Editing Sex-determination system (section)

==Environmental systems==
{{main |Environmental sex determination}}

===Temperature-dependent===
{{Main |Temperature-dependent sex determination}}
[[File:Alligator.jpg |thumb|right |All alligators determine the sex of their offspring by the temperature of the nest.]]

Many other sex-determination systems exist. In some species of reptiles, including [[alligator]]s, some [[turtle]]s, and 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. [[Megapodes]] had formerly been thought to exhibit this phenomenon, but were found to actually have different temperature-dependent embryo mortality rates for each sex.<ref>{{cite journal | vauthors = Göth A, Booth DT | title = Temperature-dependent sex ratio in a bird | journal = Biology Letters | volume = 1 | issue = 1 | pages = 31–33 | date = March 2005 | pmid = 17148121 | pmc = 1629050 | doi = 10.1098/rsbl.2004.0247 }}</ref> For some species with TSD, sex determination is achieved by exposure to hotter temperatures resulting in the offspring being one sex and cooler temperatures resulting in the other. This type of TSD is called ''Pattern&nbsp;I''. For others species using TSD, it is exposure to temperatures on both extremes that results in offspring of one sex, and exposure to moderate temperatures that results in offspring of the opposite sex, called ''Pattern&nbsp;II'' TSD. The specific temperatures required to produce each sex are known as the female-promoting temperature and the male-promoting temperature.<ref name="Torres Maldonado-2002">{{cite journal | vauthors = Torres Maldonado LC, Landa Piedra A, Moreno Mendoza N, Marmolejo Valencia A, Meza Martínez A, Merchant Larios H | title = Expression profiles of Dax1, Dmrt1, and Sox9 during temperature sex determination in gonads of the sea turtle Lepidochelys olivacea | journal = General and Comparative Endocrinology | volume = 129 | issue = 1 | pages = 20–26 | date = October 2002 | pmid = 12409092 | doi = 10.1016/s0016-6480(02)00511-7 }}</ref> When the temperature stays near the threshold during the temperature sensitive period, the [[sex ratio]] is varied between the two sexes.<ref name="Bull-1980">{{cite journal | vauthors = Bull JJ |title=Sex Determination in Reptiles |journal=The Quarterly Review of Biology |date=March 1980 |volume=55 |issue=1 |pages=3–21 |jstor=2826077 |doi=10.1086/411613|s2cid=85177125 }}</ref> Some species' temperature standards are based on when a particular 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.<ref name="Torres Maldonado-2002"/> The sex of some species, such as the [[Nile tilapia]], [[skink|Australian skink lizard]], and [[Agamidae|Australian dragon lizard]], has an initial bias, set by chromosomes, but can later be changed by the temperature of incubation.<ref name="Schartl-2004a"/>

It is unknown how exactly temperature-dependent sex determination evolved.<ref name="Valenzuela-2001">{{cite journal |vauthors=Valenzuela N, Janzen FJ |title=Nest-site philopatry and the evolution of temperature-dependent sex determination |journal=Evolutionary Ecology Research |year=2001 |volume=3 |pages=779–794 |url=http://www.public.iastate.edu/~fjanzen/pdf/01EvolEcolRes.pdf |access-date=7 December 2011 |archive-date=4 May 2013 |archive-url=https://web.archive.org/web/20130504051408/http://www.public.iastate.edu/~fjanzen/pdf/01EvolEcolRes.pdf |url-status=live }}</ref> It could have evolved through certain sexes being more suited to certain areas that fit the temperature requirements. For example, a warmer area could be more suitable for nesting, so more females are produced to increase the amount that nest next season.<ref name="Valenzuela-2001"/> 
In amniotes, environmental sex determination preceded the genetically determined systems of birds and mammals; it is thought that a temperature-dependent [[amniote]] was the [[common ancestor]] of amniotes with sex chromosomes.<ref>{{cite journal | vauthors = Janzen FJ, Phillips PC | title = Exploring the evolution of environmental sex determination, especially in reptiles | journal = Journal of Evolutionary Biology | volume = 19 | issue = 6 | pages = 1775–1784 | date = November 2006 | pmid = 17040374 | doi = 10.1111/j.1420-9101.2006.01138.x | s2cid = 15485510 }}</ref>

===Other environmental systems===
There are other [[environmental sex determination]] systems including location-dependent determination systems as seen 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, [[Bonellia viridis#Bonnelin as a biocide|bonellin]].<ref name="Gilbert-2006">{{cite book | vauthors = Gilbert S |title=Developmental biology |url=https://archive.org/details/developmentalbio00gilb_292 |url-access=limited |date=2006 |publisher=Sinauer Associates, Inc. Publishers |location=Sunderland, Mass. |isbn=9780878932504 |pages=[https://archive.org/details/developmentalbio00gilb_292/page/n569 550]–553 |edition= 8th.}}</ref> Some species, such as some [[snail]]s, practice [[Sequential hermaphroditism|sex change]]: adults start out male, then become female. In tropical [[clownfish]], the dominant individual in a group becomes female while the other ones are male, and bluehead wrasses (''[[Thalassoma bifasciatum]]'') are the reverse. 

[[File:ClownFishCycle.jpg|thumb|Life cycle of clownfish]]

Clownfish live in colonies of several small undifferentiated fish and two large fish (male and female). The male and female are the only sexually mature fish to reproduce. Clownfish are protandrous hermaphrodites, which means after they mature into males, they eventually can transform into females. They develop undifferentiated until they are needed to fill a certain role in their environment, i.e., if they receive the social and environmental cues to do so. <ref>Casas, L., Saborido-Rey, F., Ryu, T., Michell, C., Ravasi, T., & Irigoien, X. (2016). Sex Change in Clownfish: Molecular Insights from Transcriptome Analysis. Scientific Reports, 6(1). https://doi.org/10.1038/srep35461</ref>

Some species, however, have no sex-determination system. Hermaphrodite species 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 both sexually and asexually, depending on whether a mate is available.<ref name="Watts-2006">{{cite journal | vauthors = Watts PC, Buley KR, Sanderson S, Boardman W, Ciofi C, Gibson R | title = Parthenogenesis in Komodo dragons | journal = Nature | volume = 444 | issue = 7122 | pages = 1021–1022 | date = December 2006 | pmid = 17183308 | doi = 10.1038/4441021a | name-list-style = amp | s2cid = 4311088 | bibcode = 2006Natur.444.1021W }}</ref>