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Sex is a trait that determines an individual's reproductive function, typically male or female, in organisms that propagate their species through sexual reproduction. Most plants and almost all animals employ sexual reproduction. Animals are usually mobile and seek out a partner of the opposite sex for mating. Animals that mate in water can use external fertilization whereas most land-based animals, such as reptiles, birds, and mammals, including humans, use internal fertilization. Plants are generally immobile and so, 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 plants involves the recombination of genetic traits by meiosis followed by the formation of specialized haploid cells known as gametes. Pairs of gametes fuse to form diploid zygotes that develop into offspring that inherit a selection of the traits of each parent. The gametes produced by an organism define its sex. Male organisms produce smaller gametes (e.g. spermatozoa, or sperm) while female organisms produce larger gametes (ova, often called egg cells). Organisms that produce both types of gametes are called hermaphrodites.
The terms "male" and "female" are generally not applied in species where the gametes are indistinguishable in size and morphology, as the green alga Ulva lactuca. Instead, if there are functional differences between gametes, they may be referred to as mating types. Fungi have complex allelic mating systems in which the differences between gametes are most accurately described as mating types.
Male and female individuals of a species may be similar (isomorphic), or they may have physical differences (sexual dimorphism). 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 the XY sex-determination system, male mammals typically carry an X and a Y chromosome (XY), whereas female mammals typically carry two X chromosomes (XX). Other animals have various sex-determination systems, such as the ZW system in birds, the X0 system in insects, and various environmental systems, such as those relating to reptiles and crustaceans.
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 mechanisms of reproduction. Initially, reproduction was a replicating process in which organisms divided to produce new individuals that contained the same genetic information as the original or parent individual. This mode of asexual reproduction is still used by many species, particularly unicellular organisms, but it is also very common in multicellular organisms, including many of those that also employ sexual reproduction. In sexual reproduction, the genetic material of the offspring comes from two different individuals. Bacteria reproduce asexually but can undergo a parasexual process called conjugation, by which a part of the genetic material of an individual donor is transferred to another recipient.
The basic distinction between asexual and sexual reproduction is the way in which the genetic material is processed. Prior to an asexual division, a cell duplicates its genetic information, and then divides in a process of cell division called mitosis. In sexual reproduction, the genetic material is re-distributed by chromosomal crossover between parental chromosomes in a process termed meiosis, usually followed by two rounds of mitosis. The resulting four cells are called gametes, which 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. Sex comprises the arrangements that enable sexual reproduction and has evolved alongside the reproduction system,[clarification needed] 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). British biologist Richard Dawkins has stated it could be interpreted that all the differences between the sexes stem from the differences in gametes.
In multicellular 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 between male and female individuals within their populations. Conversely, there are also species, such as the green seaweed genus Ulva, in which there is no sexual specialization among the isomorphic individual plants, their sexual organs, or their gametes. In some species, the same individuals function as both male and female, and are called hermaphrodites. The majority of flowering plants are bisexual and produce perfect flowers that contain both male and female sexual organs, and are said to be monoecious. However, flowering plants may also have imperfect flowers that lack one or other type of sex organs. Sometimes, as in the tree of heaven, Ailanthus altissima the panicles can contain a mixture of functionally unisexual flowers and functionally bisexual flowers.
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. Sexual reproduction probably first evolved about a billion years ago within early single-celled eukaryotes or their prokaryotic ancestors. 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: Sex creates variation among offspring; Sex helps in the spread of advantageous traits; Sex helps in the removal of disadvantageous traits; Sex facilitates repair of germ-line DNA.
Sexual reproduction is a process specific to eukaryotes (organisms whose cells contain a nucleus). In addition to multicellular animals, plants, and fungi, unicellular eukaryotes such as the malaria parasite also engage in sexual reproduction. The defining characteristic of sexual reproduction in eukaryotes is the binary nature of fertilization. It involves the fusion between two haploid gametes that may be identical (isomorphic, isogamy) or of different types (heteromorphic, heterogamy), typically described as male or female. Multiplicity of gamete types within a species would still be considered a form of sexual reproduction. No third gamete type is known in multicellular plants or animals.
The origin of anisogamy and chromosomal sex determination may also have been fairly early in eukaryote evolution. In animals, there are four systems of sex determination, all of which depend on special sex chromosomes.
Sexual reproduction in eukaryotes is a process whereby organisms produce offspring that inherit a selection of the 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. 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 double 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. 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. This process is followed by a mitotic division, producing haploid gametes that contain one set of chromosomes. Crossing over and fertilization (the recombination 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 the life cycle of many organisms, there is no multicellular haploid phase and the gametes are the only haploid cells, specialized to recombine to form a diploid zygote that develops into a new multicellular diploid organism. In the life-cycle of plants diploid and haploid multicellular phases alternate. The diploid organism, called the sporophyte, produces haploid spores by meiosis, which, on germination, undergo mitotic 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). A more extreme version of anisogamy is where the female gamete is non-motile and the male gamete is motile (oogamy). 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. An individual that produces both types of gametes is a hermaphrodite; in some animals and many plants, hermaphrodites are able to self-fertilize and produce offspring on their own, without a second organism.
Most sexually reproducing animals spend their lives as diploid, with the haploid stage reduced to single-cell gametes. 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. 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. 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 such as fish and corals which live in the water can mate using external fertilization, where the eggs and sperm are released into and combine within the surrounding water. 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". In many other terrestrial animals, males use specialized male sex organs called intromittent organs to assist the transport of sperm. In humans and other mammals the equivalent male organ is the penis, which enters the female reproductive tract (called the vagina) to achieve insemination in 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. Within seed plants, male gametes are produced by extremely reduced multicellular microgametophytes known as pollen. The female gametes of seed plants are produced by megagametophytes contained within ovules; once fertilized by male gametes produced by pollen the ovules develop into seeds which, like eggs, contain the nutrients necessary for the development of the embryonic plant.
The flowers produced by angiosperms (flowering plants) contain their sexual organs. Most angiosperms are hermaphroditic, producing both male and female gametes on the same plant, most often from the same flowers. The female parts in the flower, are the pistils, consisting of one or more carpels; carpels consist of a style, a stigma, and an ovary. The male parts of the flower are the stamens. These consist of filaments and anthers that produce the pollen.[better source needed]
A process of double fertilization occurs in angiosperms, where one of the two sperm nuclei fertilises an egg cell to form a diploid zygote, and the other fuses with two gametophyte polar cells to produce a triploid cell that develops into the endosperm, the food source of the seed.
Within the ovary are ovules, which contain megagametophytes that produce egg cells. 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 the egg cell in an ovule that eventually develops into a seed.
In pines and other conifers the sex organs are contained in the cones. The female cones (seed cones) produce seeds and male cones (pollen cones) produce pollen. 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.
The male gametes are the only cells in plants that contain flagella. They are mobile, able to swim to the egg cells of female plants in films of water. Seed plants other than Cycads and Ginkgo have lost flagella entirely. Once their pollen is delivered to the stigma of flowering plants, or to the micropyle of gymnosperm ovules, their gametes are delivered to the egg cell by means of pollen tubes produced by one of the cells of the microgametophyte. Because seed plants are immobile, they depend upon passive methods for transporting pollen grains to female sex organs on the same or other plants. Many plants, including conifers and grasses, are anemophilous producing lightweight pollen which is carried by wind to neighboring plants. Other plants, such as orchids, have heavier, sticky pollen that is specialized for Zoophily, transportation by animals. Plants attract 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 are able to reproduce sexually and asexually. They can have both a haploid and diploid stage in their life cycles. Many fungi are typically isogamous, lacking male and female specialization.
Haploid fungi grow into contact with each other and then fuse their cells. In certain cases, the fusion is asymmetric, and the cell which donates only a nucleus (and not accompanying cellular material) could arguably be considered "male". Fungi may also have more complex allelic mating systems, with other sexes not accurately described as male, female, or hermaphroditic.
Many species of fungi have two mating types. However, species like Coprinellus disseminatus have been estimated to have about 123 mating types, and in some species there are even thousands of mating types.
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 another mating type.
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.
The most basic sexual system is one in which all organisms are hermaphrodites, producing both male and female gametes. This is true of some animals (e.g. snails) and the majority of flowering plants. 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, environmental, haplodiploidy, or by multiple factors. 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 the dioicous liverwort Marchantia polymorpha or the dioecious flowering plant genus Silene, sex may be determined by sex chromosomes. Since only about 6% of flowering plants are dioecious, the majority are bisexual. Non-genetic systems may use environmental cues, such as the temperature during early development in crocodiles, to determine the sex of the offspring.
Approximately 95% of animal species are dioecious (also referred as gonochorism). In gonochoric species, individuals are either male or female throughout their lives. Gonochorism is very common in vertebrate species, with 99% being gonochoric; the other 1% is hermaphroditic, with almost all of them being fishes. All mammals and birds are gonochoric.
Although rare, a species can have males, females, and hermaphrodites - a system called trioecy. Trioecy occurs in about 3.6% of flowering plants, such as Opuntia robusta and Fraxinus excelsior.
In some gonochoric animal species, a few individuals may have sex characteristics of both sexes, a condition called intersex. These conditions can be caused by extra sex chromosomes or by a hormonal abnormality during fetal development. Sometimes intersex individuals are called "hermaphrodite" but, unlike biological hermaphrodites, intersex individuals are atypical cases, are not typically fertile, and do not function in both male and female aspects. Some species[which?] can have gynandromorphs.
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 carrying 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.
- In the X0 sex-determination system, males have one X chromosome (X0) while females have two (XX). This system is found in most arachnids, insects such as silverfish (Apterygota), dragonflies (Paleoptera) and grasshoppers (Exopterygota), and some nematodes, crustaceans, and gastropods.
- In the Z0 sex-determination system, males have two Z chromosomes whereas females have one. This system is found in several species of moths.
- In the ZW sex-determination system, males have two Z chromosomes whereas females have one Z chromosome and one W chromosome. Therefore, it is the female gamete that determines the sex of the offspring. This system is used by birds, some fish, and some crustaceans.
- In the XY sex determination system, females have two X chromosomes whereas males have one X chromosome and one Y chromosome. Therefore it is the male gamete that determines the sex of the offspring. This system is used by most mammals, but also some insects.
No genes are shared between the avian ZW and mammal XY chromosomes, and from a chicken and human comparison, 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.
XY sex determination
Humans and most other mammals have an XY sex-determination system: the Y chromosome carries factors responsible for triggering male development, making XY sex determination mostly based on the presence or absence of the Y chromosome. Thus, XX mammals typically are female and XY typically are male. Individuals with XXY or XYY are males, while individuals with X and XXX are females.
XY sex determination is found in other organisms, including the common fruit fly and some plants. In some cases, it is the number of X chromosomes that determines sex rather than the presence of a Y chromosome. In the fruit fly (Drosophila melanogaster) 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.
ZW sex determination
In birds, which have a ZW sex-determination system, the opposite is true: the W chromosome carries factors responsible for female development, and default development is male. In this case, ZZ individuals are male and ZW are female. The majority of butterflies and moths also have a ZW sex-determination system. In species like the Lepidoptera females can have Z, ZZW, and even ZZWW.
In both XY and ZW sex determination systems, the sex chromosome carrying the critical factors is often significantly smaller, carrying little more than the genes necessary for triggering the development of a given sex.
XO sex determination
Many insects use a sex determination system based on the number of sex chromosomes. This is called X0 sex-determination whereby the "0" indicates the absence of the sex chromosome. All other chromosomes in these organisms are diploid, but organisms may inherit one or two X chromosomes. In field crickets, for example, insects with a single X chromosome develop as male, while those with two develop as female. In the nematode C. elegans most worms are self-fertilizing XX hermaphrodites, but occasional abnormalities in chromosome inheritance can give rise to individuals with only one X chromosome—these X0 individuals are fertile males (and half their offspring are male).
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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. Many reptiles, including all crocodiles and most turtles, have temperature-dependent sex determination: the temperature embryos experience during their development determines the sex of the organism. In some turtles, for example, males are produced at lower incubation temperatures than females; this difference in critical temperatures can be as little as 1–2 °C.
Many fish change sex over the course of their lifespan, a phenomenon called sequential hermaphroditism. In clownfish, smaller fish are male, and the dominant and largest fish in a group becomes female. In many wrasses the opposite is true—most fish are initially female and become male when they reach a certain size. Sequential hermaphrodites may produce both types of gametes over the course of their lifetime, but at any given point they are either female or male.
Other insects, including honey bees and ants, use a haplodiploid sex-determination system. In this case, diploid individuals 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.
Sex differences in humans include, generally a larger size and more body hair in men; women have larger breasts, wider hips, and a higher body fat percentage. In other species, the differences may be more extreme, such as differences in coloration or bodyweight.
Sexual dimorphisms in animals are often associated with sexual selection—the competition between individuals of one sex to mate with the opposite sex.[better source needed] Antlers in male deer, for example, are used in combat between males to win reproductive access to female deer. 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 drive sexual dimorphism, such as in the case of the stalk-eyed fly.
A majority of animals have larger females. This 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. For example, female southern black widow spiders are typically twice as long as the males. Occasionally this dimorphism is extreme, with males reduced to living as parasites dependent on the female, such as in the anglerfish. Some plant species also exhibit dimorphism in which the females are significantly larger than the males, such as in the moss Dicranum and the liverwort Sphaerocarpos. There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome, or to chemical signalling from females.
In birds, males often have a more colourful appearance and may have features (like the long tail of male peacocks) that would seem to put the organism 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. This hypothesis says 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.
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