This article is about sex in sexually reproducing organisms. For the act, see sexual intercourse. For other uses, see Sex ( disambiguation )
Sex is a trait that determines an individual ‘s generative officiate, male or female, in animals and plants that propagate their species through sexual reproduction. [ 1 ] [ 2 ] The type of gametes produced by an organism specify its sex. normally in plants and animals, male organisms produce smaller gametes ( sperm, sperm ) while female organisms produce larger gametes ( ova, frequently 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 choice of the traits of each parent.
Males and females of a species may be exchangeable ( sexual monomorphism ), or have forcible differences ( sexual dimorphism ). The differences reflect the different generative pressures the sexes know. For example, mate option 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 isomorphous ( look the same ) and the gametes are isogamous ( indistinguishable in size and shape ), such as the park alga Ulva lactuca. If there are rather 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 arrangement in insects. diverse environmental systems include temperature-dependent sex determination in reptiles and crustaceans. [ 7 ]
The life bicycle of sexually reproducing organism cycles through haploid and diploid stages sexual reproduction is product of offspring by the fusion of haploid gametes. [ 8 ] [ 9 ] The codes for genetic traits are contained within the deoxyribonucleic acidic ( DNA ) of chromosomes. By combining one laid of chromosomes from each rear, an organism is formed containing a double set of chromosomes. This double-chromosome degree is called “ diploid “ while the single-chromosome degree is “ haploid “. Diploid organisms can, in turning, produce haploid cells ( gametes ) that randomly contain one of each of the chromosome pair, via litotes. [ 10 ] Meiosis besides involves a stage of chromosomal crossover in which regions of deoxyribonucleic acid are exchanged between matched types of chromosomes, to form new pairs of mix chromosomes, each of which is a blend of the genes of both parents. [ 10 ] This serve is followed by a mitotic division, producing haploid gametes that contain one laid of chromosomes. Crossing over to make fresh recombinant chromosomes and fertilization ( the coalition of two gametes ) [ 11 ] result in the newly organism containing a different set of genic traits from either parent. Gametes may be outwardly exchangeable ( isogamy ) or may differ in size and other aspects ( anisogamy ). [ 6 ] Oogamy is an extreme point case of anisogamy, in which a large, non-motile gamete is fused with a smaller, normally motile one. [ 12 ] Isogamy is very common in unicellular organisms while anisogamy is coarse in multicellular organisms. [ 13 ] Individuals that entirely produce large gametes are females, and those that entirely produce humble gametes are males. [ 14 ] [ 3 ] [ 15 ] An individual that produces both types of gametes is a hermaphrodite. [ 4 ] Some hermaphrodites such as the tinea 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 can not self-fertilize. [ 17 ] Some hermaphroditic plants are self-fertile, but plants have evolved multiple different mechanisms to avoid self-fertilization, involving consecutive androgyny ( dichogamy ), self-incompatibility or morphologic mechanisms such as heterostyly ( herkogamy ). [ 18 ] : 73, 74 In the life-cycle of plants and multicellular alga, diploid and haploid multicellular phases alternate. The diploid organism is called the sporophyte because it produces haploid spores by litotes, which, on germination, undergo mitotic cell division to produce multicellular haploid organisms, the gametophytes that produce gametes by mitosis. [ 19 ]
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 sperm ( sperm ) and female ovum ( egg cells ). [ 21 ] A sperm, produced in vertebrates within the testes, is a little cell containing a single long scourge which propels it. [ 22 ] Egg cells ( ovum ) are produced within the ovaries. In oviparous species such as birds, the fertilize egg cellular telephone or zygote is provided with egg yolk, a food add 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 sexual intercourse organs which help ecstasy of sperm. [ 25 ]
In mammals the female generative tract, called the vagina, connects with the uterus, an organ which immediately supports the development of a fertilize embryo within, a procedure called gestation. In humans and early mammals the equivalent male harmonium is the penis, which enters the vagina to achieve insemination in a process called sexual sexual intercourse. The penis contains a tube through which semen ( a fluid containing sperm ) travels. [ citation needed ] In Marsupials and placental mammals the fertilize testis develops within the female, receiving nutrition directly from its mother via a specialize organ called the placenta. [ 26 ]
In 97 % of dame species, males do not have a penis. [ 25 ] alternatively in most birds, both elimination and replica are done through a single back tooth open called the sewer. Male and female birds affect sewer to transfer sperm, a action called “ cloacal snog ”. [ 27 ]
Most aquatic animals such as fish and corals mate using external fertilization, where the eggs and sperm are released into, and combine within, the wall water system. [ 28 ] however, some species like crustaceans use inner fertilization. [ 24 ] In seahorses, females use their ovipositors to deliver eggs into the males ’ bottom for fertilization and gestation. pipefish and seahorses are the alone species that entail male pregnancy. [ 29 ]
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 regurgitate 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 ]
In the green seaweed genus Ulva, there is no sexual specialization among the isomorphous person 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 alga that have flagella. They are motile, able to swim to the testis cells of female gametophyte plants in films of water. semen plants early than Cycads and Ginkgo have lost flagellum wholly and are ineffective 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 weave to neighbor plants. other plants, such as orchids, [ 36 ] have heavier, gluey 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 besides contain female generative organs, resulting in cross-pollination .
In seed plants, male gametes are produced by highly reduced multicellular microgametophytes known as pollen. The female gametes ( testis 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 necessity for the initial development of the embryonic establish. [ 18 ] : 175
 producing separate male and female cones on the same plant. Female ( left ) and male ( right ) cones contain the sex organs of pines and other conifers. Most conifers are monoecious, producing separate male and female cones on the same implant. In pines and other conifers, the sexual activity 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 long lived and typically much larger and more durable. The ovules attached to the cone scales are not enclosed in an ovary, giving wax to the appoint gymnosperm mean ‘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 ]
Flowers contain the sexual organs of flowering plants. They normally 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 bloom 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 dwell of an ovary, a style and a stigma. Within the ovary are ovules, which contain haploid megagametophytes that produce egg cells. When a pollen granulate 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 lens nucleus to fertilize the testis cell in an ovule that finally develops into a seed. At the same clock 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 human body 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 intimate organs are said to be perfective. [ 45 ] [ 46 ] Angiosperms may besides have fallible flowers, on the like or different plants, that lack one or other type of sex organs. sometimes, as in the tree of eden ( 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 unlike trees. [ 47 ] : 398, 615 Because flowering plants are fast, they evolved flowers to attract animals such as insects to help in fertilization. [ 48 ]
Mushrooms are produced as separate of fungal intimate reproduction Most fungi are able to reproduce sexually and asexually and have both haploid and diploid stages in their life sentence cycles. [ 5 ] : 214 many fungi are isogamous, lacking male and female specialization. [ 49 ] even fungi that are anisogamic are all hermaphroditic, which is why even anisogamic fungi are considered to be mating types rather than sexes. [ 50 ] [ 51 ] : 182 Fungi may have building complex allelic mating systems and many species of fungi have two felt types. [ 52 ] however, Coprinellus disseminatus has been estimated to have about 123 felt types, and in some species there are thousands of mating types. [ 49 ] For exemplar, Schizophyllum commune has about 28,000 or more match types. [ 53 ] Some fungi, including that used as baker ‘s yeast, have mating types that create a duality similar to male and female roles. [ citation needed ] Yeast with the lapp match type do not fuse to form diploid cells, lone with yeast carrying another coupling type. [ 54 ] many species of higher fungi [ clarification needed ] produce mushrooms as separate of their sexual reproduction. Within the mushroom diploid cells are formed, late dividing into haploid spores .
sexual reproduction is coarse among parasitic protozoa but rare among free-living protozoan, which normally reproduce asexually unless food is barely or the environment changes drastically. Both anisogamy and isogamy are found in free-living protozoan. [ 55 ] Ciliates are all isogamous such as Tetrahymena thermophila, which has 7 checkmate types. [ 56 ] [ original research? ]
A sexual system is a distribution of male and female functions across organisms in a species. [ 57 ]
approximately 95 % of animal species are gonochoric ( besides known as dioecious ) and about 5 % are hermaphroditic. [ 57 ] This low percentage is due to the identical large count of insect species, in which androgyny is absent. [ 44 ] Hermaphroditism however occurs in 70 % of animal phylum. [ 58 ] Gonochoric individuals are either male or female throughout their lives. [ 59 ] Gonochorism is identical common in vertebrates, about 99 % of which gonochoric. The remaining 1 % that are hermaphroditic are about all fishes. [ 60 ] All birds and mammals are gonochoric. [ 61 ]
approximately 5 to 6 % of bloom 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 lapp establish ). [ 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 ]
development of sex
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 anisogamy A ) anisogamy of motile cells, B ) oogamy ( egg cell and sperm cellular telephone ), C ) anisogamy of non-motile cells ( egg cell and spermatia ).
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A) isogamy of different forms of isogamy : A ) isogamy of motile cells, B ) isogamy of non-motile cells, C ) conjugation. intimate conflict underlies the evolutionary distinction between male and female with the distinction starting from anisogamy. [ 65 ] The evolution of anisogamy is besides synonymous to the development of male and female sexes, [ 13 ] a well the starting point toward intimate dimorphism, [ 66 ] and lead to the development of many sex differences. [ 67 ] It is broadly accepted that anisogamy has evolved from isogamy [ 68 ] several times independently in different groups of eukaryotes, including protists, alga, plants and animals, [ 44 ] but its development has left no dodo attest. [ 69 ] Until 2006 there was no genic testify for the evolutionary yoke between sexes and mating types due to plants and animals having no isogamous relatives. [ 70 ] Anisogamy evolves due to disruptive selection leading to modest gametes and boastfully gametes. [ 71 ] In anisogamic species an average gamete is ineffective to persist. [ 72 ] There should always be two gamete types, with all analyses showing that intermediate gamete sizes are eliminated due to choice. [ 73 ] [ 74 ] As of 2016, it remains indecipherable if anisogamy foremost led to the evolution of gonochorism or the development of androgyny. [ 58 ] : 213
sex helps the spread of advantageous traits through recombination. The diagram compare the evolution of allele frequency in a sexual population ( top ) and an asexual population ( bottom ). The vertical axis shows frequency and the horizontal bloc shows fourth dimension. The alleles a/A and b/B occur at random. The advantageous alleles A and B, arising independently, can be quickly combined by intimate 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 frailty versa. The biological campaign of an organism developing into one sexual activity or the other is called sex determination. The induce may be genetic, environmental, haplodiploidy, or multiple factors. [ 44 ] Within animals and other organisms that have familial 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 bloom establish genus Silene, arouse may besides be determined by arouse chromosomes. [ 75 ] Non-genetic systems may use environmental cues, such as the temperature during early development in crocodiles, to determine the sexual activity of the young. [ 76 ] sex decision is much discrete from sex differentiation, sex determination is the appellation for the development phase towards either male or female while sex differentiation is the nerve pathway towards the growth of the phenotype. [ 77 ]
Like humans and most other mammals, the common fruit fly has an XY sex-determination organization
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 by and large based on the presence or absence of the Y chromosome. It is the male gamete that determines the sexual activity of the young. [ 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 sexual activity decision is found in early 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 besides be female, and individuals with x can be males. [ 81 ]
ZW sex decision
In birds, which have a ZW sex-determination arrangement, 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 young. This organization is used by birds, some fish, and some crustaceans. [ 7 ] The majority of butterflies and moths besides have a ZW sex-determination system. In groups like the Lepidoptera, females can have Z, ZZW, and even ZZWW. [ 83 ]
XO arouse decision
In the X0 sex-determination system, males have one X chromosome ( X0 ) while females have two ( twenty ). All other chromosomes in these diploid organisms are paired, but organisms may inherit one or two ten 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 explicate 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 rebel 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 organization is found in respective species of moths. [ 88 ]
For many species, sex is not determined by inherit traits, but alternatively by environmental factors such as temperature experienced during development or late in life. The bonelliidae larva 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 land that has previously supported hermaphrodites are influenced by the pheromone antheridiogen to develop as male. [ 89 ]
Clownfishes are initially male; the largest fish in a group becomes female Some species can change sexual activity over the course of their life, a phenomenon called consecutive hermaphroditism. [ 90 ] Teleost fishes are the only vertebrate lineage where consecutive 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 spouse changes sex. In many wrasses the reverse is true—the fish are initially female and become male when they reach a certain size. [ 91 ] Sequential hermaphroditism besides occurs in plants such as Arisaema triphyllum .
Temperature-dependent sex decision
many reptiles, including all crocodiles and most turtles, have temperature-dependent sex determination. In these species, the temperature experienced by the embryo during their development determines their sexual activity. [ 44 ] In some turtles, for exemplar, 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 ]
other insects, including honey bees and ants, use a haplodiploid sex-determination system. [ 93 ] Diploid bees and ants are broadly 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 leave in males ) rather than the categorization of chromosomes during litotes. [ 94 ]
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 indeed in what has come to be known as Fisher ‘s principle. [ 95 ] [ better source needed ] This basically says the stick to :
- 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.
Anisogamy is the cardinal dispute 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 ] arouse 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 share. In early species, there may be differences in coloration or other features, and may be so pronounce that the different sexes may be mistaken for two wholly unlike taxonomic group. [ 99 ]
sex differences in behavior
The sexes across gonochoric species normally differ in demeanor. 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 besides tend to be more choosy for who they mate with, [ 102 ] such as most boo species. [ 103 ] Males tend to be more competitive for mating than females. [ 13 ]
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 excerpt —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 match systems—presumably due to choice for achiever 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 encase of the stalk-eyed fly. [ 105 ] Females are the larger sex in a majority of animals. [ 104 ] For case, female southerly black widow spiders are typically doubly a farseeing as the males. [ 106 ] This size disparity may be associated with the monetary value 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 point, with males, such as some goosefish, living parasitically on the female. Some plant species besides exhibit dimorphism in which the females are importantly larger than the males, such as in the moss genus Dicranum [ 108 ] and the liverwort genus Sphaerocarpos. [ 109 ] There is some attest that, in these genus, the dimorphism may be tied to a sex chromosome, [ 109 ] [ 110 ] or to chemical signalling from females. [ 111 ] In birds, males frequently have a more colorful appearance and may have features ( like the hanker stern 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 rationale. [ 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 a well, who will not be encumbered with such handicaps .
intimate monomorphism is when both sexes are like in appearance and social organization. [ 113 ] In chief sexual monomorphism both sexes have alike traits, and possibly exchangeable 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 lapp sum in their offspring and both sexes are choosy for whom to mate with. [ 115 ] Sexual monomorphism is besides related to moo levels of sexual choice. [ 116 ] monogamous species tend to be sexually monomorphic. [ 117 ] All pair bonded primates are sexually monomorphic, [ 118 ] including the tarsier kin. [ 119 ] however, not all monogamous primates are sexually monomorphic, [ 118 ] there being only a few primate taxonomic group where intimate 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 ]
primary coil sexual activity characteristics are organs directly involved in reproduction such as the testes or ovaries, while secondary sex characteristics in humans for exemplar are body hair, breasts, and distribution of fatness. [ 124 ] Organisms that have intermediate sex characteristics between male and female are called hermaphrodite, this can be caused by extra sexual activity chromosomes or by hormonal abnormality during fetal exploitation. [ 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 hermaphrodite. [ 127 ]
- Human Sexual Differentiation Archived 9 February 2010 at the Wayback Machine by P. C. Sizonenko
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