The ZW sex-determination system is a
chromosomal 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), the schistosome family of flatworms, and some reptiles, e.g. majority of snakes, lacertid lizards and monitors, including
Komodo dragons. It is also present in some plants, where it has probably evolved independently on several occasions.[1] The letters Z and W are used to distinguish this system from the
XY sex-determination system. In the ZW system, females have a pair of dissimilar ZW
chromosomes, and males have two similar ZZ
chromosomes.
In contrast to the XY sex-determination system and the
X0 sex-determination system, where the
sperm determines the sex, in the ZW system, the
ovum determines the sex of the offspring. Males are 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,[2] and, from a comparison between chicken and human, the Z chromosome appears similar to the
autosomal chromosome 9 in humans. It has been proposed that the ZW and XY sex determination systems do not share an origin but that the sex chromosomes are derived from autosomal chromosomes of the
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
platypus, a
monotreme mammal, has a system of five pairs of XY chromosomes. They form a multiple chain due to homologous regions in male
meiosis and finally segregates into XXXXX-sperm and YYYYY-sperm. The bird Z-like pair shows up on opposite ends of the chain. Areas homologous to the bird Z chromosome are scattered throughout X3 and X5.[4]: fig. 5 Although the sex-determination system is not necessarily linked to that of birds and definitely not to that of
therian mammals, the similarity at least allowed for the conclusion that mammals evolved sex chromosomes twice.[5] The previous report that platypus has X chromosomes similar to that of
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
dosage compensation, and instead seem to dosage compensate on a gene-by-gene basis.[8][9] 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.[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.[16] It appears possible that either condition could cause
embryonic death, or that both chromosomes could be responsible for sex selection.[17][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]
Boa and Python families are now known to probably have an
XY sex-determination system.[19] 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.[20]Python bivittatus and Boa imperator, similarly only produce female offspring; their genomes share male-specific
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
Schistosomatidae, commonly called blood flukes, are small parasitic flatworms dwelling in the blood vessels of the bladder, liver, intestines and other organs of birds and mammals. They are the only sexually heteromorphic family among the
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]
^Stiglec R, Ezaz T, Graves JA (2007). "A new look at the evolution of avian sex chromosomes". Cytogenet. Genome Res. 117 (1–4): 103–9.
doi:
10.1159/000103170.
PMID17675850.
S2CID12932564.
^Ellegren, Hans (1 March 2011). "Sex-chromosome evolution: recent progress and the influence of male and female heterogamety". Nature Reviews Genetics. 12 (3): 157–166.
doi:
10.1038/nrg2948.
ISSN1471-0056.
PMID21301475.
S2CID21098648.