Sex Determination System
A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most sexual organisms have two sexes. Occasionally there are hermaphrodites in place of one or both sexes. There are also some species that are only one sex due to parthenogenesis, the act of a female reproducing without fertilization. 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. Determination genetically is generally through chromosome combinations of XY, ZW, XO, ZO, or haplodiploid. Sexual differentiation is generally started by a main gene, a sex locus, then a multitude of other genes follow in a domino effect. 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 XX/XY sex-determination system is the most familiar, as it is found in humans. In the system, females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). Some species (including humans) have a gene SRY on the Y chromosome that determines maleness; others (such as the fruit fly) use the presence of two X chromosomes to determine femaleness. The XX/XY system is also found in most other mammals, as well as some insects. Some fish also have variants of this, as well as the regular system.
XX/X0 sex determination
In this variant of the XY system, females have two copies of the sex chromosome (XX) but males have only one (X0). The 0 denotes the absence of a second sex chromosome. Generally in this method, the sex is determined by amount of genes expressed across the two chromosomes. This system is observed in a number of insects, including the grasshoppers and crickets of order Orthoptera and in cockroaches (order Blattodea). A small number of mammals also lack a Y chromosome. These include the Amami spiny rat (Tokudaia osimensis) and the Tokunoshima spiny rat (Tokudaia tokunoshimensis) and Sorex araneus, a shrew specie. Voles also have a form of XO determination in which both genders lack a second sex chromosome. The mechanism of sex determination is not yet understood.
ZW/ZZ sex chromosomes
The ZW sex-determination system is found in birds, reptiles, some insects and other organisms. The ZW sex-determination system is reversed compared to the XY system: females have two different kinds of chromosomes (ZW), and males have two of the same kind of chromosomes (ZZ). In the chicken, this was found to be dependent on the expression of DMRT1. In birds, the genes FET1 and ASW are found on the W chromosome for females, similar to how the Y chromosome contains SRY.
Haplodiploidy is found in insects belonging to Hymenoptera, such as ants and bees. Unfertilized eggs develop into haploid individuals, which are the males. Diploid individuals are generally female but may be sterile males. Males cannot have sons or fathers. If a queen bee mates with one drone, her daughters share ¾ of their genes with each other, not ½ as in the XY and ZW systems. This is believed to be significant for the development of eusociality, as it increases the significance of kin selection, but it is debated.Most females in the Hymenoptera order can decide the sex of their offspring by holding received sperm in theirspermatheca and either releasing it into their oviduct or not. This allows them to create more workers, depending on the status of the colony.
Non-genetic determination systems
Temperature dependent sex determination
Many other sex-determination systems exist.
In some species of reptiles, including alligators, some turtles, the tuatara, sex is determined by the temperature at which the egg is incubated during a temperature sensitive period. There are no examples of temperature-dependent sex determination (TSD) in birds (including megapodes, which had formerly been thought to exhibit this phenomenon, but actually exhibit temperature-dependent embryo mortality). For some species with TSD, sex determination is achieved by hotter temperatures being one sex and cooler temperatures being the other. For others, the extreme temperatures are one sex and the middle temperature is the other.
The temperatures required for the specific sexes are known as the female promoting temperature and the male promoting temperature. When the temperature stays near the threshold during the temperature sensitive period, the sex ratio is varied between the two sexes. Some species set their temperature standards based on when an enzyme is created. These species that rely upon temperature for their sex determination do not have the SRY gene, but have other genes such as DAX1, DMRT1, and SOX9 that are expressed or not expressed depending on the temperature. Some species such as the Nile Tilapia, Australian skink lizard, and Australian dragon lizard have sex determined by chromosomes, but this can later be switched by the temperature of incubation. These species seem to be in a transitional state of evolution.
Other sex determination systems
While temperature dependent sex determination is relatively common, there are many other environmental systems. Some species, such as some snails, practice sex change: adults start out male, then become female. In the marine worm (Bonellia viridis), larvae become males if they make physical contact with a female, and females if they end up on the bare sea floor. This is triggered by the presence of a chemical produced by the females, bonellin.
Some species, however, have no sex-determination system. Hermaphrodites include the common earthworm and certain species of snails. A few species of fish, reptiles, and insects reproduce by parthenogenesis and are female altogether. There are some reptiles, such as the boa constrictor and komodo dragon that can reproduce sexually and asexually, depending if a mate is available. In the case of the boa constrictor, it can create both male and female offspring parthenogenetically.
In some arthropods, sex is determined by infection, as when 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:
- The Chironomus midge species
- The Platypus has 10 sex chromosomes but lacks the mammalian sex-determining gene SRY, meaning that the process of sex determination in the Platypus remains unknown.
- Zebrafish go through juvenile hermaphroditism, but what triggers this is unknown.
- The Platyfish has W, X, and Y chromosomes. This allows WY, WX, or XX females and YY or XY males.