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This article is about sex in sexually reproducing organisms. For the act, see Sexual intercourse. For other uses, see Sex (disambiguation).
Sex is the attribute of male or female in organisms that propagate the species through sexual reproduction.[1][2] Many plants and almost all animals employ sexual reproduction. Animals are usually mobile and seek out a partner of the opposite sex for mating. Animals which live in water can mate using external fertilization, whereas most land-based animals, such as reptiles, birds, and mammals, including humans, use internal fertilization. Plants are generally immobile, and in seed plants sexual reproduction relies on pollination, either through self-pollination, or via cross-pollination with other plants of the same species.
Sexual reproduction in animals and cross-pollination in seed plants involve the combining and mixing of genetic traits: specialized cells known as gametes combine to form offspring that inherit a selection of the traits of each parent. The gametes produced by an organism define its sex: male organisms produce small gametes (e.g. spermatozoa, or sperm, in animals) while female organisms produce large gametes (ova, or egg cells; ovules in plants). Individual organisms which produce both male and female gametes are termed hermaphroditic.[2][3]
The different sexes of a species may have physical differences, referred to as sexual dimorphism.[4] The differences reflect the different reproductive pressures the sexes experience. For instance, mate choice and sexual selection can accelerate the evolution of physical differences between the sexes.
In mammals, males typically carry an X and a Y chromosome (XY), whereas females typically carry two X chromosomes (XX), which are a part of the XY sex-determination system. Other animals have various sex-determination systems, such as the ZW system in birds, the X0 system in insects, and various environmental systems, for example in reptiles and crustaceans. Fungi may also have more complex allelic mating systems, with sexes not accurately described as male, female, or hermaphroditic[clarify].[5]
One of the basic properties of life is reproduction, the capacity to generate new individuals, and sex is an aspect of this process. Life has evolved from simple stages to more complex ones, and so have the reproduction mechanisms. Initially the reproduction was a replicating process that consists in producing new individuals that contain the same genetic information as the original or parent individual. This mode of reproduction is called asexual, and it is still used by many species, particularly unicellular, but it is also very common in multicellular organisms, including many of those with sexual reproduction.[6] In sexual reproduction, the genetic material of the offspring comes from two different individuals. Bacteria reproduce asexually, but undergo a process by which a part of the genetic material of an individual donor is transferred to another recipient.[7]
Disregarding intermediates, the basic distinction between asexual and sexual reproduction is the way in which the genetic material is processed. Typically, prior to an asexual division, a cell duplicates its genetic information content, and then divides. This process of cell division is called mitosis. In sexual reproduction, there are special kinds of cells that divide without prior duplication of its genetic material, in a process named meiosis. The resulting cells are called gametes, and contain only half the genetic material of the parent cells. These gametes are the cells that are prepared for the sexual reproduction of the organism.[8] Sex comprises the arrangements that enable sexual reproduction, and has evolved alongside the reproduction system, starting with similar gametes (isogamy) and progressing to systems that have different gamete types, such as those involving a large female gamete (ovum) and a small male gamete (sperm).[9]
In complex organisms, the sex organs are the parts that are involved in the production and exchange of gametes in sexual reproduction. Many species, both plants and animals, have sexual specialization, and their populations are divided into male and female individuals. Conversely, there are also species in which there is no sexual specialization, and the same individuals both contain masculine and feminine reproductive organs, and they are called hermaphrodites. This is very frequent in plants.[10]
Different forms of anisogamy:
A) anisogamy of motile cells, B) oogamy (egg cell and sperm cell), C) anisogamy of non-motile cells (egg cell and spermatia).
Different forms of isogamy:
A) isogamy of motile cells, B) isogamy of non-motile cells, C) conjugation.
Sexual reproduction first probably evolved about a billion years ago within ancestral single-celled eukaryotes.[11] The reason for the evolution of sex, and the reason(s) it has survived to the present, are still matters of debate. Some of the many plausible theories include: that sex creates variation among offspring, sex helps in the spread of advantageous traits, that sex helps in the removal of disadvantageous traits, and that sex facilitates repair of germ-line DNA.
Sexual reproduction is a process specific to eukaryotes, organisms whose cells contain a nucleus and mitochondria. In addition to animals, plants, and fungi, other eukaryotes (e.g. the malaria parasite) also engage in sexual reproduction. Some bacteria use conjugation to transfer genetic material between cells; while not the same as sexual reproduction, this also results in the mixture of genetic traits.
The defining characteristic of sexual reproduction in eukaryotes is the difference between the gametes and the binary nature of fertilization. Multiplicity of gamete types within a species would still be considered a form of sexual reproduction. However, no third gamete type is known in multicellular plants or animals.[12][13][14]
While the evolution of sex dates to the prokaryote or early eukaryote stage,[15] the origin of chromosomal sex determination may have been fairly early in eukaryotes. In animals there are four systems of sex determination, which depend on a special chromosome.
No genes are shared between the avian ZW and mammal XY chromosomes,[22] and from a comparison between chicken and human, the Z chromosome appeared similar to the autosomal chromosome 9 in human, rather than X or Y, suggesting that the ZW and XY sex-determination systems do not share an origin, but that the sex chromosomes are derived from autosomal chromosomes of the common ancestor of birds and mammals. A paper from 2004 compared the chicken Z chromosome with platypus X chromosomes and suggested that the two systems are related.[23]
Further information: Isogamy and Anisogamy
Sexual reproduction in eukaryotes is a process whereby organisms produce offspring that combine genetic traits from both parents. Chromosomes are passed on from one generation to the next in this process. Each cell in the offspring has half the chromosomes of the mother and half of the father.[24] Genetic traits are contained within the deoxyribonucleic acid (DNA) of chromosomes—by combining one of each type of chromosomes from each parent, an organism is formed containing a doubled set of chromosomes. This double-chromosome stage is called "diploid", while the single-chromosome stage is "haploid". Diploid organisms can, in turn, form haploid cells (gametes) that randomly contain one of each of the chromosome pairs, via meiosis.[25] Meiosis also involves a stage of chromosomal crossover, in which regions of DNA are exchanged between matched types of chromosomes, to form a new pair of mixed chromosomes. Crossing over and fertilization (the recombining of single sets of chromosomes to make a new diploid) result in the new organism containing a different set of genetic traits from either parent.
In many organisms, the haploid stage has been reduced to just gametes specialized to recombine and form a new diploid organism. In plants the diploid organism produces haploid spores that undergo cell division to produce multicellular haploid organisms known as gametophytes that produce haploid gametes at maturity. In either case, gametes may be externally similar, particularly in size (isogamy), or may have evolved an asymmetry such that the gametes are different in size and other aspects (anisogamy).[26][27] By convention, the larger gamete (called an ovum, or egg cell) is considered female, while the smaller gamete (called a spermatozoon, or sperm cell) is considered male. An individual that produces exclusively large gametes is female, and one that produces exclusively small gametes is male.[28][29] An individual that produces both types of gametes is a hermaphrodite; in some cases hermaphrodites are able to self-fertilize and produce offspring on their own, without a second organism.[30][3][31]
Most sexually reproducing animals spend their lives as diploid, with the haploid stage reduced to single-cell gametes.[32] The gametes of animals have male and female forms—spermatozoa and egg cells. These gametes combine to form embryos which develop into a new organism.
The male gamete, a spermatozoon (produced in vertebrates within the testes), is a small cell containing a single long flagellum which propels it.[33] Spermatozoa are extremely reduced cells, lacking many cellular components that would be necessary for embryonic development. They are specialized for motility, seeking out an egg cell and fusing with it in a process called fertilization.
Female gametes are egg cells (produced in vertebrates within the ovaries), large immobile cells that contain the nutrients and cellular components necessary for a developing embryo.[34] Egg cells are often associated with other cells which support the development of the embryo, forming an 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.[35] 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 is done through a single posterior opening, called the cloaca—male and female birds touch cloaca to transfer sperm, a process called "cloacal kissing".[36] 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 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.
Like animals, plants have specialized male and female gametes.[37] Within seed plants, male gametes are produced by extremely reduced multicellular gametophytes known as pollen. The female gametes of seed plants are contained within ovules; once fertilized by male gametes produced by pollen these form seeds which, like eggs, contain the nutrients necessary for the development of the embryonic plant.
Female (left) and male (right) cones are the sex organs of pines and other conifers.
Many plants have flowers and these are the sexual organs of those plants. Flowers are usually hermaphroditic, producing both male and female gametes. The female parts, in the center of a flower, are the pistils, each unit consisting of a carpel, a style and a stigma. One or more of these reproductive units may be merged to form a single compound pistil. Within the carpels are 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.
In pines and other conifers the sex organs are conifer cones and have male and female forms. The more familiar female cones are typically more durable, containing ovules within them. Male cones are smaller and produce pollen which is transported by wind to land in female cones. As with flowers, seeds form within the female cone after pollination.
Because plants are immobile, they depend upon passive methods for transporting pollen grains to other plants. Many plants, including conifers and grasses, produce lightweight pollen which is carried by wind to neighboring plants. Other plants have heavier, sticky pollen that is specialized for transportation by animals. The plants attract these insects or larger animals such as humming birds and bats with nectar-containing flowers. These animals transport the pollen as they move to other flowers, which also contain female reproductive organs, resulting in pollination.
Most fungi reproduce sexually, having both a haploid and diploid stage in their life cycles. These fungi are typically isogamous, lacking male and female specialization: haploid fungi grow into contact with each other and then fuse their cells. In some of these cases, the fusion is asymmetric, and the cell which donates only a nucleus (and not accompanying cellular material) could arguably be considered "male".[38] Fungi may also have more complex allelic mating systems, with other sexes not accurately described as male, female, or hermaphroditic.[5]
Some fungi, including baker's yeast, have mating types that create a duality similar to male and female roles. Yeast with the same mating type will not fuse with each other to form diploid cells, only with yeast carrying the other mating type.[39]
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. The height of the mushroom aids the dispersal of these sexually produced offspring.[citation needed]
The most basic sexual system is one in which all organisms are hermaphrodites, producing both male and female gametes.[citation needed] This is true of some animals (e.g. snails) and the majority of flowering plants.[40] In many cases, however, specialization of sex has evolved such that some organisms produce only male or only female gametes. The biological cause for an organism developing into one sex or the other is called sex determination. The cause may be genetic or non-genetic. Within animals and other organisms that have genetic sex determination systems, the determining factor may be the presence of a sex chromosome or other genetic differences.[vague] In plants also, such as the liverwort Marchantia polymorpha and the flowering plant genus Silene that have sexual dimorphism (monoicy or dioicy, respectively), sex may be determined by sex chromosomes.[41] Non-genetic systems may use environmental cues, such as the temperature during early development in crocodiles, to determine the sex of the offspring.[42]
In the majority of species with sex specialization (approximately 99% of all vertebrate species), organisms are either male (producing only male gametes) or female (producing only female gametes), a system called dioecy.[3][43] There are exceptions; for example, the roundworm C. elegans has an hermaphrodite and a male sex (a system called androdioecy).
Sometimes an individual organism has sex characteristics of both sexes, and these conditions are called intersex. They can be caused by an abnormal amount of sex chromosomes or a hormonal abnormality during fetal development. Sometimes intersex individuals are called "hermaphrodite"; but, unlike biological hermaphrodites, intersex individuals are atypical cases and are not typically fertile in both male and female aspects.[citation needed] Some species can have gynandromorphs.[44][45][46]
In genetic sex-determination systems, an organism's sex is determined by the genome it inherits. Genetic sex-determination usually depends on asymmetrically inherited sex chromosomes which carry genetic features that influence development; sex may be determined either by the presence of a sex chromosome or by how many the organism has. Genetic sex-determination, because it is determined by chromosome assortment, usually results in a 1:1 ratio of male and female offspring.
Humans and most other mammals have an XY sex-determination system: the Y chromosome carries factors responsible for triggering male development. The "default sex," in
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