ZW sex-determination system

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 dosage compensation, and instead seem to dosage compensate on a gene-by-gene basis.[7][8] Specific locations on the chicken Z chromosome, such as the MHM region, are thought to exhibit regional dosage compensation, though researchers have argued that this region does not actually constitute local dosage compensation.[9][10] 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.[11][12] Both transcriptional and translational gene-specific dosage compensation have been observed in avian sex chromosomes.[13] In addition, the involvement of sex-biased miRNAs was proposed to compensate for the presence of 2 Z-chromosomes in male birds.[14]

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 discovered. 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. It appears possible that either condition could cause embryonic death, or that both chromosomes could be responsible for sex selection.[15] 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.[13][16]

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

Snake W chromosomes show different levels of decay compared to their Z chromosomes. This allows for tracking the shrinking of W 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 Python show little signs of "W-shrinking".[6]

Boa and Python families are now known to probably have an XY sex-determination system.[17] Interest in looking into this came from female family members capable of parthenogenesis, or producing offspring without mating. In 2010 a female Boa constrictor that produced 22 female offspring in this manner was found in the wild. By then it was presumed that such a pattern was produced by WW chromosomes.[18] Python bivittatus and Boa imperator, similarly only produce female offspring; their genomes share male-specific single nucleotide polymorphisms identifiable by restrictive 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.[19] The female-only pattern is in contrast to the ZW Colubroidean parthenogens, which always produce male (ZZ) offspring.[20]

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).