In the first stage of sexual reproduction,meiosis, the number of chromosomes is reduced from adiploid number (2n) to ahaploid number (n). Duringfertilisation, haploid gametes come together to form a diploidzygote, and the original number of chromosomes is restored.
Sexual reproduction is a type ofreproduction that involves a complexlife cycle in which agamete (haploid reproductive cells, such as asperm oregg cell) with a single set ofchromosomes combines with another gamete to produce azygote that develops into an organism composed ofcells with two sets of chromosomes (diploid).[1] This is typical in animals, though the number of chromosome sets and how that number changes in sexual reproduction varies, especially among plants, fungi, and othereukaryotes.[2][3]
Sexual reproduction is the most common life cycle inmulticellular eukaryotes, such asanimals,fungi andplants.[6][7] Sexual reproduction also occurs in someunicellular eukaryotes.[2][8] Sexual reproduction does not occur inprokaryotes, unicellular organisms withoutcell nuclei, such asbacteria andarchaea. However, some processes in bacteria, includingbacterial conjugation,transformation andtransduction, may be considered analogous to sexual reproduction in that they incorporate new genetic information.[9] Someproteins and other features that are key for sexual reproduction may have arisen in bacteria, but sexual reproduction is believed to have developed in an ancient eukaryotic ancestor.[10]
In eukaryotes, diploid precursor cells divide to produce haploid cells in a process calledmeiosis. In meiosis, DNA is replicated to produce a total of four copies of each chromosome. This is followed by two cell divisions to generate haploid gametes. After the DNA is replicated in meiosis, thehomologous chromosomes pair up so that theirDNA sequences are aligned with each other. During this period before cell divisions, genetic information is exchanged between homologous chromosomes ingenetic recombination. Homologous chromosomes contain highly similar but not identical information, and by exchanging similar but not identical regions, genetic recombination increases genetic diversity among future generations.[11]
During sexual reproduction, two haploid gametes combine into one diploid cell known as azygote in a process calledfertilization. The nuclei from the gametes fuse, and each gamete contributes half of the genetic material of the zygote. Multiple cell divisions bymitosis (without change in the number of chromosomes) then develop into a multicellular diploid phase or generation. In plants, the diploid phase, known as thesporophyte, produces spores by meiosis. These spores then germinate and divide by mitosis to form a haploid multicellular phase, thegametophyte, which produces gametes directly by mitosis. This type of life cycle, involving alternation between two multicellular phases, the sexual haploid gametophyte and asexual diploid sporophyte, is known asalternation of generations.
Theevolution of sexual reproduction is considered paradoxical,[12] becauseasexual reproduction should be able to outperform it as every young organism created can bear its own young. This implies that an asexual population has an intrinsic capacity to grow more rapidly with each generation.[13] This 50% cost is afitness disadvantage of sexual reproduction.[14] The two-fold cost of sex includes this cost and the fact that any organism can only pass on 50% of its own genes to its offspring. However, one definite advantage of sexual reproduction is that it increases genetic diversity and impedes the accumulation of harmful geneticmutations.[15][11]
Sexual selection is a mode ofnatural selection in which some individuals out-reproduce others of a population because they are better at securingmates for sexual reproduction.[16][failed verification][17] It has been described as "a powerful evolutionary force that does not exist in asexual populations".[18]
The firstfossilized evidence of sexual reproduction in eukaryotes is from theStenian period, about 1.05 billion years old.[19][20]
Biologists studyingevolution propose several explanations for the development of sexual reproduction and its maintenance. These reasons include reducing the likelihood of theaccumulation of deleterious mutations, increasing rate ofadaptation to changing environments,[21]dealing with competition,DNA repair, masking deleterious mutations, and reducing genetic variation on the genomic level.[22][23][24][25] All of these ideas about why sexual reproduction has been maintained are generally supported, but ultimately the size of the population determines if sexual reproduction is entirely beneficial. Largerpopulations appear to respond more quickly to some of the benefits obtained through sexual reproduction than do smaller population sizes.[26]
However, newer models presented in recent years suggest a basic advantage for sexual reproduction in slowly reproducingcomplex organisms.[citation needed]
Sexual reproduction allows these species to exhibit characteristics that depend on the specificenvironment that they inhabit, and the particular survival strategies that they employ.[27]
In order to reproduce sexually, both males and females need to find amate. Generally in animalsmate choice is made by females while males compete to be chosen. This can leadorganisms to extreme efforts in order to reproduce, such as combat and display, or produce extreme features caused by apositive feedback known as aFisherian runaway. Thus sexual reproduction, as a form ofnatural selection, has an effect onevolution.Sexual dimorphism is where the basicphenotypic traits vary between males and females of the samespecies. Dimorphism is found in bothsex organs and insecondary sex characteristics, body size, physical strength and morphology,biological ornamentation,behavior and other bodily traits. However, sexual selection is only implied over an extended period of time leading to sexual dimorphism.[28]
Animals have different ways of going about sexual selection. One common example is with male peacocks fanning out their wings in order to show all their colors and attract a female mate. Lions with bigger and fuller manes are more likely to attract a female mate. Male deer with larger antlers are more likely to gain a female mate. These are just few of many examples in nature that show how sexual selection would be used in nature when females are choosing a mate.[citation needed]
A few arthropods, such asbarnacles, arehermaphroditic, that is, each can have the organs of bothsexes. However, individuals of most species remain of one sex their entire lives.[29] A few species ofinsects and crustaceans can reproduce byparthenogenesis, especially if conditions favor a "population explosion". However, most arthropods rely on sexual reproduction, and parthenogenetic species often revert to sexual reproduction when conditions become less favorable.[30] The ability to undergomeiosis is widespread among arthropods including both those that reproduce sexually and those that reproduceparthenogenetically.[31] Although meiosis is a major characteristic of arthropods, understanding of its fundamental adaptive benefit has long been regarded as an unresolved problem,[32] that appears to have remained unsettled.
Aquatic arthropods may breed by external fertilization, as for examplehorseshoe crabs do,[33] or byinternal fertilization, where theova remain in the female's body and thesperm must somehow be inserted. All known terrestrial arthropods use internal fertilization.Opiliones (harvestmen),millipedes, and some crustaceans use modified appendages such asgonopods orpenises to transfer the sperm directly to the female. However, most maleterrestrial arthropods producespermatophores, waterproof packets ofsperm, which the females take into their bodies. A few such species rely on females to find spermatophores that have already been deposited on the ground, but in most cases males only deposit spermatophores when complexcourtship rituals look likely to be successful.[29]
The nauplius larva of apenaeid shrimpMost arthropods lay eggs,[29] but scorpions areovoviviparous: they produce live young after the eggs have hatched inside the mother, and are noted for prolonged maternal care.[34] Newly born arthropods have diverse forms, and insects alone cover the range of extremes. Some hatch as apparently miniature adults (direct development), and in some cases, such assilverfish, the hatchlings do not feed and may be helpless until after their first moult. Many insects hatch as grubs orcaterpillars, which do not have segmented limbs or hardened cuticles, andmetamorphose into adult forms by entering an inactive phase in which the larval tissues are broken down and re-used to build the adult body.[35]Dragonfly larvae have the typical cuticles and jointed limbs of arthropods but are flightless water-breathers with extendable jaws.[36] Crustaceans commonly hatch as tinynauplius larvae that have only three segments and pairs of appendages.[29]
Insect species make up more than two-thirds of allextant animal species. Most insect species reproduce sexually, though some species are facultativelyparthenogenetic. Many insect species havesexual dimorphism, while in others the sexes look nearly identical. Typically they have two sexes with males producing spermatozoa and females ova. The ova develop into eggs that have a covering called thechorion, which forms before internal fertilization. Insects have very diverse mating and reproductive strategies most often resulting in the male depositing aspermatophore within the female, which she stores until she is ready for egg fertilization. After fertilization, and the formation of a zygote, and varying degrees of development, in many species the eggs are deposited outside the female; while in others, they develop further within the female and the young are born live.[37]
There are three extant kinds of mammals:monotremes,placentals andmarsupials, all with internal fertilization. In placental mammals, offspring are born as juveniles: complete animals with thesex organs present although not reproductively functional. After several months or years, depending on the species, the sex organs develop further to maturity and the animal becomessexually mature. Most female mammals are onlyfertile during certain periods during theirestrous cycle, at which point they are ready to mate.[38] For most mammals, males and femalesexchange sexual partners throughout their adult lives.[39][40][41]
The vast majority of fish species lay eggs that are then fertilized by the male.[42] Some species lay their eggs on a substrate like a rock or on plants, while others scatter their eggs and the eggs are fertilized as they drift or sink in the water column.
Some fish species use internal fertilization and then disperse the developing eggs or give birth to live offspring. Fish that have live-bearing offspring include theguppy and mollies orPoecilia. Fishes that give birth to live young can beovoviviparous, where the eggs are fertilized within the female and the eggs simply hatch within the female body, or inseahorses, the male carries the developing young within a pouch, and gives birth to live young.[43] Fishes can also beviviparous, where the female supplies nourishment to the internally growing offspring. Some fish arehermaphrodites, where a single fish is both male and female and can produce eggs and sperm. In hermaphroditic fish, some are male and female at the same time while in other fish they are serially hermaphroditic; starting as one sex and changing to the other. In at least one hermaphroditic species, self-fertilization occurs when the eggs and sperm are released together. Internal self-fertilization may occur in some other species.[44] One fish species does not reproduce by sexual reproduction but uses sex to produce offspring;Poecilia formosa is a unisex species that uses a form ofparthenogenesis calledgynogenesis, where unfertilized eggs develop into embryos that produce female offspring.Poecilia formosa mate with males of other fish species that use internal fertilization, the sperm does not fertilize the eggs but stimulates the growth of the eggs which develops into embryos.[45]
Reptiles generally reproduce sexually, though some are capable of asexual reproduction. All reproductive activity occurs through the cloaca, the single exit/entrance at the base of the tail where waste is also eliminated. Most reptiles have copulatory organs, which are usually retracted or inverted and stored inside the body. In turtles and crocodilians, the male has a single median penis, while squamates, including snakes and lizards, possess a pair of hemipenes, only one of which is typically used in each session. Tuatara, however, lack copulatory organs, and so the male and female simply press their cloacas together as the male discharges sperm. Most reptiles lay amniotic eggs covered with leathery or calcareous shells.
Asexual reproduction has been identified in squamates in six families of lizards and one snake. In some species of squamates, a population of females is able to produce a unisexual diploid clone of the mother.
Animals have life cycles with a single diploid multicellular phase that produces haploid gametes directly by meiosis. Male gametes are called sperm, and female gametes are called eggs or ova. In animals, fertilization of the ovum by a sperm results in the formation of a diploid zygote that develops by repeated mitotic divisions into a diploid adult. Plants have two multicellular life-cycle phases, resulting in analternation of generations. Plant zygotes germinate and divide repeatedly by mitosis to produce a diploid multicellular organism known as the sporophyte. The mature sporophyte produces haploid spores by meiosis that germinate and divide by mitosis to form a multicellular gametophyte phase that produces gametes at maturity. The gametophytes of different groups of plants vary in size. Mosses and other pteridophytic plants may have gametophytes consisting of several million cells, whileangiosperms have as few as three cells in each pollen grain.
Flowers contain the sexual organs of flowering plants.
Flowering plants are the dominant plant form on land[46]: 168, 173 and they reproduce either sexually or asexually. Often their most distinctive feature is their reproductive organs, commonly called flowers. Theanther producespollen grains which contain the malegametophytes that produce sperm nuclei. For pollination to occur, pollen grains must attach to the stigma of the female reproductive structure (carpel), where the female gametophytes are located within ovules enclose within theovary. After the pollen tube grows through the carpel's style, thesex cell nuclei from the pollen grain migrate into the ovule to fertilize the egg cell and endosperm nuclei within the female gametophyte in a process termeddouble fertilization. The resulting zygote develops into an embryo, while the triploid endosperm (one sperm cell plus two female cells) and female tissues of the ovule give rise to the surrounding tissues in the developing seed. The ovary, which produced the female gametophyte(s), then grows into afruit, which surrounds the seed(s). Plants may eitherself-pollinate orcross-pollinate.
In 2013, flowers dating from theCretaceous (100 million years before present) were found encased in amber, the oldest evidence of sexual reproduction in a flowering plant. Microscopic images showed tubes growing out of pollen and penetrating the flower's stigma. The pollen was sticky, suggesting it was carried by insects.[47]
Ferns produce large diploidsporophytes withrhizomes, roots and leaves. Fertile leaves producesporangia that contain haploidspores. The spores are released and germinate to produce small, thin gametophytes that are typically heart shaped and green in color. The gametophyteprothalli, produce motile sperm in theantheridia and egg cells inarchegonia on the same or different plants.[48] After rains or when dew deposits a film of water, the motile sperm are splashed away from the antheridia, which are normally produced on the top side of the thallus, and swim in the film of water to the archegonia where they fertilize the egg. To promote out crossing or cross fertilization the sperm are released before the eggs are receptive of the sperm, making it more likely that the sperm will fertilize the eggs of different thallus. After fertilization, azygote is formed which grows into a new sporophytic plant. The condition of having separate sporophyte and gametophyte plants is calledalternation of generations.
Thebryophytes, which includeliverworts,hornworts andmosses, reproduce both sexually andvegetatively. They are small plants found growing in moist locations and like ferns, have motile sperm withflagella and need water to facilitate sexual reproduction. These plants start as a haploid spore that grows into the dominant gametophyte form, which is a multicellular haploid body with leaf-like structures thatphotosynthesize. Haploid gametes are produced in antheridia (male) and archegonia (female) by mitosis. The sperm released from the antheridia respond to chemicals released by ripe archegonia and swim to them in a film of water and fertilize the egg cells thus producing a zygote. Thezygote divides by mitotic division and grows into a multicellular, diploid sporophyte. The sporophyte produces spore capsules (sporangia), which are connected by stalks (setae) to the archegonia. The spore capsules produce spores by meiosis and when ripe the capsules burst open to release the spores. Bryophytes show considerable variation in their reproductive structures and the above is a basic outline. Also in some species each plant is one sex (dioicous) while other species produce both sexes on the same plant (monoicous).[49]
Fungi are classified by the methods of sexual reproduction they employ. The outcome of sexual reproduction most often is the production ofresting spores that are used to survive inclement times and to spread. There are typically three phases in the sexual reproduction of fungi:plasmogamy,karyogamy andmeiosis. The cytoplasm of two parent cells fuse during plasmogamy and the nuclei fuse during karyogamy. New haploid gametes are formed during meiosis and develop into spores. The adaptive basis for the maintenance of sexual reproduction in theAscomycota andBasidiomycota (dikaryon)fungi was reviewed by Wallen and Perlin.[50] They concluded that the most plausible reason for maintaining this capability is the benefit ofrepairing DNA damage, caused by a variety of stresses, throughrecombination that occurs duringmeiosis.[50]
Bacterial transformation involves therecombination of genetic material and its function is mainly associated withDNA repair. Bacterial transformation is a complex process encoded by numerous bacterial genes, and is a bacterial adaptation for DNA transfer.[22][23] This process occurs naturally in at least 40 bacterial species.[51] For a bacterium to bind, take up, and recombine exogenous DNA into its chromosome, it must enter a special physiological state referred to as competence (seeNatural competence). Sexual reproduction in early single-celled eukaryotes may have evolved from bacterial transformation,[24] or from a similar process inarchaea (see below).
On the other hand, bacterial conjugation is a type of direct transfer of DNA between two bacteria mediated by an external appendage called the conjugation pilus.[52] Bacterial conjugation is controlled byplasmid genes that are adapted for spreading copies of the plasmid between bacteria. The infrequent integration of a plasmid into a host bacterial chromosome, and the subsequent transfer of a part of the host chromosome to another cell do not appear to be bacterial adaptations.[22][53]
Exposure of hyperthermophilic archaeal Sulfolobus species to DNA damaging conditions induces cellular aggregation accompanied by high frequencygenetic marker exchange[54][55] Ajon et al.[55] hypothesized that this cellular aggregation enhances species-specific DNA repair by homologous recombination. DNA transfer inSulfolobus may be an early form of sexual interaction similar to the more well-studied bacterial transformation systems that also involve species-specific DNA transfer leading to homologous recombinational repair of DNA damage.
^abBernstein, Harris; Bernstein, Carol (2010). "Evolutionary Origin of Recombination during Meiosis".BioScience.60 (7):498–505.doi:10.1525/bio.2010.60.7.5.S2CID86663600.
^Schurko, A. M.; Mazur, D. J.; Logsdon, J. M. (February 2010). "Inventory and phylogenomic distribution of meiotic genes in Nasonia vitripennis and among diverse arthropods".Insect Molecular Biology.19 (Suppl 1):165–180.doi:10.1111/j.1365-2583.2009.00948.x.PMID20167026.S2CID11617147.
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