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 ar ...

system that determines the sex of offspring in

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

s, some

fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

and

crustaceans Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean g ...

such as the giant river prawn, some

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pa ...

s (including butterflies and

moth Moths are a paraphyletic group of insects that includes all members of the order Lepidoptera that are not butterflies, with moths making up the vast majority of the order. There are thought to be approximately 160,000 species of moth, many of w ...

s), the schistosome family of flatworms, and some reptiles, e.g. majority of snakes, lacertid lizards and monitors including

Komodo dragon The Komodo dragon (''Varanus komodoensis''), also known as the Komodo monitor, is a member of the monitor lizard family Varanidae that is endemic to the Indonesian islands of Komodo, Rinca, Flores, and Gili Motang. It is the largest extant ...

s. 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 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 ar ...

, and males have two similar ZZ

. In contrast to the XY sex-determination system and the

X0 sex-determination system The XO sex-determination system (sometimes X0 sex-determination system) is a system that some species of insects, arachnids, and mammals use to determine the sex of offspring. In this system, there is only one sex chromosome, referred to as X. Male ...

, 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, ...

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 i ...

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, 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 The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or mono ...

, a monotreme mammal, has a system of five pairs of XY chromosomes. They form a multiple chain due to homologous regions in male

meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...

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 Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. Chara ...

, 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

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 Doublesex and mab-3 related transcription factor 1, also known as DMRT1, is a protein which in humans is encoded by the ''DMRT1'' gene. Function DMRT1 is a dose sensitive transcription factor protein that regulates Sertoli cells and germ cell ...

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 Sex-links are crossbred chickens whose color at hatching is differentiated by sex, thus making chick sexing an easier process. Sex-links come in several varieties. As hybrids of laying or dual-purpose breeds infused with extra vigor via hetero ...

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'' 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 Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and developmen ...

, 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 polymorphisms identifiable by

restriction enzyme A restriction enzyme, restriction endonuclease, REase, ENase or'' restrictase '' is an enzyme that cleaves DNA into fragments at or near specific recognition sites within molecules known as restriction sites. Restriction enzymes are one class ...

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 wh ...

s in other snakes. The female-only pattern is in contrast to the ZW

Colubroidea Colubroidea is a superfamily of snakes in the clade Colubroides that includes Colubridae, with some studies splitting Colubridae into multiple families that make up Colubroidea. Historically, Colubroidea also included other caenophidian snakes su ...

n parthenogens, which always produce male (ZZ) offspring.

In moths and butterflies

Lepidoptera Lepidoptera ( ) is an order of insects that includes butterflies and moths (both are called lepidopterans). About 180,000 species of the Lepidoptera are described, in 126 families and 46 superfamilies, 10 percent of the total described speci ...

(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 Trematoda is a class of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is usually a snail. The definitive h ...

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 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), several are known to have a ZW system of sex determination. These include pistachio, several species of strawberry such as '' Fragaria virginiana'' and '' Fragaria chiloensis,'' and several species of willow including '' Salix viminalis'' and '' Salix purpurea''.

References

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