Sex-determination system
[citation needed] Such analysis of biological systems could also signal whether the fetus is hermaphrodite, which includes total or partial of both male and female reproduction organs.
[4][5][6] In 1694, J.R. Camerarius, conducted early experiments on pollination and reported the existence of male and female characteristics in plants(Maize).
Among animals, the most common chromosomal sex determination systems are XY, XO, ZW, ZO, but with numerous exceptions.
In some species, such as humans, organisms remain sex indifferent for a time during development (embryogenesis); in others, however, such as fruit flies, sexual differentiation occurs as soon as the egg is fertilized.
Once the SRY gene is activated, cells create testosterone and anti-müllerian hormone which typically ensures the development of a single, male reproductive system.
[9] In typical XX embryos, cells secrete estrogen, which drives the body toward the female pathway.
In XY mice, lack of the gene DAX1 on the X chromosome results in sterility, but in humans it causes adrenal hypoplasia congenita.
[11] Even when there are normal sex chromosomes in XX females, duplication or expression of SOX9 causes testes to develop.
[22] Also, while mammals deactivate one of their extra X chromosomes when female, it appears that in the case of Lepidoptera, the males produce double the normal amount of enzymes, due to having two Z's.
[33][34] The mating type in microorganisms is analogous to sex in multi-cellular organisms, and is sometimes described using those terms, though they are not necessarily correlated with physical body structures.
Unfertilized eggs develop into haploid individuals which have a single, hemizygous copy of the csd locus and are therefore males.
Fertilized eggs develop into diploid individuals which, due to high variability in the csd locus, are generally heterozygous females.
[36][37][38] Most females in the Hymenoptera order can decide the sex of their offspring by holding received sperm in their spermatheca and either releasing it into their oviduct or not.
They then develop into male or female adults, with the determination based on a complex interaction genes on multiple chromosomes, but not affected by environmental variations.
Homomorphic sex chromosomes exist among pufferfish, ratite birds, pythons, and European tree frogs.
This is hypothesized to be because the X inactivation means any change would cause serious disruption, thus subjecting it to strong purifying selection.
[67] It is an evolutionary puzzle why certain sex chromosomes remain homomorphic over millions of years, especially among lineages of fishes, amphibians, and nonavian reptiles.
While in humans, sex reversal (such as the XX male syndrome) are often infertile, sex-reversed individuals of some species are fertile under some conditions.
For example, some XY-individuals in population of Chinook salmon in the Columbia River became fertile females, producing YY sons.
In some species of reptiles, including alligators, some turtles, and the tuatara, sex is determined by the temperature at which the egg is incubated during a temperature-sensitive period.
Megapodes had formerly been thought to exhibit this phenomenon, but were found to actually have different temperature-dependent embryo mortality rates for each sex.
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.
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.
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.
The Wolbachia genus of parasitic bacteria lives inside the cytoplasm of its host, and is vertically transmitted from parents to children.
[80] Mitochondrial male sterility: In many flowering plants, the mitochondria can cause hermaphrodite individuals to be unable to father offsprings, effectively turning them into exclusive females.
[80] The first published case of mitochondrial male sterility among metazoan was reported in 2022 in the hermaphroditic snail Physa acuta.
[83] The accepted hypothesis of XY and ZW sex chromosome evolution in amniotes is that they evolved at the same time, in two different branches.
These two groups both evolved the ZW system separately, as evidenced by the existence of different sex chromosomal locations.
[16] Non-inverted Y chromosomes with long histories are found in pythons and emus, each system being more than 120 million years old, suggesting that inversions are not necessarily an eventuality.
