A male (♂) male reproductive organs.
Not all species share a common sex-determination system. In most animals, including humans, sex is determined genetically, but in some species it can be determined due to social, environmental or other factors. For example, Cymothoa exigua changes sex depending on the number of females present in the vicinity.
- Overview 1
- Symbol 2
Sex determination 3
- Genetic determination 3.1
- Environmental determination 3.2
- Secondary sex characteristics 4
- See also 5
- References 6
The existence of two sexes seems to have been selected independently across different evolutionary lineages (see convergent evolution). The repeated pattern is sexual reproduction in isogamous species with two or more mating types with gametes of identical form and behavior (but different at the molecular level) to anisogamous species with gametes of male and female types to oogamous species in which the female gamete is very much larger than the male and has no ability to move. There is a good argument that this pattern was driven by the physical constraints on the mechanisms by which two gametes get together as required for sexual reproduction.
Accordingly, sex is defined operationally across species by the type of gametes produced (i.e.: spermatozoa vs. ova) and differences between males and females in one lineage are not always predictive of differences in another.
sporophytes that give rise to male and female plants.
A common symbol used to represent the male sex is the Mars symbol, ♂ (Unicode: U+2642 Alt codes: Alt+11)—a circle with an arrow pointing northeast. The symbol is identical to the planetary symbol of Mars. It was first used to denote sex by Carolus Linnaeus in 1751. The symbol is often called a stylized representation of the Roman god Mars' shield and spear. According to Stearn, however, all the historical evidence favours that it is derived from θρ, the contraction of the Greek name for the planet, Thouros.
The sex of a particular organism may be determined by a number of factors. These may be genetic or environmental, or may naturally change during the course of an organism's life. Although most species with male and female sexes have individuals that are either male or female, hermaphroditic animals, such as worms, have both male and female reproductive organs.
Most mammals, including humans, are genetically determined as such by the XY sex-determination system where males have an XY (as opposed to XX) sex chromosome. It is also possible in a variety of species, including human beings, to be XXY or have other intersex/hermaphroditic qualities. During reproduction, a male can give either an X sperm or a Y sperm, while a female can only give an X egg. A Y sperm and an X egg produce a male, while an X sperm and an X egg produce a female.
The part of the Y-chromosome which is responsible for maleness is the sex-determining region of the Y-chromosome, the SRY. The SRY activates Sox9, which forms feedforward loops with FGF9 and PGD2 in the gonads, allowing the levels of these genes to stay high enough in order to cause male development; for example, Fgf9 is responsible for development of the spermatic cords and the multiplication of Sertoli cells, both of which are crucial to male sexual development.
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. In tropical clown fish, the dominant individual in a group becomes female while the other ones are male.
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.
Secondary sex characteristics
In those species with two sexes, males may differ from females in ways other than production of spermatozoa. In many insects and fish the male is smaller than the female. In seed plants, which exhibit plumage that attracts females.
- Creighton, Jolene. "Meet The Sex-Changing, Tongue-Eating Parasite:". From Quarks to Quasars. Retrieved 7 April 2014.
- Dusenbery, David B. (2009). Living at Micro Scale, Chapter 20. Harvard University Press, Cambridge, Mass. ISBN 978-0-674-03116-6.
- The Origin of the Male and Female Symbols of Biology, William T. Stearn, Taxon, Vol. 11, No. 4 (May, 1962), pp. 109-113
- Moniot, Brigitte; Declosmenil, Faustine; Barrionuevo, Francisco; Scherer, Gerd; Aritake, Kosuke; Malki, Safia; Marzi, Laetitia; Cohen-Solal, Ann; Georg, Ina; Klattig, Jürgen; Englert, Christoph; Kim, Yuna; Capel, Blanche; Eguchi, Naomi; Urade, Yoshihiro; Boizet-Bonhoure, Brigitte; Poulat, Francis (2009). "The PGD2 pathway, independently of FGF9, amplifies SOX9 activity in Sertoli cells during male sexual differentiation". Development 136 (11): 1813–1821.
- Kim, Y.; Kobayashi, A.; Sekido, R.; Dinapoli, L.; Brennan, J.; Chaboissier, M. C.; Poulat, F.; Behringer, R. R.; Lovell-Badge, R.; Capel, B. (2006). "Fgf9 and Wnt4 Act as Antagonistic Signals to Regulate Mammalian Sex Determination". PLoS Biology 4 (6): e187.