Sexual reproduction
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/* Reproduction in fungi */
'''Sexual reproduction''' is characterized by processes that pass a [[Genetic recombination|combination]] of [[Genetics|genetic]] material to [[offspring]], resulting in [[Genetic diversity|diversity]]. The main two processes are: [[meiosis]], involving the halving of the number of [[chromosome]]s; and [[fertilization]], involving the fusion of two [[gamete]]s and the restoration of the original number of chromosomes. During meiosis, the chromosomes of each pair usually [[chromosomal crossover|cross over]] to achieve [[genetic recombination]].
The [[evolution of sex]] is a major puzzle. The first [[fossil]]ized evidence of sexually reproducing [[organism]]s is from [[eukaryote]]s of the [[Stenian]] period, about 1.2 to 1 billion years ago.<ref>{{cite web | last = Orgel | first = Leslie E. | url = http://proxy.arts.uci.edu/~nideffer/Hawking/early_proto/orgel.html | title = The Origin of Life on the Earth | publisher = Scientific American | accessdate = 2007-01-03 }}</ref> Sexual reproduction is the primary method of reproduction for the vast majority of visible organisms, including almost all [[animal]]s and [[plant]]s. [[Bacterial conjugation]], the transfer of [[DNA]] between two [[bacteria]], is often mistakenly confused with sexual reproduction, because the mechanics are similar.
A major question is why sexual reproduction persists when [[parthenogenesis]] appears in some ways to be a superior form of [[reproduction]]. Contemporary evolutionary thought proposes some explanations. It may be due to selection pressure on the [[clade]] itself—the ability for a population to radiate more rapidly due to a changing environment through sexual recombination than parthenogenesis allows. Alternatively, sexual reproduction may allow for the "ratcheting" of evolutionary speed as one clade competes with another for a limited resource.
[[Image:Sexual cycle.svg|thumb|250px|right|In the first stage of sexual reproduction, "meiosis," the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n). During "fertilization," haploid gametes come together to form a diploid zygote and the original number of chromosomes (2n) is restored.]]
==Reproduction in insects==
Insect species make-up more than two-thirds of all [[extant]] animal species, and most insect species use sex for reproduction, though some species are facultatively [[parthenogenetic]] . Typically they have two sexes with males producing spermatozoa and females ovum, the ovums develop into eggs that have a covering called the [[chorion]] forms before internal fertilization. Insects have very diverse mating and reproductive strategies most often resulting in the male depositing [[spermatophore]] within the female, which stores the sperm until she is ready for egg fertilization. After fertilization, and the formation of a zygote, and varying degrees of development; the eggs are deposited outside the female or they develop further within the female and live born offspring are produced.
==Reproduction in fungi==
[[Fungi]] are classified by the methods of sexual reproduction they employ. The outcome of sexual reproduction most often is the production of [[resting spore]]s that are used to survive inclement times and to spread. There are typically three phases in the sexual reproduction of fungi, [[plasmogarny]], [[karyogamy]] and meiosis.
==Reproduction in plants==
{{main|Plant sexuality}}
In [[flowering plant]]s,the [[Stamen|anther]] produces male [[gametophyte]]s, the sperm is produced in [[pollen|pollen grains]], which attach to the stigma on top of a [[carpel]], in which the female gametophytes (inside ovules) are located. After the pollen tube grows through the carpel's style, the [[sex]] 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 termed ''double 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 a [[fruit]], which surrounds the seed(s). Plants may either [[Self-pollination|self-pollinate]] or [[pollination|cross-pollinate]]. Nonflowering plants like [[fern]]s, [[moss]] and [[liverwort]]s use other means of sexual reproduction.
Ferns typically produce large diploid [[sporophyte]]s with [[rhizome]]s, roots and leaves; and on fertile leaves called [[sporangium]], [[spore]]s are produced. The spores are released and germinate to produce short, thin gametophytes that are typically heart shaped, small and green in color. The gametophytes or [[thallus]], produce both motile sperm in the [[antheridia]] and egg cells in separate [[archegonia]]. After rains or when dew deposits a film of water, the motile sperm are splashed away from the antheridia, which are normally produce on the top side of the thallus, and swim in the film of water to the antheridia 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 diffrent thallus. A [[zygote]] is formed after fertilization, which grows into a new sporophytic plant. The condition of having separate sporephyte and gametophyte plants is call [[alternation of generation]]s. Other plants with similar reproductive means include the ''[[Psilotum]]'', ''[[Lycopodium]]'', ''[[Selaginella]]'' and ''[[Equisetum]]''.
==Reproduction in mammals==
There are three kind of mammals; Monotremes, Placentals and Marsupials, all with internal fertilisation yet do still differ from each other. In [[placenta]]l mammals, [[offspring]] are born as juveniles: complete animals with the [[sex organ]]s present although not reproductively functional. After several months or years, the sex organs develop further to maturity and the animal becomes [[Sexual maturity|sexually mature]]. Most female mammals are only [[fertility|fertile]] during certain periods and during those times, they are said to be "in heat". At this point, the animal is ready to mate. Individual male and female mammals meet and carry out [[copulation]]. For most mammals, males and females exchange sexual partners throughout their adult lives.
===The mammalian male===
{{seedetails|Male reproductive system (human)}}
The male reproductive system contains two main divisions: the [[penis]], and the [[testicle|testicles]], the latter of which is where sperm are produced. In humans, both of these organs are outside the [[abdominal cavity]], but they can be primarily housed within the abdomen in other animals (for instance, in [[dog]]s, the penis is internal except when mating). Having the testicles outside the abdomen best facilitates [[Thermoregulation|temperature regulation]] of the sperm, which require specific temperatures to survive.
Sperm are the smaller of the two gametes and are generally very short-lived, requiring males to produce them continuously from the time of [[sexual maturity]] until death. Prior to ejaculation the produced sperm are stored in the seminal vesicle, a small gland that is located just behind the bladder.<br />A sperm cell is motile and swims via [[chemotaxis]], using its flagellum to propel itself towards the ovum.
===The mammalian female===
{{seedetails|Female reproductive system (human)}}
The female reproductive system likewise contains two main divisions: the [[vagina]] and [[uterus]], which act as the receptacle for the sperm, and the [[Ovary|ovaries]], which produce the female's [[Ovum|ova]]. All of these parts are always internal. The vagina is attached to the uterus through the [[cervix]], while the uterus is attached to the ovaries via the [[Fallopian tube]]s. At certain intervals, the ovaries release an ovum, which passes through the fallopian tube into the uterus.
If, in this transit, it meets with [[Spermatozoon|sperm]], the sperm penetrate and merge with the egg, [[fertilization|fertilizing]] it. The fertilization usually occurs in the oviducts, but can happen in the uterus itself. The zygote then [[implantation|implants]] itself in the wall of the uterus, where it begins the processes of [[embryogenesis]] and [[morphogenesis]]. When developed enough to survive outside the womb, the cervix dilates and contractions of the uterus propel the fetus through the birth canal, which is the vagina.
The ova, which are the female sex cells, are much larger than the sperm and are normally formed with in the ovaries of the fetus before its birth. They are mostly fixed in location with in the ovary until their transit to the uterus, and contain nutrients for the later [[zygote]] and [[embryo]]. Over a regular interval, in response to hormonal signals, a process of [[oogenesis]] matures one ovum which is released and sent down the Fallopian tube. If not fertilized, this egg is flushed out of the system through [[menstrual cycle|menstruation]] in humans and other [[Hominidae|great apes]] and reabsorbed in other mammals in the [[estrus cycle]].
===Gestation===
{{main|Pregnancy (mammals)|Pregnancy}}
Gestation, called ''pregnancy'' in humans, is the period of time during which the fetus develops, dividing via [[mitosis]] inside the female. During this time, the fetus receives all of its [[nutrition]] and oxygenated blood from the female, filtered through the [[placenta]], which is attached to the fetus' [[abdomen]] via an [[umbilical cord]]. This drain of [[nutrient]]s can be quite taxing on the female, who is required to ingest slightly higher levels of [[calorie]]s. In addition, certain [[vitamin]]s and other nutrients are required in greater quantities than normal, often creating abnormal eating habits. The length of gestation, called the [[gestation period]], varies greatly from species to species; it is 40 weeks in humans, 56–60 in [[giraffe]]s and 16 days in [[hamster]]s.
===Birth===
{{main|Childbirth}}
Once the fetus is sufficiently developed, chemical signals start the process of birth, which begins with contractions of the uterus and the dilation of the cervix. The fetus then descends to the cervix, where it is pushed out into the vagina, and eventually out of the female. The newborn, which is called an [[infant]] in humans, should typically begin [[Respiration (physiology)|respiration]] on its own shortly after birth. Not long after, the placenta is passed as well. Most mammals eat this, as it is a good source of protein and other vital nutrients needed for caring for the young. The end of the umbilical cord attached to the young’s abdomen eventually falls off on its own.
===Monotremes===
[[Monotreme]]s, only five species of which exist, all from [[Australia]] and [[New Guinea]], lay [[egg (biology)|egg]]s. They have one opening for excretion and reproduction called the [[cloaca]]. They hold the eggs internally for several weeks, providing nutrients, and then lay them and cover them like [[bird]]s. After less than two weeks the young hatches and crawls into its mother’s pouch, much like marsupials, where it nurses for several weeks as it grows.
==Marsupials==
[[Marsupial]]s reproduce in essentially the same manner, though their young are born at a far earlier stage of development than other mammals. After birth, marsupial [[Joey (marsupial)|joeys]] crawl into their mother’s pouch and attach to a [[teat]], where they receive nourishment and finish developing into self-sufficient animals.
==Fish==
The vast majority of fish species lay eggs that are then fertilized by the male,<ref>[http://www.seaworld.org/animal-info/info-books/bony-fish/reproduction.htm BONY FISHES - Reproduction<!-- Bot generated title -->]</ref> 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. Fishes that have live-bearing offspring include the [[Guppy]] and Mollies or ''[[Poecilia]]''. Fishes that give birth to live young can be [[ovoviviparous]], were the eggs are fertilized within the female and the eggs simply hatch within the female body, or they can be [[viviparous]], were the female supplies nourishment to the internally growing offspring. Some fish are hermaphrodites, 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.<ref> Orlando EF, Katsu Y, Miyagawa S, Iguchi T. J Mol Endocrinol. 2006 Oct;37(2):353-65. ''Cloning and differential expression of estrogen receptor and aromatase genes in the self-fertilizing hermaphrodite and male mangrove rivulus, Kryptolebias marmoratus.''
Department of Biological Sciences, Florida Atlantic University, North, Ft. Pierce, Florida 34946, USA. eorlando@fau.edu</ref> One fish species does not need sexual reproduction to produce offspring; ''[[Poecilia formosa]]'' can use [[parthenogenesis]] for reproduction, where unfertilized eggs develop into embryos that produce female offspring.<ref> I. Schlupp, J. Parzefall, J. T. Epplen, M. Schartl
''Limia vittata as host species for the Amazon molly: no evidence for sexual reproduction''
Journal of Fish Biology
Volume 48 Issue 4 Page 792-795, April 1996 (1996)
Journal of Fish Biology 48 (4), 792–795.
doi:10.1111/j.1095-8649.1996.tb01472.x </ref>
==See also==
*[[Asexual reproduction]]
*[[Biological reproduction]]
*[[Mating in fungi]]
*[[Operational sex ratio]]
*[[Reptile]]
*[[Sex organ]]
*[[Sexual intercourse]]
*[[Anisogamy]]
*[[Isogamy]]
==References==
<references/>
# Pang, K. "Certificate Biology: New Mastering Basic Concepts", Hong Kong, 2004
# [http://www.biolreprod.org/ Journal of Biology of Reproduction], accessed in August 2005.
[[Category:Developmental biology]]
[[Category:Reproduction]]
[[Category:Sexuality]]
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