platypus may indicate that ancestors of snakes had a bird-like ZW system.[6]

Across species

In birds

While there has not been extensive research on other organisms with the ZW sex-determination system, in 2007, researchers announced that chickens' and zebra finches' sex chromosomes do not exhibit any type of chromosome-wide

MHM region, are thought to exhibit regional dosage compensation, though researchers have argued that this region does not actually constitute local dosage compensation.[10][11] Further research expanded the list of birds that do not exhibit any type of chromosome-wide dosage compensation to crows and ratites, thus implying that all avian chromosomes lack chromosome-wide dosage compensation.[12][13] Both transcriptional and translational gene-specific dosage compensation have been observed in avian sex chromosomes.[14] In addition, the involvement of sex-biased miRNAs was proposed to compensate for the presence of two Z-chromosomes in male birds.[15]

It is unknown whether it might be that the presence of the W chromosome induces female features, or whether instead it is the duplication of the Z chromosome that induces male ones; unlike mammals, no birds with a double W chromosome (ZWW) or a single Z (Z0) have been satisfactorily documented. However, it is known that the removal or damage to the ovaries of female birds can lead to the development of male plumage, suggesting that female hormones repress the expression of male characteristics in birds.

better source needed] One possible gene that could determine sex in birds is the DMRT1 gene. Studies have shown that two copies of the gene are necessary for male sex determination.[14][18]

The ZW sex-determination system makes it possible to create sex-link chickens in which color at hatching is differentiated by sex, thus making chick-sexing an easier process.

In snakes

Snakes' W chromosomes show different levels of decay compared to their Z chromosomes. This allows for tracking the shrinking of W chromosomes (analogous to the shrinking of Y chromosomes) by comparing across species. Mapping of specific genes reveals that the snake system is different from the bird system. It is not yet known which gene is the sex-determining one in snakes. One thing that stood out was that pythons show little signs of "W-shrinking".[7]

single nucleotide polymorphisms identifiable by restriction enzyme digestion. Their chromosomal origins, however, differ: Python's XY are similar to other snakes' ZW, while Boa XY maps to microchromosomes in other snakes.[21] The female-only pattern is in contrast to the ZW Colubroidean parthenogens, which always produce male (ZZ) offspring.[22]

In moths and butterflies

In Lepidoptera (moths and butterflies), females can have Z, ZZW, or ZZWW.[23]

In schistosomes

The family

trematode class, and depend on remaining biochemically paired in copula to complete their life cycle.[24] The heterogametic sex chromosomes in females of nine species of schistosomes were first described by geneticist Margaret Menzel and parasitologist Robert B. Short of Florida State University in 1960.[25][26]
The difference in the sex chromosomes was noted during the pachytene stage of meiotic prophase, when the chromosomes thicken and align with their homologous partner.

In turtles

Trionychidae turtles possess a ZZ-ZW sex determinate system, which originated sometime between the beginning of the Jurassic and the Early Cretaceous.[27]

In plants

Among the approximately 5% of plant species that have separate male and female individuals (

pistachio, several species of strawberry such as Fragaria virginiana and Fragaria chiloensis, and several species of willow including Salix viminalis and Salix purpurea.[1][28]

See also

References

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