Sex

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Sex is the

ova, often called egg cells) are called female.[4] An organism that produces both types of gamete is hermaphrodite.[3][5]

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 (

identified through observation of that individual's sexual characteristics. Sexual selection or mate choice
can accelerate the evolution of differences between the sexes.

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

green alga Ulva lactuca. Some kinds of functional differences between individuals, such as in fungi,[7] may be referred to as mating types.[8]

Sexual reproduction

The life cycle of a sexually reproducing species cycles through haploid and diploid stages

Sexual reproduction, in which two individuals produce an offspring that possesses a selection of the genetic traits of each parent, is exclusive to

fertilization, bringing the two single sets of chromosomes together to make a new diploid zygote
, results in a new organism that contains a different set of the genetic traits of each parent.

In

motile gamete combines with a much larger, non-motile gamete.[12]

In anisogamic organisms, by convention, the larger gamete (called an

self-fertilize
and produce an offspring on its own.

Animals

Hoverflies mating

Most sexually reproducing animals spend their lives as diploid, with the haploid stage reduced to single-cell gametes.

embryos
which develop into new organisms.

The male gamete, a

fertilization
.

Female gametes are egg cells. In vertebrates, they are produced within the

egg
. In mammals, the fertilized embryo instead develops within the female, receiving nutrition directly from its mother.

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,

animal sexual behavior can involve coercive sex. Traumatic insemination
, for example, is used by some insect species to inseminate females through a wound in the abdominal cavity—a process detrimental to the female's health.

Plants

Flowers contain the sexual organs of flowering plants. They are usually hermaphrodite, containing both male and female parts.

Like animals,

gametophytes that are contained within pollen which have hard coats that protect the male gamete forming cells during transport from the anthers to the stigma. The female gametes of seed plants are contained within ovules. Once fertilized, these form seeds
which, like eggs, contain the nutrients necessary for the initial development of the embryonic plant.

Female (left) and male (right) cones contain the sex organs of pines and other conifers.

The

ovules which develop into seeds after fertilization. The male parts of the flower are the stamens: these consist of long filaments arranged between the pistil and the petals that produce pollen in anthers at their tips. When a pollen grain lands upon the stigma on top of a carpel's style, it germinates to produce a pollen tube
that grows down through the tissues of the style into the carpel, where it delivers male gamete nuclei to fertilize an ovule that eventually develops into a seed.

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

seed plants are immobile, they depend upon passive methods for transporting pollen grains to other plants. Many, including conifers and grasses, produce lightweight pollen which is carried by wind to neighboring plants. Some flowering plants have heavier, sticky pollen that is specialized for transportation by insects or larger animals such as hummingbirds and bats, which may be attracted to flowers containing rewards of nectar and pollen. These animals transport the pollen as they move to other flowers, which also contain female reproductive organs, resulting in pollination
.

Fungi

Mushrooms are produced as part of fungal sexual reproduction.

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

gonochoric. About 5% of animal species are hermaphroditic.[26] This low percentage is partially attributable to the very large number of insect species, in which hermaphroditism is absent.[27] About 99% of vertebrates are gonochoric, and the remaining 1% that are hermaphroditic are almost all fishes.[28]

Plants

The majority of plants are

conifers are monoecious.[34]

Evolution of sex

motile cells
, B) isogamy of non-motile cells, C) conjugation.

It is generally accepted that

evolved several times independently in different groups of eukaryotes, including protists, algae, plants, and animals.[27] The evolution of anisogamy is synonymous with the origin of male and the origin of female.[36] It is also the first step towards sexual dimorphism[37] and influenced the evolution of various sex differences.[38]

However, the evolution of anisogamy has left no fossil evidence

: 213 

But a 1.2 billion year old fossil from

Bangiomorpha pubescens has provided the oldest fossil record for the differentiation of male and female reproductive types and shown that sexes evolved early in eukaryotes.[41]

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]

Sex-determination systems

Sex helps the spread of advantageous traits through recombination. The diagrams compare the evolution of allele frequency in a sexual population (top) and an asexual population (bottom). The vertical axis shows frequency and the horizontal axis shows time. The alleles a/A and b/B occur at random. The advantageous alleles A and B, arising independently, can be rapidly combined by sexual reproduction into the most advantageous combination AB. Asexual reproduction takes longer to achieve this combination because it can only produce AB if A arises in an individual which already has B or vice versa.

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.[44] Non-genetic systems may use environmental cues, such as the temperature during early development in crocodiles, to determine the sex of the offspring.[45]

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.[46]

Genetic

XY sex determination

The common fruit fly has an XY sex-determination system, as do humans and most mammals.

Humans and most other

XXX are females.[6] Unusually, the platypus, a monotreme mammal, has ten sex chromosomes; females have ten X chromosomes, and males have five X chromosomes and five Y chromosomes. Platypus egg cells all have five X chromosomes, whereas sperm cells can either have five X chromosomes or five Y chromosomes.[48]

XY sex determination is found in other organisms, including insects like the common fruit fly,[49] and some plants.[50] 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.[51]

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.[52] 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.[53]

XO sex determination

In the

Paleoptera) and grasshoppers (Exopterygota), and some nematodes, crustaceans, and gastropods.[54][55]

In field crickets, for example, insects with a single X chromosome develop as male, while those with two develop as female.[56]

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).[57]

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.[58]

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.[59]

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.[60] The bonelliidae larvae can only develop as males when they encounter a female.[27]

Sequential hermaphroditism

Clownfishes are initially male; the largest fish in a group becomes female.

Some species can change sex over the course of their lifespan, a phenomenon called sequential hermaphroditism.[61]

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.[62]

Sequential hermaphroditism also occurs in plants such as Arisaema triphyllum.

Temperature-dependent sex determination

sex chromosomes.[63]
Instead, whether these eggs will produce male or female crocodiles depends on the temperature of the eggs.


Many

turtles, have temperature-dependent sex determination. In these species, the temperature experienced by the embryos during their development determines their sex.[27]

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.[64]

Haplodiploidy

Certain insects, such as honey bees and ants, use a haplodiploid sex-determination system.[65] 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.[66]

Sex ratio

A

parthenogenic species, periodically mating organisms such as aphids, some eusocial wasps, bees, ants, and termites.[69]

The
human sex ratio is of particular interest to anthropologists and demographers. In human societies, sex ratios at birth may be considerably skewed by factors such as the age of mother at birth[70] and by sex-selective abortion and infanticide. Exposure to pesticides and other environmental contaminants may be a significant contributing factor as well.[71] As of 2014, the global sex ratio at birth is estimated at 107 boys to 100 girls (1,000 boys per 934 girls).[72]

Sex differences

Anisogamy is the fundamental difference between male and female.[73][74] Richard Dawkins has stated that it is possible to interpret all the differences between the sexes as stemming from this.[75]

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.[76]

Sexual dimorphism

Common pheasants are sexually dimorphic in both size and appearance.
The common hill myna is sexually monomorphic, meaning that the external appearance of males and females is very similar.[77]

In many animals and some plants, individuals of male and female sex differ in size and appearance, a phenomenon called sexual dimorphism.[78] Sexual dimorphism in animals is often associated with sexual selection—the mating competition between individuals of one sex vis-à-vis the opposite sex.[76] In many cases, the male of a species is larger than the female. Mammal species with extreme sexual size dimorphism tend to have highly polygynous mating systems—presumably due to selection for success in competition with other males—such as the elephant seals. Other examples demonstrate that it is the preference of females that drives sexual dimorphism, such as in the case of the stalk-eyed fly.[79]

Females are the larger sex in a majority of animals.

southern black widow spiders are typically twice as long as the males.[80] This size disparity may be associated with the cost of producing egg cells, which requires more nutrition than producing sperm: larger females are able to produce more eggs.[81][78]

Sexual dimorphism can be extreme, with males, such as some

parasitically on the female. Some plant species also exhibit dimorphism in which the females are significantly larger than the males, such as in the moss genus Dicranum[82] and the liverwort genus Sphaerocarpos.[83] There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome,[83][84] or to chemical signalling from females.[85]

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.[86] 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.

Sexual characteristics

secondary sex characteristics
.

Sex differences in behavior

The sexes across gonochoric species usually differ in behavior. In most animal species females invest more in parental care,[87] although in some species, such as some coucals, the males invest more parental care.[88] Females also tend to be more choosy for who they mate with,[89] such as most bird species.[90] Males tend to be more competitive for mating than females.[36]

See also

References

  1. . Retrieved 23 March 2018. 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.
  2. . Retrieved 3 October 2023.
  3. ^ . Retrieved 23 March 2018. 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.
  4. . 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.
  5. . Retrieved 18 September 2020.
  6. ^ a b c d Hake L, O'Connor C. "Genetic Mechanisms of Sex Determination | Learn Science at Scitable". www.nature.com. Archived from the original on 19 August 2017. Retrieved 13 April 2021.
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  8. . Anisogamy can be defined as a mode of sexual reproduction in which fusing gametes, formed by participating parents, are dissimilar in size.
  9. ^ Alberts et al. (2002), "V. 20. Meiosis", U.S. NIH, V. 20. Meiosis Archived 25 January 2017 at the Wayback Machine
  10. ^ Alberts et al. (2002), U.S. National Institutes of Health, "V. 20. The Benefits of Sex Archived 22 May 2009 at the Wayback Machine".
  11. ^ Gilbert (2000), "1.2. Multicellularity: Evolution of Differentiation". 1.2.Mul Archived 8 March 2021 at the Wayback Machine, NIH.
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  13. ^ Gee, Henry (22 November 1999). "Size and the single sex cell". Nature. Archived from the original on 11 October 2017. Retrieved 4 June 2018.
  14. ^ Alberts et al. (2002), "3. Mendelian genetics in eukaryotic life cycles", U.S. NIH, 3. Mendelian/eukaryotic Archived 2 April 2017 at the Wayback Machine.
  15. ^ Alberts et al. (2002), "V.20. Sperm", U.S. NIH, V.20. Sperm Archived 29 June 2009 at the Wayback Machine.
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  17. ^ Alberts et al. (2002), "V.20. Fertilization", U.S. NIH, V.20. Fertilization Archived 19 December 2008 at the Wayback Machine.
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  71. ^ "CIA Fact Book". The Central Intelligence Agency of the United States. Archived from the original on 13 June 2007.
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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.
  73. .
  74. . 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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  76. . 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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