Gametes of both mating individuals can be the same size and shape, a condition known asisogamy. By contrast, in the majority of species, the gametes are of different sizes, a condition known asanisogamy orheterogamy that applies to humans and other mammals. The human ovum has approximately 100,000 times the volume of a single human sperm cell. The type of gamete an organism produces determines itssex[4] and sets the basis for the sexual roles andsexual selection.[5]
In humans and other species that produce twomorphologically distinct types of gametes, and in whicheach individual produces only one type, afemale is any individual that produces the larger type of gamete called anovum, and amale produces the smaller type, called asperm cell or spermatozoon. Sperm cells are small and motile due to the presence of a tail-shaped structure, theflagellum, that provides propulsion. In contrast, each egg cell or ovum is comparably large and non-motile.[2]
Oogenesis, the process of female gamete formation in animals, involvesmeiosis (includingmeiotic recombination) of a diploid primaryoocyte to produce a haploidovum.Spermatogenesis, the process of male gamete formation in animals, involves meiosis in a diploid primaryspermatocyte to produce haploidspermatozoa. In animals, ova are produced in the ovaries of females and sperm develop in the testes of males. During fertilization, a spermatozoon and an ovum, each carrying half of thegenetic information of an individual, unite to form azygote that develops into a newdiploid organism.[2]
It is generally accepted thatisogamy is the ancestral state from whichanisogamy andoogamy evolved, although its evolution has left no fossil records.[6][7][8] There are almost invariably only two gamete types, all analyses showing that intermediate gamete sizes are eliminated due to selection.[9][10] Since intermediate sized gametes do not have the same advantages as small or large ones,[11] they do worse than small ones in mobility and numbers, and worse than large ones in supply.[12]
In contrast to a gamete, which has only one set of chromosomes, a diploidsomatic cell has two sets ofhomologous chromosomes, one of which is a copy of thechromosome set from the sperm and one a copy of the chromosome set from the egg cell.Recombination of the genes during meiosis ensures that the chromosomes of gametes are not exact duplicates of either of the sets of chromosomes carried in the parental diploid chromosomes but a mixture of the two.[13]
A human spermatozoon fusing with a human ovum. The spermatozoon is approximately 100,000 times smaller in size than the human ovum.
Artificial gametes, also known as in vitro derived gametes (IVD), stem cell-derived gametes (SCDGs), and in vitro generated gametes (IVG), are gametes derived fromstem cells. The use of such artificial gametes would "necessarily requireIVF techniques".[14] Research shows that artificial gametes may be a reproductive technique for same-sex male couples, although asurrogate mother would still be required for the gestation period.[14] Women who have passedmenopause may be able to produce eggs and bear genetically related children with artificial gametes.[14] Robert Sparrow wrote, in theJournal of Medical Ethics, that embryos derived from artificial gametes could be used to derive new gametes and this process could be repeated to create multiple human generations in the laboratory.[15] This technique could be used to createcell lines for medical applications and for studying the heredity ofgenetic disorders.[15] Additionally, this technique could be used forhuman enhancement byselectively breeding for a desired genome or by usingrecombinant DNA technology to create enhancements that have not arisen in nature.[15]
Plants that reproduce sexually also produce gametes. However, since plants have a life cycle involvingalternation of diploid and haploid generations some differences from animal life cycles exist. Plants usemeiosis to produce spores that develop intomulticellular haploidgametophytes which produce gametes by mitosis. In animals there is no corresponding multicellular haploid phase. The sperm of plants that reproduce using spores are formed by mitosis in an organ of the gametophyte known as theantheridium and the egg cells by mitosis in a flask-shaped organ called thearchegonium.[16] Plant sperm cells are their only motile cells, often described as flagellate, but more correctly as ciliate.[17]
Bryophytes have 2 flagella, horsetails have up to 200 and the mature spermatozoa of thecycadZamia pumila has up to 50,000 flagella.[18] Cycads andGinkgo biloba are the only gymnosperms with motile sperm.[17]
In theflowering plants, the female gametophyte is produced inside theovule within theovary of the flower. When mature, the haploid gametophyte produces female gametes which are ready for fertilization. The male gametophyte is produced inside apollen grain within theanther and is non-motile, but can be distributed by wind, water or animal vectors. When a pollen grain lands on a maturestigma of a flower it germinates to form apollen tube that grows down the style into the ovary of the flower and then into the ovule.
The pollen then produces non-motile sperm nuclei by mitosis that are transported down the pollen tube to the ovule where they are released for fertilization of the egg cell.
^Sporne KR (2022).The morphology of Pteridophytes; the structure of ferns and allied plants. Legare Street Press.ISBN978-1-01-550566-7.
^abWolniak SM, Klink VP, Hart PE, Tsai CW (June 2000). "Control of development and motility in the spermatozoids of lower plants".Gravitational and Space Biology Bulletin.13 (2):85–93.PMID11543285.
