Sex

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.[1][2] 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).[3] Organisms that produce both types of gametes are called hermaphrodites.[2][4] 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.

Males and females 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 (look the same) and the gametes are isogamous (indistinguishable in size and shape), such as the green alga Ulva lactuca. If there are instead functional differences between gametes, such as in fungi,[5] they may be referred to as mating types.[6]

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.[7]

Reproduction

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

Sexual reproduction is production of offspring by the fusion of haploid gametes.[8][9] 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, produce haploid cells (gametes) that randomly contain one of each of the chromosome pairs, via meiosis.[10] 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.[10] 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)[11] result in the new organism containing a different set of genetic traits from either parent.

Gametes may be externally similar (isogamy) or may differ in size and other aspects (anisogamy).[6] Oogamy is an extreme example of anisogamy, in which a large, non-motile gamete is fused with a smaller, usually motile one.[12] Isogamy is very common in unicellular organisms while anisogamy is common in multicellular organisms.[13] Individuals that exclusively produce large gametes are females, and those that exclusively produce small gametes are males.[14][3][15]

An individual that produces both types of gametes is a hermaphrodite.[4] Some hermaphrodites such as the roundworm Caenorhabditis elegans are able to self-fertilize and produce offspring on their own, without a second organism.[16] Some hermaphrodite animals such as Helix pomatia and Cepaea cannot self-fertilize.[17]

Some hermaphroditic plants are self-fertile, but plants have evolved multiple different mechanisms to avoid self-fertilization, involving sequential hermaphroditism (dichogamy), self-incompatibility or morphological mechanisms such as heterostyly (herkogamy).[18]: 73, 74 

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.[19]

Animals

Hoverflies mating

Sexually reproducing animals are diploid, and their single-celled gametes are the only haploid cells in their life cycles.[20] Animals have two gamete types: male spermatozoa (sperm) and female ova (egg cells).[21]

A spermatozoon, produced in vertebrates within the testes, is a small cell containing a single long flagellum which propels it.[22] Egg cells (ova) are produced within the ovaries. In oviparous species such as birds, the fertilized egg cell or zygote is provided with yolk, a nutrient supply which supports the development of the embryo.[23]

All animals that live outside of water use internal fertilization to transfer sperm directly into the female, thereby preventing the gametes from drying up.[24] Intromittent organs are the male copulation organs which help transport of sperm.[25]

Mammals

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 Marsupials and placental mammals the fertilized egg develops within the female, receiving nutrition directly from its mother via a specialized organ called the placenta.[26]

Birds

In 97% of bird species, males do not have a penis.[25] 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".[27]

Aquatic animals

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.[28] However, some species like crustaceans use internal fertilization.[24] 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.[29]

Insects

Most insects reproduce through oviparity, where a female mates with a male and the females lays the egg outside of her body.[30] A few groups of insects such as the Strepsiptera reproduce through traumatic insemination, where a male pierces a female's exoskeleton with his aedeagus.[31] In some harvester ants, a queen needs to mate with two types of males: one to reproduce queens and another to reproduce worker ants; these ants may be considered to have three or four sexes.[32]

Plants

In the green seaweed genus Ulva, there is no sexual specialization among the isomorphic individual plants, their sexual organs, or their isogamous gametes.[33] However, the majority of plants have specialized male and female gametes.[34][35]

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,[36] 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.[37] These animals transport the pollen as they move to other flowers, which also contain female reproductive organs, resulting in cross-pollination.

Spermatophytes

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.[18]: 175 

Conifers
Female (left) and male (right) cones contain the sex organs of pines and other conifers. Most conifers are monoecious,[38] producing separate male and female cones on the same plant.

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.[39] 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.[40]

Angiosperms
Flowers contain the sexual organs of flowering plants. They usually contain both male and female parts, organs to attract pollinators and organs that provide rewards to pollinators.

The sex organs of flowering plants are contained in flowers. The male parts of the flower are the stamens, which consist of the filaments supporting the anthers that produce the pollen.[41][42] 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.[43]

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.[44] Bisexual flowers that contain both male and female sexual organs are said to be perfect.[45][46]

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.[47]: 398, 615 

Because flowering plants are immobile, they evolved flowers to attract animals such as insects to help in fertilization.[48]

Fungi

Mushrooms are produced as part of fungal sexual reproduction

Most fungi are able to reproduce sexually and asexually and have both haploid and diploid stages in their life cycles.[5]: 214  Many fungi are isogamous, lacking male and female specialization.[49] Even fungi that are anisogamous are all hermaphroditic, which is why even anisogamous fungi are considered to be mating types rather than sexes.[50][51]: 182 

Fungi may have complex allelic mating systems and many species of fungi have two mating types.[52] However, Coprinellus disseminatus has been estimated to have about 123 mating types, and in some species there are thousands of mating types.[49] For example, Schizophyllum commune has about 28,000 or more mating types.[53]

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.[54]

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.

Protozoa

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.[55] Ciliates are all isogamous such as Tetrahymena thermophila, which has 7 mating types.[56]

Sexual systems

A sexual system is a distribution of male and female functions across organisms in a species.[57]

Animals

Approximately 95% of animal species are gonochoric (also known as dioecious) and about 5% are hermaphroditic.[57] This low percentage is due to the very large number of insect species, in which hermaphroditism is absent.[44] Hermaphroditism nevertheless occurs in 70% of animal phyla.[58]

Gonochoric individuals are either male or female throughout their lives.[59] Gonochorism is very common in vertebrates, about 99% of which gonochoric. The remaining 1% that are hermaphroditic are almost all fishes.[60] All birds and mammals are gonochoric.[61]

Plants

Roughly 5 to 6% of flowering plants are dioecious, resulting from between 871 and 5000 independent origins.[62] Consequently the majority are bisexual,[58] either hermaphrodite (with both stamens and pistil in the same flower) or monoecious (with separate male and female flowers on the same plant).[46][63] In dioecious species male and female sexes are on separate plants.[64] Dioecy is common in gymnosperms, in which 65% of species are dioecious, but most conifers are monoecious.[38]

Evolution of sex

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 conflict underlies the evolutionary distinction between male and female with the distinction starting from anisogamy, the form of sexual reproduction that involves the union or fusion of two gametes that differ in size or form.[65] The evolution of anisogamy is also synonymous to the evolution of male and female sexes,[13] as well the starting point toward sexual dimorphism,[66] and lead to the evolution of many sex differences.[67]

It is generally accepted that anisogamy has evolved from isogamy[68] several times independently in different groups of eukaryotes, including protists, algae, plants and animals,[44] but its evolution has left no fossil evidence.[69] Until 2006 there was no genetic evidence for the evolutionary link between sexes and mating types due to plants and animals having no isogamous relatives.[70]

Anisogamy evolves due to disruptive selection leading to small gametes and large gametes.[71] In anisogamous species an intermediate gamete is unable to persist.[72] There should always be two gamete types, with all analyses showing that intermediate gamete sizes are eliminated due to selection.[73][74] As of 2016, it remains unclear if anisogamy first led to the evolution of gonochorism or the evolution of hermaphroditism.[58]: 213 

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.[44] 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.[75] Non-genetic systems may use environmental cues, such as the temperature during early development in crocodiles, to determine the sex of the offspring.[76]

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.[77]

Genetic

Like humans and most other mammals, the common fruit fly has an XY sex-determination system.

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.[78] In this system XX mammals typically are female and XY typically are male.[44] However, individuals with XXY or XYY are males, while individuals with X and XXX are females.[7]

XY sex determination is found in other organisms, including insects like the common fruit fly,[79] and some plants.[80] In some cases, it is the number of X chromosomes that determines sex rather than the presence of a Y chromosome.[7] 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.[81]

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.[82] 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.[7]

The majority of butterflies and moths also have a ZW sex-determination system. In groups like the Lepidoptera, females can have Z, ZZW, and even ZZWW.[83]

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.[84][85]

In field crickets, for example, insects with a single X chromosome develop as male, while those with two develop as female.[86]

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).[87]

ZO sex determination

In the Z0 sex-determination system, males have two Z chromosomes whereas females have one. This system is found in several species of moths.[88]

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.

The bonelliidae larvae can only develop as males when they encounter a female.[44]

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.[89]

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.[90] 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.[91] 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.[44] 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.[92]

Haplodiploidy

Other insects, including honey bees and ants, use a haplodiploid sex-determination system.[93] 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.[94]

Sex ratio

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.[95] This essentially says the following:

  1. Suppose male births are less common than female.
  2. A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
  3. Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
  4. Therefore the genes for male-producing tendencies spread, and male births become more common.
  5. As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
  6. The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.

Sex differences

Anisogamy is the fundamental difference between male and female.[96][97] Richard Dawkins has stated that it is possible to interpret all the differences between the sexes as stemming from this.[98]

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.[99]

Sex differences in behavior

The sexes across gonochoric species usually differ in behavior. In most animal species females invest more in parental care,[100] although in some species, such as some coucals, the males invest more parental care.[101] Females also tend to be more choosy for who they mate with,[102] such as most bird species.[103] Males tend to be more competitive for mating than females.[13]

Sexual dimorphism

Common pheasants are sexually dimorphic in both size and appearance.

In many animals and some plants, individuals of male and female sex differ in size and appearance, a phenomenon called sexual dimorphism.[104] Sexual dimorphism in animals is often associated with sexual selection—the mating competition between individuals of one sex vis-à-vis the opposite sex.[99] 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.[105]

Females are the larger sex in a majority of animals.[104] For instance, female southern black widow spiders are typically twice as long as the males.[106] 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.[107][104]

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[108] and the liverwort genus Sphaerocarpos.[109] There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome,[109][110] or to chemical signalling from females.[111]

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.[112] 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 monomorphism

Sexual monomorphism is when both sexes are similar in appearance and structure.[113] In primary sexual monomorphism both sexes have similar traits, and possibly similar genes, while in secondary sexual monomorphism both sexes have a trait that was historically in one sex.[114] In sexually monomorphic species both parents invest the same amount in their offspring and both sexes are choosy for whom to mate with.[115] Sexual monomorphism is also related to low levels of sexual selection.[116]

Monogamous species tend to be sexually monomorphic.[117] All pair bonded primates are sexually monomorphic,[118] including the tarsier family.[119] However, not all monogamous primates are sexually monomorphic,[118] there being only a few primate taxa where sexual monomorphism is associated with socially monogamous groups.[120]

Many bird species are sexually monomorphic.[121] Birds that are larger in size are more sexually monomorphic than smaller birds.[122]

In plants many sexually monomorphic species are hermaphrodites.[123]

Sex characteristics

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.[124]

Organisms that have intermediate sex characteristics between male and female are called intersex, this can be caused by extra sex chromosomes or by hormonal abnormality during fetal development.[125] The term intersex typically applies to abnormal members of gonochoric species rather than to hermaphroditic species.[126] Some species, such as the fruit fly (Drosophila melanogaster), and some crustaceans may have gynandromorphs.[127]

See also

References

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Further reading

  • Arnqvist G, Rowe L (2005). Sexual conflict. Princeton University Press. ISBN 978-0-691-12217-5.
  • Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2002). Molecular Biology of the Cell (4th ed.). New York: Garland Science. ISBN 978-0-8153-3218-3.
  • Ellis H (1933). Psychology of Sex. London: W. Heinemann Medical Books. N.B.: One of many books by this pioneering authority on aspects of human sexuality.
  • Gilbert SF (2000). Developmental Biology (6th ed.). Sinauer Associates, Inc. ISBN 978-0-87893-243-6.
  • Maynard-Smith J (1978). The Evolution of Sex. Cambridge University Press. ISBN 978-0-521-29302-0.
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