Sex is a trait that determines an individual's reproductive function, male or female, in animals and plants that propagate their species through sexual reproduction. The type of gametes produced by an organism defines its sex. Commonly in plants and animals, male organisms produce smaller gametes (spermatozoa, sperm) while female organisms produce larger gametes (ova, often called egg cells). Organisms that produce both types of gametes are called hermaphrodites. During sexual reproduction, male and female gametes fuse to form zygotes that develop into offspring that inherit a selection of the traits of each parent.
Male and female individuals of a species may be similar (sexual monomorphism), or 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.
The terms "male" and "female" typically do not apply in sexually undifferentiated species in which the individuals are isomorphic and the gametes are isogamous (indistinguishable in size and morphology), such as the green alga Ulva lactuca. If there are instead functional differences between gametes, such as in fungi, they may be referred to as mating types.
Sex is genetically determined in most mammals by the XY sex-determination system, where male mammals carry an X and a Y chromosome (XY), whereas female mammals carry two X chromosomes (XX). Other chromosomal sex-determination systems in animals include the ZW system in birds, and the X0 system in insects. Various environmental systems include temperature-dependent sex determination in reptiles and crustaceans.
Approximately 95% of animal species are gonochoric (also known as dioecious, or dioicous especially in plants) and about 5% are hermaphroditic. This low percentage is due to the very large number of insect species, in which hermaphroditism is absent. Hermaphroditism nevertheless occurs in 70% of animal phyla.
Gonochoric individuals are either male or female throughout their lives. Gonochorism is very common in vertebrates, about 99% of which gonochoric. The remaining 1%, almost all fishes, are hermaphroditic. All birds and mammals are gonochoric.
Roughly 5 to 6% of flowering plants are dioecious, resulting from between 871 and 5000 independent origins. Consequently the majority are bisexual, either hermaphrodite (with both stamens and pistil in the same flower) or monoecious (with separate male and female flowers on the same plant). In dioecious species male and female sexes are on separate plants. Dioecy is common in gymnosperms, in which 65% of species are dioecious, but most conifers are monoecious.
Mixed breeding systems
Androdioecy, gynodioecy, and trioecy are sometimes called mixed breeding systems. The roundworm Caenorhabditis elegans has a hermaphrodite and a male sex - a system called androdioecy. If there separate male and hermaphrodite flowers on the same plant, as in Solanum carolinense, the species is described as andromonoecious.
The flowering plant Salvia pratensis is gynodioecious, where a species has females and hermaphrodites. If female and hermaphrodite flowers exist on the same plant, the species is described as gynomonoecious.
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 Pachycereus pringlei and Fraxinus excelsior.
Sexual reproduction in eukaryotes produces offspring that inherit a selection of the genetic traits of both parents. In this process, chromosomes are passed on from one generation to the next. Each of the offspring's cells has half the chromosomes of the mother and half of the father. The codes for genetic traits are contained within the deoxyribonucleic acid (DNA) of chromosomes. By combining one set 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 new pairs of mixed chromosomes, each of which is a blend of the genes of both parents. This process is followed by a mitotic division, producing haploid gametes that contain one set of chromosomes. Crossing over to make new recombinant chromosomes and fertilization (the fusion of two gametes) result in the new organism containing a different set of genetic traits from either parent.
In the life cycle of most multicellular 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.
Gametes may be externally similar (isogamy) or may differ in size and other aspects (anisogamy). Oogamy is an extreme example of anisogamy, in which a large, non-motile gamete is fused with a smaller, usually motile one. Isogamy is very common in unicellular organisms while anisogamy is common in multicellular organisms. An individual that exclusively produces large gametes is female, and one that exclusively produces small gametes is male.
An individual that produces both types of gametes is a hermaphrodite. Some hermaphrodites, particularly hermaphroditic plants, are able to self-fertilize and produce offspring on their own, without a second organism. However, some hermaphrodite animals such as Helix pomatia and Cepaea cannot self-fertilize.
In the life-cycle of plants and multicellular algae, diploid and haploid multicellular phases alternate. The diploid organism is called the sporophyte because it produces haploid spores by meiosis, which, on germination, undergo mitotic cell division to produce multicellular haploid organisms, the gametophytes that produce gametes by mitosis.
Sexually reproducing animals are diploid, and their single-celled gametes are the only haploid cells in their life cycles. Animals have two gamete types: male spermatozoa (sperm) and female ova (egg cells). During fertilization, the gametes combine to form diploid zygotes that develop into embryos, which in turn develop into new organisms. Animals are usually mobile and seek out a partner of the opposite sex for mating.
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 and fertilizing an egg cell.
Egg cells (ova) are produced within the ovaries. They are large, immobile cells that contain the nutrients necessary for a developing embryo. Egg cells are often associated with other cells which support the development of the embryo, forming an organic vessel called an egg.
All animals that live outside of water use internal fertilization to transfer sperm directly into the female, thereby preventing the gametes from drying up. Specialized organs called intromittent organs are commonly used by males to assist the transport of sperm.
With the exception of monotremes, mammals are viviparous, where the fertilized egg develops into an embryo within the female, receiving nutrition directly from its mother. In mammals the female reproductive tract, called the vagina, connects with the uterus, an organ which directly supports the development of a fertilized embryo within, a process called gestation. In humans and other mammals the equivalent male organ is the penis, which enters 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 97% of bird species, males do not have a penis. Instead in most birds, both excretion and reproduction are done through a single posterior opening called the cloaca. Male and female birds touch cloacae to transfer sperm, a process called "cloacal kissing".
Most aquatic animals such as fish and corals mate using external fertilization, where the eggs and sperm are released into, and combine within, the surrounding water. However, some species like crustaceans use internal fertilization. In seahorses, females use their ovipositors to deliver eggs into the males’ underside for fertilization and gestation. Pipefish and seahorses are the only species that entail male pregnancy.
Most insects reproduce through oviparity, where a female mates with a male and the females lays the egg outside of her body. A few groups of insects such as the Strepsiptera reproduce through traumatic insemination, where a male pierces a female's exoskeleton with his aedeagus. In some harvester ants, a queen needs to mate with two types of males: one to reproduce queens and another to reproduce worker ants, this species may be constituting three or four sexes.
In the green seaweed genus Ulva, there is no sexual specialization among the isomorphic individual plants, their sexual organs, or their isogamous gametes. However, the majority of plants and animals have specialized male and female gametes.
The male gametes are the only cells in plants and green algae that have flagella. They are motile, able to swim to the egg cells of female gametophyte plants in films of water. Seed plants other than Cycads and Ginkgo have lost flagella entirely and are unable to swim in water. Once their pollen is delivered to the stigma of flowering plants, or 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. 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 such as bees or larger animals such as humming birds and bats with flowers containing rewards of nectar or resin. These animals transport the pollen as they move to other flowers, which also contain female reproductive organs, resulting in cross-pollination.
In seed plants, male gametes are produced by extremely reduced multicellular microgametophytes known as pollen. The female gametes (egg cells) of seed plants are produced by larger megagametophytes contained within ovules. Once the egg cells are fertilized by male gametes produced by pollen, the ovules develop into seeds which contain the nutrients necessary for the initial development of the embryonic plant.:175
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 female cones are longer lived and typically much larger and more durable. The ovules attached to the cone scales are not enclosed in an ovary, giving rise to the name gymnosperm meaning 'naked seed'. The smaller male cones produce pollen which is transported by wind to land in female cones. Naked seeds form after pollination, protected by the scales of the female cone.
The sex organs of flowering plants are contained in flowers. The majority of flowers are hermaphroditic (bisexual) and produce both male and female gametophytes in the same flowers. The male gametophytes form inside pollen grains and produce male gametes. The female gametophytes form inside ovules and produce female gametes.Bisexual flowers that contain both male and female sexual organs are said to be perfect. Angiosperms may also have imperfect flowers, on the same or different plants, that lack one or other type of sex organs. Sometimes, as in the tree of heaven (Ailanthus altissima) and the European ash (Fraxinus excelsior) the panicles can produce different mixtures of functionally unisexual and functionally bisexual flowers on the same or different trees.:398, 615
The male parts of the flower are the stamens, which consist of the filaments supporting the anthers that produce the pollen. The female parts in the flower, are the pistils, composed of one or more carpels. Carpels consist of an ovary, a style and a stigma. Within the ovary are ovules, which contain haploid 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. At the same time the ovary develops into a fruit. Because flowering plants are immobile, they evolved flowers to attract animals such as insects to help in fertilization.
Most fungi are able to reproduce sexually and asexually and have both haploid and diploid stages in their life cycles.:214 Many fungi are isogamous, lacking male and female specialization. Even fungi that are anisogamous are all hermaphroditic, which is why even anisogamous fungi are considered to be mating types rather than sexes.:182
Fungi may have complex allelic mating systems and many species of fungi have two mating types. However, Coprinellus disseminatus has been estimated to have about 123 mating types, and in some species there are thousands of mating types. For example, Schizophyllum commune has about 28,000 or more mating types.
Some fungi, including that used as baker's yeast, have mating types that create a duality similar to male and female roles. Yeast with the same mating type do not fuse to form diploid cells, only with yeast carrying another mating type.
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Sexual reproduction is common among parasitic protozoa but rare among free-living protozoa, which usually reproduce asexually unless food is scarce or the environment changes drastically. Both anisogamy and isogamy are found in free-living protozoa. Ciliates are all isogamous such as Tetrahymena thermophila, which has 7 mating types.
Evolution of sex
Anisogamy most likely evolved from isogamy, but its evolution has left no fossil evidence. Its evolution was due to disruptive selection leading to small gametes and large gametes. In anisogamous species an intermediate gamete is unable to persist. There should always be two gamete types, with all analyses showing that intermediate gamete sizes are eliminated due to selection.
Evolution of sex determination
Chromosomal sex determination may have evolved early in the history of eukaryotes. No genes are shared between the avian ZW and mammal XY chromosomes and the chicken Z chromosome is similar to the human autosomal chromosome 9, rather than X or Y. This suggests not that the ZW and XY sex-determination systems share an origin but that the sex chromosomes are derived from autosomal chromosomes of the common ancestor of birds and mammals. In the platypus, a monotreme, the X1 chromosome shares homology with therian mammals, while the X5 chromosome contains an avian sex-determination gene, further suggesting an evolutionary link.
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. 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 liverwort Marchantia polymorpha or the dioecious species in the flowering plant genus Silene, sex may also be determined by sex chromosomes. Non-genetic systems may use environmental cues, such as the temperature during early development in crocodiles, to determine the sex of the offspring.
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.
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.[vague] Sex may be determined either by the presence of a sex chromosome or by the number of sex chromosomes 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.
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. It is the male gamete that determines the sex of the offspring. In this system XX mammals typically are female and XY typically are male. However, individuals with XXY or XYY are males, while individuals with X and XXX are females.
XY sex determination is found in other organisms, including insects like 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 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. It is the female gamete that determines the sex of the offspring. This system is used by birds, some fish, and some crustaceans.
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.[better source needed]
XO sex determination
In the X0 sex-determination system, males have one X chromosome (X0) while females have two (XX). All other chromosomes in these diploid organisms are paired, but organisms may inherit one or two X chromosomes. 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 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 X0 individuals are fertile males (and half their offspring are male).
ZO sex determination
This section relies largely or entirely upon a single source. (June 2021)
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.
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.
Some species can change sex over the course of their lifespan, a phenomenon called sequential hermaphroditism. 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. In many wrasses the opposite is true—the fish are initially female and become male when they reach a certain size. Sequential hermaphroditism also occurs in plants such as Arisaema triphyllum.
Temperature-dependent sex determination
Many reptiles, including all crocodiles and most turtles, have temperature-dependent sex determination. In these species, the temperature experienced by the embryos during their development determines their sex. 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.
Other insects, including honey bees and ants, use a haplodiploid sex-determination system. 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.
Most organisms which reproduce sexually have a 1:1 sex ratio of male and female births. The English statistician and biologist Ronald Fisher outlined why this is so in what has come to be known as Fisher's principle.[better source needed] This essentially says the following:
- Suppose male births are less common than female.
- A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
- Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
- Therefore the genes for male-producing tendencies spread, and male births become more common.
- As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
- The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.
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.
Primary sex characteristics are organs directly involved in reproduction such as the testes or ovaries, while secondary sex characteristics in humans for example are body hair, breasts, and distribution of fat.
In some species,[which?] a few individuals may have a mixture of characteristics from both sexes, a condition called intersex. This can be caused by extra sex chromosomes or by a hormonal abnormality during fetal development. The term intersex typically applies to abnormal members of gonochoric species rather than to hermaphroditic species. Some species, such as the fruit fly (Drosophila melanogaster), and some crustaceans may have gynandromorphs (also known as sexual mosaics).[clarification needed]
In many animals and some plants, individuals of male and female sex differ in size and appearance, a phenomenon called sexual dimorphism. Sexual dimorphism in animals is often associated with sexual selection—the mating competition between individuals of one sex vis-à-vis the opposite sex. 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.
Females are the larger sex in a majority of animals. For instance, female southern black widow spiders are typically twice as long as the males. 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.
Sexual dimorphism can be extreme, with males, such as some anglerfish, living 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 and the liverwort genus 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 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. 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.
By contrast sexual monomorphism (also called sexual isomorphism) is when males and females of a certain species are similar in external appearance. Many sexually monomorphic species are hermaphrodites.
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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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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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However, species are also considered gonochoric if (as develop mental abnormalities[clarification needed] or because of genetic mutations) intersex individuals accidentally occur, i.e. ones with a mix of both male and female phenotypic characters ( Section 6.1.1 ).
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