Sex
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Sex is the
In non-hermaphroditic species, the sex of an individual is determined through one of several biological sex-determination systems. Most mammalian species have the XY sex-determination system, where the male usually carries an X and a Y chromosome (XY), and the female usually carries two X chromosomes (XX). Other chromosomal sex-determination systems in animals include the ZW system in birds, and the XO system in insects. Various environmental systems include temperature-dependent sex determination in reptiles and crustaceans.[6]
The male and female of a species may be physically alike (sexual monomorphism) or have physical differences (
The terms male and female typically do not apply in sexually undifferentiated species in which the individuals are isomorphic (look the same) and the gametes are
Sexual reproduction
Sexual reproduction, in which two individuals produce an offspring that possesses a selection of the genetic traits of each parent, is exclusive to
In
In anisogamic organisms, by convention, the larger gamete (called an
Animals
Most sexually reproducing animals spend their lives as diploid, with the haploid stage reduced to single-cell gametes.
The male gamete, a
Female gametes are egg cells. In vertebrates, they are produced within the
Animals are usually mobile and seek out a partner of the opposite sex for mating. Animals which live in the water can mate using external fertilization, where the eggs and sperm are released into and combine within the surrounding water.[17] Most animals that live outside of water, however, use internal fertilization, transferring sperm directly into the female to prevent the gametes from drying up.
In most birds, both excretion and reproduction are done through a single posterior opening, called the cloaca—male and female birds touch cloaca to transfer sperm, a process called "cloacal kissing".[18] In many other terrestrial animals, males use specialized sex organs to assist the transport of sperm—these male sex organs are called intromittent organs. In humans and other mammals, this male organ is known as the penis, which enters the female reproductive tract (called the vagina) to achieve insemination—a process called sexual intercourse. The penis contains a tube through which semen (a fluid containing sperm) travels. In female mammals, the vagina connects with the uterus, an organ which directly supports the development of a fertilized embryo within (a process called gestation).
Because of their motility,
Plants
Like animals,
The
Some hermaphroditic plants are self-fertile, but plants have evolved multiple different self-incompatibility mechanisms to avoid self-fertilization, involving sequential hermaphroditism, molecular recognition systems and morphological mechanisms such as heterostyly.[22]: 73, 74
In pines and other conifers, the sex organs are produced within cones that have male and female forms. Male cones are smaller than female ones and produce pollen, which is transported by wind to land in female cones. The larger and longer-lived female cones are typically more durable, and contain ovules within them that develop into seeds after fertilization.
Because
Fungi
Most species of fungus can reproduce sexually and have life cycles with both haploid and diploid phases. These species of fungus are typically isogamous, i.e. lacking male and female specialization. One haploid fungus grows into contact with another, and then they fuse their cells. In some cases, the fusion is asymmetric, and the cell which donates only a nucleus (and no accompanying cellular material) could arguably be considered male.[23] Fungi may also have more complex allelic mating systems, with other sexes not accurately described as male, female, or hermaphroditic.[24]
Some fungi, including baker's yeast, have mating types that determine compatibility. Yeasts with the same mating types will not fuse with each other to form diploid cells, only with yeast carrying another mating type.[25]
Many species of higher fungi produce mushrooms as part of their sexual reproduction. Within the mushroom, diploid cells are formed, later dividing into haploid spores.
Sexual systems
A sexual system is a distribution of male and female functions across organisms in a species.[26]
Animals
Approximately 95% of
Plants
The majority of plants are
Evolution of sex
It is generally accepted that
However,
Studies on
The original form of sex was external fertilization. Internal fertilization, or sex as we know it, evolved later[42] and became dominant for vertebrates after their emergence on land.[43]
Adaptive function of sex
The most basic role of meiosis appears to be conservation of the integrity of the genome that is passed on to progeny by parents.[44][45] The two most fundamental aspects of sexual reproduction, meiotic recombination and outcrossing, are likely maintained respectively by the adaptive advantages of recombinational repair of genomic DNA damage and genetic complementation which masks the expression of deleterious recessive mutations.[46] Genetic variation, often produced as a byproduct of these processes, may provide long-term advantages in those sexual lineages that favor outcrossing.[46]
Sex-determination systems
The biological cause of an organism developing into one sex or the other is called sex determination. The cause may be genetic, environmental, haplodiploidy, or multiple factors.[27] Within animals and other organisms that have genetic sex-determination systems, the determining factor may be the presence of a sex chromosome. In plants that are sexually dimorphic, such as Ginkgo biloba,[22]: 203 the liverwort Marchantia polymorpha or the dioecious species in the flowering plant genus Silene, sex may also be determined by sex chromosomes.[47] Non-genetic systems may use environmental cues, such as the temperature during early development in crocodiles, to determine the sex of the offspring.[48]
Sex determination is often distinct from sex differentiation. Sex determination is the designation for the development stage towards either male or female while sex differentiation is the pathway towards the development of the phenotype.[49]
Genetic
XY sex determination
Humans and most other
XY sex determination is found in other organisms, including insects like the common fruit fly,[52] and some plants.[53] In some cases, it is the number of X chromosomes that determines sex rather than the presence of a Y chromosome.[6] In the fruit fly individuals with XY are male and individuals with XX are female; however, individuals with XXY or XXX can also be female, and individuals with X can be males.[54]
ZW sex determination
In birds, which have a ZW sex-determination system, the W chromosome carries factors responsible for female development, and default development is male.[55] In this case, ZZ individuals are male and ZW are female. It is the female gamete that determines the sex of the offspring. This system is used by birds, some fish, and some crustaceans.[6]
The majority of butterflies and moths also have a ZW sex-determination system. Females can have Z, ZZW, and even ZZWW.[56]
XO sex determination
In the
In field crickets, for example, insects with a single X chromosome develop as male, while those with two develop as female.[59]
In the nematode Caenorhabditis elegans, most worms are self-fertilizing hermaphrodites with an XX karyotype, but occasional abnormalities in chromosome inheritance can give rise to individuals with only one X chromosome—these XO individuals are fertile males (and half their offspring are male).[60]
ZO sex determination
In the ZO sex-determination system, males have two Z chromosomes whereas females have one. This system is found in several species of moths.[61]
Environmental
For many species, sex is not determined by inherited traits, but instead by environmental factors such as temperature experienced during development or later in life.[62]
In the fern Ceratopteris and other homosporous fern species, the default sex is hermaphrodite, but individuals which grow in soil that has previously supported hermaphrodites are influenced by the pheromone antheridiogen to develop as male.[63] The bonelliidae larvae can only develop as males when they encounter a female.[27]
Sequential hermaphroditism
Some species can change sex over the course of their lifespan, a phenomenon called sequential hermaphroditism.[64]
Teleost fishes are the only vertebrate lineage where sequential hermaphroditism occurs. In clownfish, smaller fish are male, and the dominant and largest fish in a group becomes female; when a dominant female is absent, then her partner changes sex from male to female. In many wrasses the opposite is true: the fish are initially female and become male when they reach a certain size.[65]
Sequential hermaphroditism also occurs in plants such as Arisaema triphyllum.
Temperature-dependent sex determination
Many
In some turtles, for example, males are produced at lower temperatures than females; but Macroclemys females are produced at temperatures lower than 22 °C or above 28 °C, while males are produced in between those temperatures.[67]
Haplodiploidy
Certain insects, such as honey bees and ants, use a haplodiploid sex-determination system.[68] Diploid bees and ants are generally female, and haploid individuals (which develop from unfertilized eggs) are male. This sex-determination system results in highly biased sex ratios, as the sex of offspring is determined by fertilization (arrhenotoky or pseudo-arrhenotoky resulting in males) rather than the assortment of chromosomes during meiosis.[69]
Sex ratio
A
Sex differences
Anisogamy is the fundamental difference between male and female.[76][77] Richard Dawkins has stated that it is possible to interpret all the differences between the sexes as stemming from this.[78]
Sexual characteristics
Sexual dimorphism
In many animals and some plants, individuals of male and female sex differ in size and appearance, a phenomenon called sexual dimorphism.[80] Sexual dimorphism in animals is often associated with sexual selection: the mating competition between individuals of one sex vis-à-vis the opposite sex.[81] Other examples demonstrate that it is the preference of females that drives sexual dimorphism, such as in the case of the stalk-eyed fly.[82]
Sex differences in humans include a generally larger size and more body hair in men, while women have larger breasts, wider hips, and a higher body fat percentage. In other species, there may be differences in coloration or other features, and may be so pronounced that the different sexes may be mistaken for two entirely different taxa.[81]
Females are the larger sex in a majority of animals.
Sexual dimorphism can be extreme, with males, such as some
In birds, males often have a more colorful appearance and may have features (like the long tail of male peacocks) that would seem to put them at a disadvantage (e.g. bright colors would seem to make a bird more visible to predators). One proposed explanation for this is the handicap principle.[89] This hypothesis argues that, by demonstrating he can survive with such handicaps, the male is advertising his genetic fitness to females—traits that will benefit daughters as well, who will not be encumbered with such handicaps.
Sex differences in behavior
The sexes across gonochoric species usually differ in behavior. In most animal species females invest more in parental care,[90] although in some species, such as some coucals, the males invest more parental care.[91] Females also tend to be more choosy for who they mate with,[92] such as most bird species.[93] Males tend to be more competitive for mating than females.[36]
See also
References
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Sex: Either of the two main categories (male and female) into which humans and most other living things are divided on the basis of their reproductive functions. The fact of belonging to one of these categories. The group of all members of either sex.
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A single body can function as both male and female. Sexual reproduction requires both male and female haploid gametes. In most species, these gametes are produced by individuals that are either male or female. Species that have male and female members are called dioecious (from the Greek for 'two houses'). In some species, a single individual may possess both female and male reproductive systems. Such species are called monoecious ("one house") or hermaphroditic.
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The answer is that there is an agreement by convention: individuals producing the smaller of the two gamete types – sperm or pollen – are males, and those producing larger gametes – eggs or ovules – are females.
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Anisogamy can be defined as a mode of sexual reproduction in which fusing gametes, formed by participating parents, are dissimilar in size.
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One thing biologists do agree on is that males and females count as different sexes. And they also agree that the main difference between the two is gamete size: males make lots of small gametes—sperm in animals, pollen in plants—and females produce a few big eggs.
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However, there is one fundamental feature of the sexes which can be used to label males as males, and females as females, throughout animals and plants. This is that the sex cells or 'gametes' of males are much smaller and more numerous than the gametes of females. This is true whether we are dealing with animals or plants. One group of individuals has large sex cells, and it is convenient to use the word female for them. The other group, which it is convenient to call male, has small sex cells. The difference is especially pronounced in reptiles and in birds, where a single egg cell is big enough and nutritious enough to feed a developing baby for. Even in humans, where the egg is microscopic, it is still many times larger than the sperm. As we shall see, it is possible to interpret all the other differences between the sexes as stemming from this one basic difference.
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Many species of birds are, however, monomorphic and difficult to sex visually, particularly in the field and some even in hand. Some examples are the Hill Mynah, Gracula religiosa and the Black-capped Chickadee, Parus atricapillus.
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Further reading
- Arnqvist G, Rowe L (2005). Sexual conflict. Princeton University Press. ISBN 978-0-691-12217-5.
- ISBN 978-0-8153-3218-3.
- Ellis H (1933). Psychology of Sex. London: W. Heinemann Medical Books. N.B.: One of many books by this pioneering authority on aspects of human sexuality.
- ISBN 978-0-87893-243-6.
- ISBN 978-0-521-29302-0.
External links
- Human Sexual Differentiation (Archived (2010)) – Geneva Foundation for Medical Education and Research (GFMER)
- Sexual Reproduction and the Evolution of Sex (Archived (2023)) − Nature journal (2008)