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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.
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]
In 2007, a time where there had not been extensive research on other organisms with the ZW sex-determination system, 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.
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 Lepidoptera (moths and butterflies), females can have Z, ZZW, or ZZWW. [23]
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.
Trionychidae turtles possess a ZZ-ZW sex determinate system, which originated sometime between the beginning of the Jurassic and the Early Cretaceous. [27]
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 . [1] [28]
Sex is the biological trait that determines whether a sexually reproducing organism produces male or female gametes. During sexual reproduction, a male and a female gamete fuse to form a zygote, which develops into an offspring that inherits traits from each parent. By convention, organisms that produce smaller, more mobile gametes are called male, while organisms that produce larger, non-mobile gametes are called female. An organism that produces both types of gamete is hermaphrodite.
The XY sex-determination system is a sex-determination system present in many mammals, including humans, some insects (Drosophila), some snakes, some fish (guppies), and some plants.
A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two common sexes and a few less common intersex variations.
The Y chromosome is one of two sex chromosomes in therian mammals and other organisms. Along with the X chromosome, it is part of the XY sex-determination system, in which the Y is the sex-determining chromosome because the presence of the Y chromosome causes offspring produced in sexual reproduction to be of male sex. In mammals, the Y chromosome contains the SRY gene, which triggers development of male gonads. The Y chromosome is passed only from male parents to male offspring.
Haldane's rule is an observation about the early stage of speciation, formulated in 1922 by the British evolutionary biologist J. B. S. Haldane, that states that if — in a species hybrid — only one sex is inviable or sterile, that sex is more likely to be the heterogametic sex. The heterogametic sex is the one with two different sex chromosomes; in therian mammals, for example, this is the male.
Sexual differentiation is the process of development of the sex differences between males and females from an undifferentiated zygote. Sex determination is often distinct from sex differentiation; sex determination is the designation for the development stage towards either male or female, while sex differentiation is the pathway towards the development of the phenotype.
Sex-determining region Y protein (SRY), or testis-determining factor (TDF), is a DNA-binding protein encoded by the SRY gene that is responsible for the initiation of male sex determination in therian mammals. SRY is an intronless sex-determining gene on the Y chromosome. Mutations in this gene lead to a range of disorders of sex development with varying effects on an individual's phenotype and genotype.
Dosage compensation is the process by which organisms equalize the expression of genes between members of different biological sexes. Across species, different sexes are often characterized by different types and numbers of sex chromosomes. In order to neutralize the large difference in gene dosage produced by differing numbers of sex chromosomes among the sexes, various evolutionary branches have acquired various methods to equalize gene expression among the sexes. Because sex chromosomes contain different numbers of genes, different species of organisms have developed different mechanisms to cope with this inequality. Replicating the actual gene is impossible; thus organisms instead equalize the expression from each gene. For example, in humans, female (XX) cells randomly silence the transcription of one X chromosome, and transcribe all information from the other, expressed X chromosome. Thus, human females have the same number of expressed X-linked genes per cell as do human males (XY), both sexes having essentially one X chromosome per cell, from which to transcribe and express genes.
Male is the sex of an organism that produces the gamete known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilisation. A male organism cannot reproduce sexually without access to at least one ovum from a female, but some organisms can reproduce both sexually and asexually. Most male mammals, including male humans, have a Y chromosome, which codes for the production of larger amounts of testosterone to develop male reproductive organs.
The pseudoautosomal regions or PARs are homologous sequences of nucleotides found within the sex chromosomes of species with an XY or ZW mechanism of sex determination.
The XO 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. Males only have one X chromosome (XO), while females have two (XX). The letter O signifies the lack of a Y chromosome. Maternal gametes always contain an X chromosome, so the sex of the animals' offspring depends on whether a sex chromosome is present in the male gamete. Its sperm normally contains either one X chromosome or no sex chromosomes at all.
In embryology, the genital ridge is the developmental precursor to the gonads. The genital ridge initially consists mainly of mesenchyme and cells of underlying mesonephric origin. Once oogonia enter this area they attempt to associate with these somatic cells. Development proceeds and the oogonia become fully surrounded by a layer of cells.
Sex chromosomes are chromosomes that carry the genes that determine the sex of an individual. The human sex chromosomes are a typical pair of mammal allosomes. They differ from autosomes in form, size, and behavior. Whereas autosomes occur in homologous pairs whose members have the same form in a diploid cell, members of an allosome pair may differ from one another.
Parthenogenesis is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means development of an embryo from an unfertilized egg cell. In plants, parthenogenesis is a component process of apomixis. In algae, parthenogenesis can mean the development of an embryo from either an individual sperm or an individual egg.
The western clawed frog is a species of frog in the family Pipidae, also known as tropical clawed frog. It is the only species in the genus Xenopus to have a diploid genome. Its genome has been sequenced, making it a significant model organism for genetics that complements the related species Xenopus laevis, a widely used vertebrate model for developmental biology. X. tropicalis also has a number of advantages over X. laevis in research, such as a much shorter generation time, smaller size, and a larger number of eggs per spawn.
An organism's sex is female if it produces the ovum, the type of gamete that fuses with the male gamete during sexual reproduction.
A microchromosome is a chromosome defined for its relatively small size. They are typical components of the karyotype of birds, some reptiles, fish, amphibians, and monotremes. As many bird genomes have chromosomes of widely different lengths, the name was meant to distinguish them from the comparatively large macrochromosomes. The distinction referred to the measured size of the chromosome while staining for karyotype, and while there is not a strict definition, chromosomes resembling the large chromosomes of mammals were called macrochromosomes, while the much smaller ones of less than around 0.5 μm were called microchromosomes. In terms of base pairs, by convention, those of less than 20Mb were called microchromosomes, those between 20 and 40 Mb are classified as intermediate chromosomes, and those larger than 40Mb are macrochromosomes. By this definition, all normal chromosomes in organisms with relatively small genomes would be considered microchromosomes.
Sex reversal is a biological process whereby the pathway directed towards the already determined-sex fate is flipped towards the opposite sex, creating a discordance between the primary sex fate and the sex phenotype expressed. The process of sex reversal occurs during embryonic development or before gonad differentiation. In GSD species, sex reversal means that the sexual phenotype is discordant with the genetic/chromosomal sex. In TSD species, sex reversal means that the temperature/conditions that usually trigger the differentiation towards one sexual phenotype are producing the opposite sexual phenotype.
Silene is a flowering plant genus that has evolved a dioecious reproductive system. This is made possible through heteromorphic sex chromosomes expressed as XY. Silene recently evolved sex chromosomes 5-10 million years ago and are widely used by geneticists and biologists to study the mechanisms of sex determination since they are one of only 39 species across 14 families of angiosperm that possess sex-determining genes. Silene are studied because of their ability to produce offspring with a plethora of reproductive systems. The common inference drawn from such studies is that the sex of the offspring is determined by the Y chromosome.
The evolution of sex-determining mechanisms, characterized by the evolutionary transition to genetic sex determination or temperature-dependent sex determination from the opposite mechanism, has frequently and readily occurred among multiple taxa across a transitionary continuum.
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