Sex-determination system: Encyclopedia - Sex-determination system

Sex-determination system

A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. In many cases, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology. In animals, this is often accompanied by chromosomal differences. In other cases, sex is determined by environmental variables (such as temperature) or social variables (the size of an organism relative to other members of its population). The details of some sex-determination systems are not yet fully understood.

Sex-determination system - Chromosomal determination

Sex-determination system - Mammals

The most familiar sex-determination system is the XY sex-determination system found in human beings and most other mammals. See the XY sex-determination system article for a detailed discussion of this system.

Sex-determination system - Other chromosomal systems

The WZ sex-determination system is found in birds and some insects and other organisms. In the WZ sex-determination system, the situation is reversed: females have two different kinds of chromosomes (WZ), and males have two of the same kind of chromosomes (ZZ).

Chromosomes in the WZ region in birds are autosomal in mammals, and vice-versa; therefore, it's theorized that the WZ and XY couples come from different chromosomes of the common ancestor.

Until recently, it was thought that the WZ system was unrelated to the XY system common to most mammals. A paper published in 2004 (Frank Grützner et al, Nature; doi:10.1038/nature03021) suggests that the two systems may be related. It would seem that platypuses have a ten-chromosome–based system, where the chromosomes form a multivalent chain in male meiosis, segregating into XXXXX-sperm and YYYYY-sperm, with XY-equivalent chromosomes at one end of this chain and the WZ-equivalent chromosomes at the other end.

It is unknown whether the presence of the W chromosome induces female features or the duplication of the Z chromosome induces male ones; Unlike mammals, no birds with a double W chromosome (WWZ) or a single Z (Z0) have been discovered. It's probable that either condition causes embryonic death, and both chromosomes are responsible for gender selection.

Haplodiploidy is found in Hymenoptera. Haploid individuals are male. Diploid individuals are generally female but may be sterile males. Thus, if a queen bee mates with one drone, her daughters share 3/4 of their genes with each other, not 1/2 as in the XY and WZ systems. This is believed to be significant for the development of eusociality, as it increases the significance of kin selection.

Sex-determination system - Non-genetic sex-determination systems

Many other exotic sex-determination systems exist. In some species of reptiles, including alligators, sex is determined by the temperature at which the egg is incubated. Other species, such as some snails, practice sex change: adults start out male, then become female.

Some species have no sex-determination system. Earthworms and some snails are hermaphrodites; a few species of lizard, fish, and insect are all female and reproduce by parthenogenesis.

In some arthropods, sex is determined by infection. Bacteria of the genus Wolbachia alter their sexuality; some species consist entirely of ZZ individuals, with sex determined by the presence of Wolbachia.

Other unusual systems [this section still being researched]:

• Swordtail fish?
• The Chironomus midge species
• etc

See also

• testis-determining factor
• Barr body
• Protandry
• Protogyny
• For humans:
  • Sex organ, or primary sexual characteristic
  • Secondary sex characteristic

Sex-determination system - Reference

• (2004) Evolution of Sex Chromosomes: The Case of the White Campion. PLoS Biol 3(1): e28.

Categories: Epigenetics | Sex

Adapted from the Wikipedia article "Sex-determination system", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki