The ZW sex-determination system is a

chromosomal A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

system that determines the sex of offspring in birds, some fish and crustaceans such as the giant river prawn, some insects (including

butterflies Butterflies are insects in the macrolepidopteran clade Rhopalocera from the Order (biology), order Lepidoptera, which also includes moths. Adult butterflies have large, often brightly coloured wings, and conspicuous, fluttering flight. The ...

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 used in some plants where it has probably evolved independently on several occasions. The letters Z and W are used to distinguish this system from the XY sex-determination system. In this 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 Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...

determines the sex, in the ZW system, the

ovum The egg cell, or ovum (plural ova), is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female gamete is ...

determines the sex of the offspring. Males are the

homogametic sex Heterogametic sex (digametic sex) refers to the individuals of a species in which the sex chromosomes are not the same. For example, in humans, males with an X and a Y sex chromosome would be referred to as the heterogametic sex, and females ...

(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.

Significance of the ZW and XY systems

No genes are shared between the avian ZW and mammalian XY chromosomes, 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. The platypus, a

monotreme Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their brain ...

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. 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. The previous report that platypus has X chromosomes similar to that of therian mammals is now considered a mistake. Bird and snake ZW are unrelated, having evolved from different autosomes. However, the bird-like chromosomes of platypus may indicate that ancestors of snakes had a bird-like ZW system.

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. 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. 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. Both transcriptional and translational gene-specific dosage compensation have been observed in avian sex chromosomes. In addition, the involvement of sex-biased miRNAs was proposed to compensate for the presence of two Z-chromosomes in male birds. 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. It appears possible that either condition could cause

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

nic death, or that both chromosomes could be responsible for sex selection. 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. 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". ''

Boa Kwon Bo-ah (; born November 5, 1986), known professionally as BoA, is a South Korean singer, songwriter, dancer, record producer and actress. One of the most successful and influential Korean entertainers, she has been dubbed the " Queen of K- ...

'' and '' Python'' families are now known to probably have an XY sex-determination system. 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. ''

Python bivittatus The Burmese python (''Python bivittatus'') is one of the largest species of snakes. It is native to a large area of Southeast Asia and is listed as Vulnerable on the IUCN Red List. Until 2009, it was considered a subspecies of the Indian python ...

'' and '' Boa imperator,'' similarly only produce female offspring; their genomes share male-specific

single nucleotide polymorphism In genetics, a single-nucleotide polymorphism (SNP ; plural SNPs ) is a germline substitution of a single nucleotide at a specific position in the genome. Although certain definitions require the substitution to be present in a sufficiently larg ...

s identifiable by restriction enzyme digestion. Their chromosomal origins, however, differ: Python's XY are similar to other snakes' ZW, while Boa XY maps to

microchromosome A microchromosome (μChr) is a type of very small chromosome which is a typical component of the karyotype of birds, some reptiles, fish, and amphibians; they have yet to be found in mammals. They are less than 20 Mb in size; chromosomes which a ...

s in other snakes. The female-only pattern is in contrast to the ZW Colubroidean parthenogens, which always produce male (ZZ) offspring.

In moths and butterflies

Lepidoptera Lepidoptera ( ) is an order (biology), order of insects that includes butterfly, butterflies and moths (both are called lepidopterans). About 180,000 species of the Lepidoptera are described, in 126 Family (biology), families and 46 Taxonomic r ...

(moths and butterflies), females can have Z, ZZW, or ZZWW.

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. The heterogametic sex chromosomes in females of nine species of schistosomes were first described by geneticist

Margaret Menzel Margaret Young Menzel was a geneticist known for her research on chromosomes and meiosis in a range of organisms including tomatoes, flowering plants, and worms. Menzel was also an advocate for equal opportunities for women and led a 1972 class ac ...

and parasitologist Robert B. Short of Florida State University in 1960. 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.

In plants

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

dioecious Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproductio ...

), several are known to have a ZW system of sex determination. These include

pistachio The pistachio (, ''Pistacia vera''), a member of the cashew family, is a small tree originating from Central Asia and the Middle East. The tree produces seeds that are widely consumed as food. ''Pistacia vera'' is often confused with other sp ...

, several species of strawberry such as '' Fragaria virginiana'' and '' Fragaria chiloensis,'' and several species of willow including '' Salix viminalis'' and ''

Salix purpurea ''Salix purpurea'', the purple willow purpleosier willow or purple osier, is a species of willow native to most of Europe and western Asia north to the British Isles, Poland, and the Baltic States.Flora Europaea''Salix purpurea''/ref>Meikle, R. D ...

''.

References

{{DEFAULTSORT:Zw Sex-Determination System Sex-determination systems Insect genetics Lepidopterology