Sex chromosomes (also referred to as allosomes, heterotypical chromosome, gonosomes, heterochromosomes, or idiochromosomes) 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.
Nettie Stevens and Edmund Beecher Wilson both independently discovered sex chromosomes in 1905. However, Stevens is credited for discovering them earlier than Wilson.
Nomenclature and glossary By convention, when sex-determination is karyotypically determined (i.e. when there are sex chromosomes), the homogametic sex chromosome is called "X" if two copies of it (heterogamety) leads to a female phenotype, and "Z" if two copies of it leads to a male phenotype. If the other sex has a different, sex-specific chromosome (heterogametic), this other sex chromosome is called "Y" or "W" respectively. If there is none, it is called "O" or "0". The naming of the sex chromosomes is purely based on its effect on an organism's sex; different chromosomes of the same name are not necessarily evolutionarily related as sex chromosomes have independently arisen in different lineages of organisms. Additionally, if the female and male individuals do not share any sex chromosome, the female one is named "U" and the male one "V".
In humans and most mammals, the Y chromosome is visibly smaller in size than the X chromosome, a condition known as heteromorphic sex chromosomes. Likewise in birds, the W chromosome is often visibly smaller than the Z chromosome. In contrast, it is nearly impossible to visually differentiate between the sex chromosomes of amphibians and some reptiles; as a result, these sex chromosome pairs are said to be homomorphic.
Differentiation
In humans, each cell nucleus contains 23 pairs of chromosomes, a total of 46 chromosomes. The first 22 pairs are called autosomes. Autosomes are homologous chromosomes i.e. chromosomes which contain the same genes (regions of DNA) in the same order along their chromosomal arms. The 23rd pair of chromosomes are called allosomes. These consist of two X chromosomes in females, and an X chromosome and a Y chromosome in males. Females therefore have 23 homologous chromosome pairs, while males have 22. The X and Y chromosomes have small regions of homology called pseudoautosomal regions.
An X chromosome is always present as the 23rd chromosome in the ovum, while either an X or Y chromosome may be present in an individual sperm. Early in female embryonic development, in cells other than egg cells, one of the X chromosomes is randomly and permanently partially deactivated: In some cells, the X chromosome inherited from the mother deactivates; in other cells, it is the X chromosome inherited from the father. This ensures that both sexes always have exactly one functional copy of an X chromosome in each body cell. The deactivated X chromosome is silenced by repressive heterochromatin that compacts the DNA and prevents expression of most genes. This compaction is regulated by PRC2 (Polycomb Repressive Complex 2).
Sex determination Animals Mammals
All diploid organisms with allosome-determined sex get half of their allosomes from each of their parents. In most mammals, females are XX, and can pass along either of their Xs; since males are XY they can pass along either an X or a Y. Females in such species receive an X chromosome from each parent while males receive an X chromosome from their mother and a Y chromosome from their father. It is thus the male's sperm that determines the sex of each offspring in such species.
A small percentage of humans have divergent sexual development, known as intersex. This can result from a genotype that is neither XX nor XY. It can also occur when two fertilized embryo fuse, producing a chimera that might contain two different sets of DNA, one XX and the other XY. It could also result from exposure, often in utero, to chemicals that disrupt the normal influence of the allosomes on production of sex hormones, and lead to the development of either ambiguous outer genitalia or internal organs.
There is a gene in the Y chromosome that has regulatory sequences that control genes that code for maleness, called the SRY gene.Graves, Jennifer A. Marshall (2015) Twenty-five years of the sex-determining gene , Nature 528, 343–344. https://doi.org/10.1038/528343a Retrieved 24 Jan 2022. This gene produces a testis-determining factor ("TDF"), which initiates testis development in humans and other mammals. The SRY sequence's prominence in sex determination was discovered when the genetics of sex-reversed XX men (i.e. humans who possess biological male-traits but actually have XX allosomes) were studied. After examination, it was discovered that the difference between a typical XX individual (traditional female) and a sex-reversed XX man was that the typical individuals lacked the SRY gene. It is theorized that in sex-reversed XX men, the SRY gets translocated to an X chromosome in the XX pair during meiosis.
Other vertebrates Diverse mechanisms are involved in the determination of sex in animals. For mammals, sex determination is carried by the genetic contribution of the spermatozoon. Many lower chordates, such as fish, amphibians and reptiles, have systems that are influenced by the environment. Fish and amphibians, for example, have genetic sex determination but their sex can also be influenced by externally available steroids and incubation temperature of eggs. In some reptiles, e.g. sea turtles, only the incubation temperature determines sex (temperature-dependent sex determination).
Genetic sex determination has independently evolved in many groups of vertebrates. Some species in the early stage of transition exhibit a hybrid genetic/temperature sex determination system.
Plants Many scientists argue that sex determination in plants is more complex than that in humans. This is because even flowering plants have a variety of mating systems, their sex determination primarily regulated by MADS-box genes. These genes code for proteins that form the sex organs in flowers.
Plant sex chromosomes are most common in bryophytes, relatively common in vascular plants and unknown in ferns and lycophytes. The diversity of plants is reflected in their sex-determination systems, which include XY and UV systems as well as many variants. Sex chromosomes have evolved independently across many plant groups. Recombination of chromosomes may lead to heterogamety before the development of sex chromosomes, or recombination may be reduced after sex chromosomes develop. Only a few pseudoautosomal regions normally remain once sex chromosomes are fully differentiated. When chromosomes do not recombine, neutral sequence divergences begin to accumulate, which has been used to estimate the age of sex chromosomes in various plant lineages. Even the oldest estimated divergence, in the liverwort Marchantia polymorpha, is more recent than mammal or bird divergence. Due to this recency, most plant sex chromosomes also have relatively small sex-linked regions. Current evidence does not support the existence of plant sex chromosomes more ancient than those of M. polymorpha.
The high prevalence of autopolyploidy in plants also impacts the structure of their sex chromosomes. Polyploidization can occur before and after the development of sex chromosomes. If it occurs after sex chromosomes are established, dosage should stay consistent between the sex chromosomes and autosomes, with minimal impact on sex differentiation. If it occurs before sex chromosomes become heteromorphic, as is likely in the octoploid red sorrel Rumex acetosella, sex is determined in a single XY system. In a more complicated system, the sandalwood species Viscum fischeri has X1X1X2X2 chromosomes in females, and X1X2Y chromosomes in males.
Sequence composition and evolution Amplification of transposable elements, tandem repeat, and retrotransposons are responsible for plant sex chromosome evolution. The insertion of retrotransposons is probably the major cause of y-chromosome expansion and plant genome size evolution. Retrotransposones contribute in size determination of sex chromosomes and its proliferation varies even in closely related species. LTR and tandom repeats play dominant role in the evolution of S. latifolia sex chromosomes.
Athila is new family of retroelements, discovered in Arabidopsis thaliana, present in heterochromatin region only. Athila retroelements overrepresented in X but absent in Y while tandem repeats enriched in Y-chromosome. Some chloroplast sequences have also been identified in the Y-chromosome of S. latifolia. S. vulgaris has more retroelements in their sex chromosomes compare to S. latifolia. Microsatellite data shows that there is no significant difference between X and Y-chromosome microsatellites in both Silene species. This would conclude that microsatellites do not participate in Y-chromosome evolution. The portion of Y-chromosome that never recombine with X-chromosome faces selection reduction. This reduced selection leads to insertion of transposable elements and accumulation of deleterious mutation. The Y become larger and smaller than X due to insertion of retroelement and deletion of genetic material respectively.
The genus Humulus is also used as model for the study of sex chromosomes evolution. Based on the phylogenetic topology distribution there are three regions on sex chromosomes. One region that has stopped recombining in the ancestor of H. lupulus, another that has stopped recombining in modern H. lupulus, and the third region called pseudoautosomal region. H. lupulus is the rare case in plants in which Y is smaller than X, while its ancestor plant has the same size of both X and Y chromosomes. This size difference should be caused by deletion of genetic material in Y but that is not the case. This is because of complex dynamics like the larger size of X than Y-chromosome may be due to duplication or retrotransposition and size of Y remains same.
Non-vascular plants
Ferns and lycophytes have bisexual gametophytes, so there is no evidence for sex chromosomes. In the bryophytes, including liverworts, hornworts and mosses, sex chromosomes are common. The sex chromosomes in bryophytes affect what type of gamete is produced by the gametophyte, and there is wide diversity in gametophyte type. Unlike seed plants, where gametophytes are always unisexual, in bryophytes they may produce male, female, or both types of gamete.
Bryophytes most commonly employ a U/V sex-determination system, where U produces female gametophytes and V produces male gametophytes. The U and V chromosomes are heteromorphic with U larger than V and are frequently both larger than the autosomes. There is variation even within this system, including UU/V and U/VV chromosome arrangements. In some bryophytes, microchromosomes have been found to co-occur with sex chromosomes and likely impact sex determination.
Gymnosperms
Dioecy is common among gymnosperms, found in an estimated 36% of species. However, heteromorphic sex chromosomes are relatively rare, with only five species known as of 2014. Five of these use an XY system, and one (Ginkgo biloba) uses a WZ system. Some gymnosperms, such as Johann's Pine (Pinus johannis), have homomorphic sex chromosomes that are almost indistinguishable through karyotyping.
Angiosperms