Sex chromosome: a chromosome involved with determining the sex of an individual. Insects are a tremendously successful group that accounts for a great majority of animal species Mora et al. Additionally, the X chromosome is transcriptionally inactivated during spermatogenesis in many species [meiotic sex chromosome inactivation MSCI ], and it has been suggested that MSCI evolved either in response to sexually antagonistic selection, driving off spermatogenesis genes from the female-biased Xor to prevent the recurrent emergence of meiotic drive elements on the X chromosome This indicates that sex chromosome turnovers can take place recurrently in some lineages.
Second, some taxa may have few genes under sexually antagonistic selection on their sex chromosomes and thus avoid selection to suppress recombination between the X and Y . The formation of palindromes is not rare in genomes, and it occurs when part of a duplicated chromosome inverts during recombination.
Evolution of alternative sex-determining mechanisms in teleost fishes. The availability of sequence data, and of more sophisticated molecular and cytological techniques, will revolutionize comparative studies of sex chromosomes and sex-determining genes. From the cytogenetic standpoint, seven distinct karyotype forms or karyomorphs A—G in H.
The genetic architecture suggests that either the Y chromosome has an X-inactivating gene, or that the Y h chromosome has an X-activating gene. Morphologically distinguishable sex chromosomes are relatively rare [ 1112 ], but studying a wider variety of species with more sophisticated cytogenetic tools, such as comparative genome hybridization, may reveal cryptic morphological differences heteromorphy between sex chromosomes [ 1314 ].
Conflicts of Interest The authors declare no conflict of interest.
Kikuchi K. Y fuse? Bitencourt J. Balancing the bipotential gonad between alternative organ fates: a new perspective on an old problem. Evidence for different origins of sex chromosomes in closely related Oryzias fishes: substitution of the master sex-determining gene. A single gene - SRY - sets off the mammalian testis-determining cascade.
The conservation of a large Z chromosome in all families, but morphological differences among the W chromosomes homologous to the Z in boids such as pythons and boas; rearranged and partly heterochromatic in colubrids, the majority of snakes, and a heterochromatic rump in the evolutionarily advanced vipers led Ohno [ 50 ] to propose that snake sex chromosomes differentiated from an original autosome pair as a sex-specific W became genetically isolated and degenerated.
Expansion of microsatellites on evolutionary young Y chromosome.
There are more than 20 known SOX genes in mammals and, although they have functions related to various aspects of embryogenesis, most of them are not involved in sex determination Prior and Walter, ; Pevny and Lovell-Badge, Evolution of the Sry genes.
Additional information Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Transitions to female heterogamety from an ancestral state of male heterogamety might be difficult to achieve, as they result in offspring that are homozygous for the Y chromosome Bull ; Bachtrog et al.
The evolutionary dynamics of sex determination.