View source for Fertilisation

{{short description|Union of opposite-sex gametes in sexual reproduction to form a zygote}}
{{about|fertilisation in animals and plants|fertilisation in humans specifically|Human fertilization|soil improvement|Fertilizer}}
{{redirect2|impregnate|impregnation|the process of making materials water-resistant|waterproofing}}
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{{Use British English|date=September 2025}}

[[File:Sperm-egg.jpg|right|299px|thumb|Sperm and ovum fusing]]
'''Fertilisation''' or '''fertilization''' (see [[American and British English spelling differences#-ise, -ize (-isation, -ization)|spelling differences]]), also known as '''generative fertilisation''', '''syngamy''' and '''impregnation''',<ref>{{cite web |title=impregnation |url=https://www.oxfordlearnersdictionaries.com/definition/english/impregnation |website=Oxford Advanced Learner's Dictionary }}</ref> is the fusion of [[gametes]] to give rise to a [[zygote]] and initiate its development into a new individual organism or offspring.<ref>{{Cite journal |url=https://rupress.org/jcb/article/220/10/e202102146/212606/The-cell-biology-of-fertilization-Gamete |access-date=2023-01-14 |journal=Journal of Cell Biology|doi=10.1083/jcb.202102146 |title=The cell biology of fertilization: Gamete attachment and fusion |date=2021 |last1=Siu |first1=Karen K. |last2=Serrão |first2=Vitor Hugo B. |last3=Ziyyat |first3=Ahmed |last4=Lee |first4=Jeffrey E. |volume=220 |issue=10 |pmid=34459848 |pmc=8406655 }}</ref> While processes such as [[insemination]] or [[pollination]], which happen before the fusion of gametes, are also sometimes informally referred to as fertilisation,<ref>{{cite web |title=Fertilization |url=https://www.merriam-webster.com/dictionary/fertilization |website=Merriam-Webster |access-date=July 10, 2018}}</ref> these are technically separate processes. The cycle of fertilisation and development of new individuals is called [[sexual reproduction]]. During [[double fertilisation]] in [[angiosperms]], the [[haploid]] male gamete combines with two haploid [[polar nuclei]] to form a [[triploid]] primary [[endosperm]] nucleus by the process of vegetative fertilisation.

==Discovery==
In antiquity, [[Aristotle]] conceived the formation of new individuals through fusion of male and female fluids, with form and function emerging gradually, in a mode called by him as [[Epigenesis (biology)|epigenetic]].<ref name=Maienschein>{{cite book |doi=10.1007/978-3-319-53258-5_4 |chapter=The First Century of Cell Theory: From Structural Units to Complex Living Systems |title=Integrated History and Philosophy of Science |series=Vienna Circle Institute Yearbook |year=2017 |last1=Maienschein |first1=Jane |volume=20 |pages=43–54 |isbn=978-3-319-53257-8 |s2cid=89731001 }}</ref>

In 1784, [[Spallanzani]] established the need of interaction between the female's ovum and male's sperm to form a zygote in frogs.<ref name=Birkhead/> In 1827, [[Karl Ernst von Baer]] observed a [[theria]]n mammalian egg for the first time.<ref name=Maienschein/> [[Oscar Hertwig]] (1876), in Germany, described the fusion of nuclei of spermatozoa and of ova from [[sea urchin]].<ref name="Birkhead">{{cite book |doi=10.1016/B978-0-12-372568-4.00001-X |chapter=Three centuries of sperm research |title=Sperm Biology |year=2009 |last1=Birkhead |first1=Tim R. |last2=Montgomerie |first2=Robert |pages=1–42 |isbn=978-0-12-372568-4 }}</ref>

==Evolution==
The evolution of fertilisation is related to the [[Origin and function of meiosis|origin of meiosis]], as both are part of [[sexual reproduction]], originated in [[eukaryote]]s. One hypothesis states that [[meiosis]] originated from mitosis.<ref name="Wilkins_2009">{{cite journal | vauthors = Wilkins AS, Holliday R | title = The evolution of meiosis from mitosis | journal = Genetics | volume = 181 | issue = 1 | pages = 3–12 | date = January 2009 | pmid = 19139151 | pmc = 2621177 | doi = 10.1534/genetics.108.099762 }}</ref>

==Fertilisation in plants==
[[File:DEV035048A.jpg|thumb|In the [[bryophyte]] land plants, fertilisation takes place within the [[archegonium]]. This [[moss]] has been [[GUS reporter system|genetically modified]] so that the unfertilised egg within the archegonium produces a blue colour.]]

The gametes that participate in fertilisation of plants are the sperm (male) and the egg (female) cell. Various plant groups have differing methods by which the gametes produced by the male and female [[gametophyte]]s come together and are fertilised. In [[bryophyte]]s and [[pteridophyte|pteridophytic]] land plants, fertilisation of the sperm and egg takes place within the [[archegonium]]. In [[seed plant]]s, the male gametophyte is formed within a [[pollen]] grain. After [[pollination]], the [[pollen]] grain [[Pollen germination|germinates]], and a [[pollen tube]] grows and penetrates the [[ovule]] through a tiny pore called a [[Ovule|micropyle]]. The sperm are transferred from the pollen through the pollen tube to the ovule where the egg is fertilised. In [[flowering plant]]s, two sperm cells are released from the pollen tube, and a second fertilisation event occurs involving the second sperm cell and the [[central cell]] of the ovule, which is a second female gamete.<ref name=Faure1999>{{cite book |last=Faure |first=J.E. |date=1999 |chapter=Double fertilization in flowering plants: origin, mechanisms and new information from in vitro fertilization |editor1=Cresti, M. |editor2=Cai, G. |editor3=Moscatelli, A. |title=Fertilization in Higher Plants. |pages=79–89 |publisher=Springer |location=Berlin, Heidelberg. |isbn=978-3-642-59969-9 }}</ref>

===Pollen tube growth===
Unlike animal sperm which is motile, the sperm of most seed plants is immotile and relies on the [[pollen tube]] to carry it to the ovule where the sperm is released.<ref name="Duan et al 2014">{{cite journal |doi=10.1038/ncomms4129 |pmid=24451849 |title=Reactive oxygen species mediate pollen tube rupture to release sperm for fertilization in Arabidopsis |journal=Nature Communications |volume=5 |page=3129 |year=2014 |last1=Duan |first1=Qiaohong |last2=Kita |first2=Daniel |last3=Johnson |first3=Eric A |last4=Aggarwal |first4=Mini |last5=Gates |first5=Laura |last6=Wu |first6=Hen-Ming |last7=Cheung |first7=Alice Y |bibcode=2014NatCo...5.3129D |doi-access=free }}</ref> The pollen tube penetrates the [[Stigma (botany)|stigma]] and elongates through the extracellular matrix of the style before reaching the ovary. Then near the receptacle, it breaks through the [[ovule]] through the micropyle (an opening in the ovule wall) and the pollen tube "bursts" into the embryo sac, releasing sperm.<ref name="Cheung-1995">{{cite journal |doi=10.1016/0092-8674(95)90427-1 |pmid=7634328 |title=A floral transmitting tissue-specific glycoprotein attracts pollen tubes and stimulates their growth |journal=Cell |volume=82 |issue=3 |pages=383–93 |year=1995 |last1=Cheung |first1=Alice Y |author-link1=Alice Cheung |last2=Wang |first2=Hong |last3=Wu |first3=Hen-Ming |doi-access=free }}</ref> The growth of the pollen tube has been believed to depend on chemical cues from the pistil, however these mechanisms were poorly understood until 1995. Work done on [[Nicotiana|tobacco plants]] revealed a family of [[glycoprotein]]s called TTS proteins that enhanced growth of pollen tubes.<ref name="Cheung-1995" /> Pollen tubes in a sugar free pollen germination medium and a medium with purified TTS proteins both grew. However, in the TTS medium, the tubes grew at a rate 3x that of the sugar-free medium.<ref name="Cheung-1995" /> TTS proteins were also placed on various locations of semi [[in vivo]] pollinated pistils, and pollen tubes were observed to immediately extend toward the proteins. Transgenic plants lacking the ability to produce TTS proteins had slower pollen tube growth and reduced fertility.<ref name="Cheung-1995" />

===Rupture of pollen tube===
The rupture of the pollen tube to release sperm in ''[[Arabidopsis]]'' has been shown to depend on a signal from the female gametophyte. Specific proteins called FER protein kinases present in the ovule control the production of highly reactive derivatives of oxygen called [[reactive oxygen species]] (ROS). ROS levels have been shown via [[Green fluorescent protein|GFP]] to be at their highest during floral stages when the ovule is the most receptive to pollen tubes, and lowest during times of development and following fertilisation.<ref name="Duan et al 2014" /> High amounts of ROS activate Calcium ion channels in the pollen tube, causing these channels to take up Calcium ions in large amounts. This increased uptake of calcium causes the pollen tube to rupture, and release its sperm into the ovule.<ref name="Duan et al 2014" /> Pistil feeding assays in which plants were fed diphenyl iodonium chloride (DPI) suppressed ROS concentrations in ''Arabidopsis'', which in turn prevented pollen tube rupture.<ref name="Duan et al 2014" />

===Flowering plants===
{{main|Double fertilisation}}

After being fertilised, the ovary starts to swell and develop into the [[fruit]].<ref name="facts_and_practice_for_a_level">{{cite book |last1=Johnstone |first1=Adam |title=Biology: facts & practice for A level |publisher=Oxford University Press |isbn=0-19-914766-3 |page=[https://archive.org/details/biologyfactsprac0000john/page/95 95] |year=2001 |url=https://archive.org/details/biologyfactsprac0000john/page/95 }}</ref> With multi-seeded fruits, multiple grains of pollen are necessary for syngamy with each ovule. The growth of the pollen tube is controlled by the vegetative (or tube) cytoplasm. Hydrolytic [[enzymes]] are secreted by the pollen tube that digest the female tissue as the tube grows down the stigma and style; the digested tissue is used as a nutrient source for the pollen tube as it grows. During pollen tube growth towards the ovary, the generative nucleus divides to produce two separate sperm nuclei (haploid number of chromosomes)<ref name="handbook_of_plant_science">{{cite book |title=Handbook of plant science |year=2007 |publisher=John Wiley |location=Chichester, West Sussex, England |isbn=978-0-470-05723-0 |page=466}}</ref> – a growing pollen tube therefore contains three separate nuclei, two sperm and one tube.<ref name="biology_today_a01">{{cite book |last1=Kirk |first1=David |last2=Starr |first2=Cecie |title=Biology today |year=1975 |publisher=CRM |location=Del Mar, Calif. |isbn=978-0-394-31093-0 |page=[https://archive.org/details/biologytoday00kirk/page/93 93] |url=https://archive.org/details/biologytoday00kirk |url-access=registration }}</ref> The sperms are interconnected and dimorphic, the large one, in a number of plants, is also linked to the tube nucleus and the interconnected sperm and the tube nucleus form the "male germ unit".<ref>{{cite book |last1=Raghavan |first1=Valayamghat |title=Double fertilization: embryo and endosperm development in flowering plant |url=https://archive.org/details/fertilizationhig00ragh |url-access=limited |year=2006 |publisher=Springer-Verlag |location=Berlin |isbn=978-3-540-27791-0 |page=[https://archive.org/details/fertilizationhig00ragh/page/n235 12]}}</ref>

Double fertilisation is the process in [[angiosperms]] (flowering plants) in which two [[sperm]] from each pollen tube fertilise two [[cell (biology)|cells]] in a female [[gametophyte]] (sometimes called an embryo sac) that is inside an ovule. After the pollen tube enters the gametophyte, the pollen tube nucleus disintegrates and the two sperm cells are released; one of the two sperm cells ''fertilises'' the egg cell (at the bottom of the gametophyte near the micropyle), forming a [[diploid]] (2n) [[zygote]]. This is the point when fertilisation actually occurs; pollination and fertilisation are two separate processes. The nucleus of the other sperm cell fuses with two haploid polar nuclei (contained in the central cell) in the centre of the gametophyte. The resulting cell is [[triploid]] (3n). This triploid cell divides through [[mitosis]] and forms the [[endosperm]], a [[nutrient]]-rich [[tissue (biology)|tissue]], inside the [[seed]].<ref name=Faure1999/> The two central-cell maternal nuclei (polar nuclei) that contribute to the endosperm arise by mitosis from the single meiotic product that also gave rise to the egg. Therefore, maternal contribution to the genetic constitution of the triploid endosperm is double that of the embryo.

One primitive species of flowering plant, ''[[Nuphar|Nuphar polysepala]]'', has endosperm that is diploid, resulting from the fusion of a sperm with one, rather than two, maternal nuclei. It is believed that early in the development of angiosperm lineages, there was a duplication in this mode of reproduction, producing seven-celled/eight-nucleate female gametophytes, and triploid endosperms with a 2:1 maternal to paternal genome ratio.<ref name=Friedman2003>{{cite journal |doi=10.1111/j.0014-3820.2003.tb00257.x |pmid=12683519 |title=Modularity of the Angiosperm Female Gametophyte and Its Bearing on the Early Evolution of Endosperm in Flowering Plants |journal=Evolution |volume=57 |issue=2 |pages=216–30 |year=2003 |last1=Friedman |first1=William E |last2=Williams |first2=Joseph H |doi-access=free }}</ref>

In many plants, the development of the flesh of the fruit is proportional to the percentage of fertilised ovules. For example, with [[watermelon]], about a thousand grains of pollen must be delivered and spread evenly on the three lobes of the stigma to make a normal sized and shaped fruit.{{citation needed|date=January 2024}}

===Self-pollination and outcrossing===
{{main|Self-pollination}}{{see also|Cleistogamy|Autogamy}}
[[Outcrossing]], or cross-fertilisation, and [[Autogamy|self-fertilisation]] represent different strategies with differing benefits and costs. An estimated 48.7% of plant species are either dioecious or self-incompatible obligate outcrossers.<ref name="pmid16817548">{{cite journal |vauthors=Igic B, Kohn JR |title=The distribution of plant mating systems: study bias against obligately outcrossing species |journal=Evolution |volume=60 |issue=5 |pages=1098–103 |year=2006 |pmid=16817548 |doi= 10.1554/05-383.1|s2cid=40964 }}</ref> It is also estimated that about 42% of flowering plants exhibit a mixed mating system in nature.<ref>{{cite journal | vauthors = Goodwillie C, Kalisz S, Eckert CG | s2cid = 3755371 | year = 2005 | title = The evolutionary enigma of mixed mating systems in plants: Occurrence, theoretical explanations, and empirical evidence | journal = Annu. Rev. Ecol. Evol. Syst. | volume = 36 | pages = 47–79 | doi = 10.1146/annurev.ecolsys.36.091704.175539 }}</ref>

In the most common kind of mixed mating system, individual plants produce a single type of flower and fruits may contain self-fertilised, outcrossed or a mixture of progeny types. The transition from cross-fertilisation to self-fertilisation is the most common evolutionary transition in plants, and has occurred repeatedly in many independent lineages.<ref name="pmid23595268">{{cite journal |doi=10.1098/rspb.2013.0133 |pmid=23595268 |pmc=3652455 |title=Evolutionary consequences of self-fertilization in plants |journal=Proceedings of the Royal Society B: Biological Sciences |volume=280 |issue=1760 |article-number=20130133 |year=2013 |last1=Wright |first1=S. I |last2=Kalisz |first2=S |last3=Slotte |first3=T }}</ref> About 10–15% of flowering plants are predominantly self-fertilising.<ref name="pmid23595268" />

Under circumstances where [[pollinator]]s or mates are rare, self-fertilisation offers the advantage of [[Reproductive success|reproductive assurance]].<ref name="pmid23595268" /> Self-fertilisation can therefore result in improved colonisation ability. In some species, self-fertilisation has persisted over many generations. ''[[Capsella rubella]]'' is a self-fertilising species that became self-compatible 50,000 to 100,000 years ago.<ref name="pmid24068948">{{cite journal |doi=10.1371/journal.pgen.1003754 |pmid=24068948 |pmc=3772084 |title=Genomic Identification of Founding Haplotypes Reveals the History of the Selfing Species Capsella rubella |journal=PLOS Genetics |volume=9 |issue=9 |article-number=e1003754 |year=2013 |last1=Brandvain |first1=Yaniv |last2=Slotte |first2=Tanja |last3=Hazzouri |first3=Khaled M |last4=Wright |first4=Stephen I |last5=Coop |first5=Graham |bibcode=2013arXiv1307.4118B |arxiv=1307.4118 |doi-access=free }}</ref> ''[[Arabidopsis thaliana]]'' is a predominantly self-fertilising plant with an [[Allogamy|out-crossing]] rate in the wild of less than 0.3%;<ref>{{cite journal | last1 = Abbott | first1 = RJ | last2 = Gomes | first2 = MF | year = 1989 | title = Population genetic structure and outcrossing rate of Arabidopsis thaliana (L.) Heynh | journal = Heredity | volume = 62 | issue = 3| pages = 411–418 | doi=10.1038/hdy.1989.56| doi-access = free | bibcode = 1989Hered..62..411A }}</ref> a study suggested that self-fertilisation evolved roughly a million years ago or more in ''A. thaliana''.<ref name="pmid17656687">{{cite journal |vauthors=Tang C, Toomajian C, Sherman-Broyles S, Plagnol V, Guo YL, Hu TT, Clark RM, Nasrallah JB, Weigel D, Nordborg M |title=The evolution of selfing in Arabidopsis thaliana |journal=Science |volume=317 |issue=5841 |pages=1070–2 |year=2007 |pmid=17656687 |doi=10.1126/science.1143153 |bibcode=2007Sci...317.1070T |s2cid=45853624 }}</ref> In long-established self-fertilising plants, the masking of deleterious [[mutation]]s and the production of genetic variability is infrequent and thus unlikely to provide a sufficient benefit over many generations to maintain the meiotic apparatus. Consequently, one might expect self-fertilisation to be replaced in nature by an ameiotic asexual form of reproduction that would be less costly. However the actual persistence of meiosis and self-fertilisation as a form of reproduction in long-established self-fertilising plants may be related to the immediate benefit of efficient [[DNA repair#Double-strand breaks|recombinational repair]] of DNA damage during formation of germ cells provided by meiosis at each generation.{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}}

{{anchor#Animals}}
{{anchor|Fertilization in animals}}
== Fertilisation in animals ==
{{see also|Sexual reproduction in animals|Animal sexual behaviour}}
The mechanics behind fertilisation has been studied extensively in sea urchins and mice. This research addresses the question of how the [[spermatozoon|sperm]] and the appropriate egg find each other and the question of how only one sperm gets into the egg and delivers its contents. There are three steps to fertilisation that ensure species-specificity:

# Chemotaxis
# Sperm activation/acrosomal reaction
# Sperm/egg adhesion

=== Internal vs. external ===
Consideration as to whether an animal (more specifically a vertebrate) uses [[Internal fertilization|internal]] or [[external fertilisation]] is often dependent on the method of birth. [[Oviparity|Oviparous]] animals laying eggs with thick calcium shells, such as [[chickens]], or thick leathery shells generally reproduce via internal fertilisation so that the sperm fertilises the egg without having to pass through the thick, protective, tertiary layer of the egg. [[Ovoviviparity|Ovoviviparous]] and [[viviparity|viviparous]] animals also use internal fertilisation. Although some organisms reproduce via [[amplexus]], they may still use internal fertilisation, as with some salamanders. Advantages of internal fertilisation include minimal waste of gametes, greater chance of individual egg fertilisation, longer period of egg protection, and selective fertilisation. Many females have the ability to store sperm for extended periods of time and can fertilise their eggs at their own desire.{{fact|date=August 2021}}

Oviparous animals producing eggs with thin tertiary membranes or no membranes at all, on the other hand, use external fertilisation methods. Such animals may be more precisely termed ovuliparous.<ref>{{cite book |last1=Lodé |first1=Thierry |title=Les stratégies de reproduction des animaux |trans-title=Reproduction Strategies in Animal Kingdom |language=fr |date=2001 |publisher=Dunod |isbn=978-2-10-005739-9 }}{{pn|date=August 2021}}</ref> External fertilisation is advantageous in that it minimises contact (which decreases the risk of disease transmission), and greater genetic variation.

=== Sea urchins ===
[[File:Acrosome reaction diagram en.svg|thumb|400px|Acrosome reaction on a sea urchin cell.]]

Sperm find the eggs via [[chemotaxis]], a type of ligand/receptor interaction. Resact is a 14 amino acid peptide purified from the jelly coat of ''A. punctulata'' that attracts the migration of sperm.<ref>{{Cite web |last=Hardin |first=Jeff |title=Dynamics of Development:  Sperm chemotaxis |url=https://worms.zoology.wisc.edu/dd2/echino/fert/chemotaxis/chemotaxis.html |access-date=2025-10-07 |website=worms.zoology.wisc.edu}}</ref>

After finding the egg, the sperm penetrates the [[egg jelly|jelly coat]] through a process called sperm activation. In another ligand/receptor interaction, an oligosaccharide component of the egg binds and activates a receptor on the sperm and causes the [[acrosomal reaction]]. The acrosomal vesicles of the sperm fuse with the plasma membrane and are released. In this process, molecules bound to the acrosomal vesicle membrane, such as bindin, are exposed on the surface of the sperm. These contents digest the jelly coat and eventually the vitelline membrane. In addition to the release of acrosomal vesicles, there is explosive polymerisation of [[actin]] to form a thin spike at the head of the sperm called the [[acrosomal process]].<ref>{{Cite web |last=Hardin |first=Jeff |title=Dynamics of Development:  Acrosome reaction |url=https://worms.zoology.wisc.edu/dd2/echino/fert/acrosome/acrosome.html |access-date=2025-10-07 |website=worms.zoology.wisc.edu}}</ref>

The sperm binds to the egg through another ligand reaction between receptors on the [[vitelline membrane]]. The sperm surface protein bindin, binds to a receptor on the vitelline membrane identified as EBR1.

Fusion of the plasma membranes of the sperm and egg are likely mediated by bindin. At the site of contact, fusion causes the formation of a fertilisation cone.<ref>{{Cite web |last=Hardin |first=Jeff |title=Dynamics of Development:  Pronuclei |url=https://worms.zoology.wisc.edu/dd2/echino/fert/pronuclear/pronuclear.html |access-date=2025-10-07 |website=worms.zoology.wisc.edu}}</ref>

=== Mammals ===
Male [[mammal]]s internally fertilise females and [[ejaculation|ejaculate]] [[semen]] through the [[penis]] during [[copulation (zoology)|copulation]].<ref name="Hyman1992">{{cite book |author=Libbie Henrietta Hyman |url=https://books.google.com/books?id=VKlWjdOkiMwC&pg=PA583 |title=Hyman's Comparative Vertebrate Anatomy |date=15 September 1992 |publisher=University of Chicago Press |isbn=978-0-226-87013-7 |pages=583–}}</ref><ref>{{Cite book |last=Naguib |first=Marc |url=https://books.google.com/books?id=KgTeDwAAQBAJ&pg=PA65 |title=Advances in the Study of Behavior |date=2020-04-19 |publisher=Academic Press |isbn=978-0-12-820726-0 |language=en}}</ref> After ejaculation, many sperm move to the upper vagina (via contractions from the vagina) through the [[cervix]] and across the length of the [[uterus]] to meet the ovum. In cases where fertilisation occurs, the female usually [[ovulation|ovulates]] during a period that extends from hours before copulation to a few days after; therefore, in most mammals, it is more common for ejaculation to precede ovulation than vice versa.{{Citation needed|date=July 2024}}

When sperm are deposited into the anterior vagina, they are not capable of fertilisation (i.e., non-capacitated){{clarification needed|reason=statements appear contradictory|date=January 2024}} and are characterised by slow linear motility patterns. This motility, combined with muscular contractions enables sperm transport towards the uterus and [[oviduct]]s.<ref>{{Cite journal|title=Sperm transport in the female reproductive tract|journal=Human Reproduction Update|volume=12|issue=1|pages=23–37|doi=10.1093/humupd/dmi047|pmid=16272225|year=2006|last1=Suarez|first1=S.S.|last2=Pacey|first2=A. A.|doi-access=free}}</ref> There is a pH gradient within the micro-environment of the female reproductive tract such that the pH near the vaginal opening is lower (approximately 5) than the oviducts (approximately 8).<ref>{{cite journal |last1=Ng |first1=Ka Ying Bonnie |last2=Mingels |first2=Roel |last3=Morgan |first3=Hywel |last4=Macklon |first4=Nick |last5=Cheong |first5=Ying |title=In vivo oxygen, temperature and pH dynamics in the female reproductive tract and their importance in human conception: a systematic review |journal=Human Reproduction Update |date=1 January 2018 |volume=24 |issue=1 |pages=15–34 |doi=10.1093/humupd/dmx028 |pmid=29077897 |doi-access=free }}</ref> The sperm-specific pH-sensitive calcium transport protein called CatSper increases the sperm cell permeability to calcium as it moves further into the reproductive tract. Intracellular calcium influx contributes to sperm capacitation and hyperactivation, causing a more violent and rapid non-linear motility pattern as sperm approach the oocyte. The [[capacitation|capacitated]] spermatozoon and the oocyte meet and interact in the ''ampulla'' of the [[fallopian tube]]. Rheotaxis, thermotaxis and chemotaxis are known mechanisms that guide sperm towards the egg during the final stage of sperm migration.<ref>{{cite journal |doi=10.1530/JOE-16-0302 |pmid=27875265 |title=Oviduct: Roles in fertilization and early embryo development |journal=Journal of Endocrinology |volume=232 |issue=1 |pages=R1–R26 |year=2016 |last1=Li |first1=Shuai |last2=Winuthayanon |first2=Wipawee |doi-access=free }}</ref> Spermatozoa respond (see [[Sperm thermotaxis]]) to the temperature gradient of ~2&nbsp;°C between the oviduct and the ampulla,<ref>{{cite journal |doi=10.1016/j.mce.2006.03.027 |pmid=16672171 |title=Sperm thermotaxis |journal=Molecular and Cellular Endocrinology |volume=252 |issue=1–2 |pages=115–9 |year=2006 |last1=Bahat |first1=Anat |last2=Eisenbach |first2=Michael |s2cid=22094855 |url=https://zenodo.org/record/995804 }}</ref> and [[Chemotaxis#Eukaryotic chemotaxis|chemotactic]] gradients of [[progesterone]] have been confirmed as the signal emanating from the [[cumulus oophorus]] cells surrounding rabbit and human oocytes.<ref>{{cite journal |doi=10.1371/journal.pone.0008211 |pmid=19997608 |pmc=2782141 |title=Molecular Mechanism for Human Sperm Chemotaxis Mediated by Progesterone |journal=PLOS ONE |volume=4 |issue=12 |article-number=e8211 |year=2009 |last1=Teves |first1=Maria E |last2=Guidobaldi |first2=Hector A |last3=Uñates |first3=Diego R |last4=Sanchez |first4=Raul |last5=Miska |first5=Werner |last6=Publicover |first6=Stephen J |last7=Morales Garcia |first7=Aduén A |last8=Giojalas |first8=Laura C |bibcode=2009PLoSO...4.8211T |doi-access=free }}</ref> Capacitated and hyperactivated sperm respond to these gradients by changing their behaviour and moving towards the cumulus-oocyte complex. Other chemotactic signals such as formyl Met-Leu-Phe (fMLF) may also guide spermatozoa.<ref name=Gnessi1986>{{Cite journal |vauthors=Gnessi L, Fabbri A, Silvestroni L, Moretti C, Fraioli F, Pert CB, Isidori A | year = 1986 | title = Evidence for the presence of specific receptors for N-formyl chemotactic peptides on human spermatozoa| journal = J Clin Endocrinol Metab| volume = 63| pages = 841–846| pmid =3018025 |doi = 10.1210/jcem-63-4-841| issue = 4 }}</ref>

The [[zona pellucida]], a thick layer of extracellular matrix that surrounds the egg and is similar to the role of the vitelline membrane in sea urchins, binds the sperm. Unlike sea urchins, the sperm binds to the egg before the acrosomal reaction. [[ZP3]], a glycoprotein in the zona pellucida, is responsible for egg/sperm adhesion in humans. The receptor [[galactosyltransferase]] (GalT) binds to the N-acetylglucosamine residues on the ZP3 and is important for binding with the sperm and activating the acrosome reaction. ZP3 is sufficient though unnecessary for sperm/egg binding. Two additional sperm receptors exist: a 250kD protein that binds to an oviduct secreted protein, and SED1, which independently binds to the zona. After the acrosome reaction, the sperm is believed to remain bound to the zona pellucida through exposed ZP2 receptors. These receptors are unknown in mice but have been identified in guinea pigs.{{fact|date=August 2021}}

In mammals, the binding of the spermatozoon to the GalT initiates the [[acrosome reaction]]. This process releases the [[hyaluronidase]] that digests the matrix of [[hyaluronic acid]] in the vestments around the oocyte. Additionally, heparin-like glycosaminoglycans (GAGs) are released near the oocyte that promote the acrosome reaction.<ref>{{cite journal |last1=Lee |first1=C. N. |last2=Clayton |first2=M. K. |last3=Bushmeyer |first3=S. M. |last4=First |first4=N. L. |last5=Ax |first5=R. L. |title=Glycosaminoglycans in Ewe Reproductive Tracts and Their Influence on Acrosome Reactions in Bovine Spermatozoa in Vitro |journal=Journal of Animal Science |date=1 September 1986 |volume=63 |issue=3 |pages=861–867 |doi=10.2527/jas1986.633861x |pmid=3759713 |s2cid=853558 }}</ref> Fusion between the oocyte [[plasma membrane]]s and sperm follows and allows the sperm [[cell nucleus|nucleus]], the typical [[centriole]], and atypical [[centriole]] that is attached to the [[flagellum]], but not the [[mitochondrion|mitochondria]], to enter the oocyte.<ref>{{cite journal |doi=10.1038/s41467-018-04678-8 |pmid=29880810 |title=A novel atypical sperm centriole is functional during human fertilization |journal=Nature Communications |volume=9 |issue=1 |page=2210 |year=2018 |last1=Fishman |first1=Emily L |last2=Jo |first2=Kyoung |last3=Nguyen |first3=Quynh P. H |last4=Kong |first4=Dong |last5=Royfman |first5=Rachel |last6=Cekic |first6=Anthony R |last7=Khanal |first7=Sushil |last8=Miller |first8=Ann L |last9=Simerly |first9=Calvin |last10=Schatten |first10=Gerald |last11=Loncarek |first11=Jadranka |last12=Mennella |first12=Vito |last13=Avidor-Reiss |first13=Tomer |bibcode=2018NatCo...9.2210F |pmc=5992222 }}</ref> The protein [[CD9]] likely mediates this fusion in mice (the binding homologue). The egg "[[egg activation|activates]]" itself upon fusing with a single sperm cell and thereby changes its cell membrane to prevent fusion with other sperm. [[Zinc]] atoms are released during this activation.{{citation needed|date=July 2019}}
{{Commons|Fertilization#Mammalian fertilization|Mammalian fertilisation}}
This process ultimately leads to the formation of a [[diploid]] cell called a [[zygote]]. The zygote divides to form a [[blastocyst]] and, upon entering the uterus, [[Implantation (human embryo)|implant]]s in the endometrium, beginning [[pregnancy]]. Embryonic implantation not in the [[uterus|uterine]] wall results in an [[ectopic pregnancy]] that can kill the mother.

In such animals as rabbits, coitus induces ovulation by stimulating the release of the pituitary hormone gonadotropin; this release greatly increases the likelihood of pregnancy.

==== Humans ====
[[File:Human Fertilization.png|thumb|Fertilisation in humans. The sperm and ovum unite through fertilisation, creating a zygote that (over the course of 8–9 days) implants in the uterine wall, where it resides for nine months.]]
{{main|Human fertilisation}}

Fertilisation in humans is the union of a human [[ovum|egg]] and [[spermatozoon|sperm]], usually occurring in the [[ampulla of the fallopian tube]], producing a single celled [[zygote]], the first stage in the development of a genetically unique organism,<ref>{{Cite web |title=Zygote {{!}} Definition, Development, Example, & Facts {{!}} Britannica |url=https://www.britannica.com/science/zygote |access-date=2023-01-14 |website=www.britannica.com |language=en}}</ref> and initiating [[Human embryonic development|embryonic development]]. Scientists discovered the dynamics of [[human fertilisation]] in the nineteenth century.

The term ''conception'' commonly refers to "the process of becoming pregnant involving fertilisation or implantation or both".<ref>{{cite web |title=Conception |url=https://www.merriam-webster.com/dictionary/conception |website=Merriam-Webster |access-date=January 27, 2019}}</ref> Its use makes it a subject of [[beginning of pregnancy controversy|semantic arguments about the beginning of pregnancy]], typically in the context of the [[abortion]] debate.
<!-- In a statement by the [[American Association of Pro-Life Obstetricians & Gynecologists]] (AAPLOG), regarding the controversial [[morning-after pill]], AAPLOG claims:-->
<!-- NOTE: This is a quote for a developing section, if you have a problem with it, hide the text (like this text is) but don't remove it, until I can rework it. Sinreg, -->
<!--:"[Again,] one must be careful of the terminology. Many now speak of "conception" as that moment when the human blastocyst, the early ball of approximately 100 cells, implants in the mother's uterus (womb).  The time from actual fertilisation (egg and sperm unite in the Fallopian Tube) until implantation, a period of about 7-10 days, is ignored, even though no genetic change occurs in the cells during this time period.  Many family planning specialists who have supported the terminology change can thus rationalise that the destruction of the human embryo between fertilisation and implantation should be labelled "contraception", rather than "early abortion".-->
Upon [[gastrulation]], which occurs around 16 days after fertilisation, the implanted blastocyst develops three germ layers, the endoderm, the ectoderm and the mesoderm, and the genetic code of the father becomes fully involved in the development of the embryo; later twinning is impossible. Additionally, interspecies hybrids survive only until gastrulation and cannot further develop.
However, some human developmental biology literature refers to the ''[[conceptus]]'' and such medical literature refers to the "products of conception" as the post-implantation embryo and its surrounding membranes.<ref>{{cite book |author1=Moore, K. L. |author2=T. V. M. Persaud |year=2003 |title=The Developing Human: Clinically Oriented Embryology |publisher=W. B. Saunders Company |isbn=0-7216-6974-3 |url=https://archive.org/details/developinghumanc00moor_0 }}{{pn|date=August 2021}}</ref> The term "conception" is not usually used in scientific literature because of its variable definition and connotation.

=== Insects ===
[[File:Voo nupcial detail.jpg|thumb|upright|Red-veined darters (''[[Sympetrum fonscolombii]]'') flying "in cop" (male ahead), enabling the male to prevent other males from mating. The eggs are fertilised as they are laid, one at a time.]]
{{expand section|date=February 2015}}<!--at least cover dragonflies, bees-->

Insects in different groups, including the [[Odonata]] ([[dragonflies]] and [[damselflies]]) and the [[Hymenoptera]] ([[ant]]s, [[bee]]s, and [[wasp]]s) practise delayed fertilisation. Among the Odonata, females may mate with multiple males, and store sperm until the eggs are laid. The male may hover above the female during egg-laying (oviposition) to prevent her from mating with other males and replacing his sperm; in some groups such as the darters, the male continues to grasp the female with his claspers during egg-laying, the pair flying around in tandem.<ref>{{cite book |last=Dijkstra |first=Klaas-Douwe B. |title=Field Guide to the Dragonflies of Britain and Europe |year=2006 |publisher=British Wildlife Publishing |isbn=0-9531399-4-8|pages=8–9}}</ref> Among social Hymenoptera, [[honeybee]] queens mate only on mating flights, in a short period lasting some days; a queen may mate with eight or more [[drone (bee)|drones]]. She then stores the sperm for the rest of her life, perhaps for five years or more.<ref>{{cite book |last1=Waldbauer |first1=Gilbert |title=The Birder's Bug Book|date=1998 |publisher=Harvard University Press}}</ref><ref>{{cite web|last1=Agriculture and Consumer Protection|title=Beekeeping in Africa: Colony life and social organization|url=http://www.fao.org/docrep/t0104e/t0104e05.htm|publisher=FAO}}</ref><!--other bees ...-->

==Fertilisation in fungi==
{{main|Mating in fungi}}
In many [[fungi]] (except [[chytrid]]s), as in some protists, fertilisation is a two step process. First, the cytoplasms of the two gamete cells fuse (called [[plasmogamy]]), producing a [[dikaryon|dikaryotic]] or [[Heterokaryon|heterokaryotic]] cell with multiple nuclei. This cell may then divide to produce dikaryotic or heterokaryotic [[hyphae]]. The second step of fertilisation is [[karyogamy]], the fusion of the nuclei to form a diploid zygote.

In [[chytrid]] fungi, fertilisation occurs in a single step with the fusion of gametes, as in animals and plants.

==Fertilisation in protists==

===Fertilisation in protozoa===
There are three types of fertilisation processes in protozoa:<ref>{{cite book |last1=Tarin |first1=Juan J. |last2=Cano |first2=Antonio |title=Fertilization in Protozoa and Metazoan Animals: Cellular and Molecular Aspects |date=2000 |publisher=Springer Science & Business Media |isbn=978-3-540-67093-3 }}{{pn|date=August 2021}}</ref>
* gametogamy;
* [[Reproduction#Autogamy|autogamy]];<ref>{{Cite web | url=https://www.britannica.com/EBchecked/topic/44777/autogamy |title = Autogamy &#124; biology}}</ref>
* gamontogamy.

===Fertilisation in algae===
Algae, like some land plants, undergo [[alternation of generations]]. Some algae are isomorphic, where both the sporophyte (2n) and gameteophyte (n) are the same morphologically. When algae reproduction is described as oogamous, the male and female gametes are different morphologically, where there is a large non-motile egg for female gametes, and the male gamete are uniflagellate (motile). Via the process of syngamy, these will form a new zygote, regenerating the sporophyte generation again.

== Fertilisation and genetic recombination ==
[[Meiosis]] results in a random segregation of the genes that each parent contributes. Each parent organism is usually identical save for a fraction of their genes; each [[gamete]] is therefore genetically unique. At fertilisation, parental [[chromosome]]s combine. In [[human]]s, (2²²)² = 17.6x10<sup>12</sup> chromosomally different [[zygote]]s are possible for the non-sex chromosomes, even assuming no [[chromosomal crossover]]. If crossover occurs once, then on average (4²²)² = 309x10<sup>24</sup> genetically different zygotes are possible for every couple, not considering that crossover events can take place at most points along each chromosome. The X and Y chromosomes undergo no crossover events{{citation needed|date=April 2015}} and are therefore excluded from the calculation. The [[mitochondrial DNA]] is only inherited from the maternal parent.

== The sperm aster and zygote centrosomes ==
Shortly after the sperm fuse with the egg, the two sperm [[centriole]]s form the embryo first [[centrosome]] and microtubule [[Aster (cell biology)|aster]].<ref>{{cite journal |last1=Avidor-Reiss |first1=Tomer |last2=Mazur |first2=Matthew |last3=Fishman |first3=Emily L. |last4=Sindhwani |first4=Puneet |title=The Role of Sperm Centrioles in Human Reproduction – The Known and the Unknown |journal=Frontiers in Cell and Developmental Biology |date=1 October 2019 |volume=7 |page=188 |doi=10.3389/fcell.2019.00188 |pmid=31632960 |pmc=6781795 |doi-access=free }}</ref> The sperm centriole, found near the male pronucleus, recruit egg [[Pericentriolar material]] proteins forming the zygote first centrosome.<ref>{{cite journal |last1=Fishman |first1=Emily L. |last2=Jo |first2=Kyoung |last3=Nguyen |first3=Quynh P. H. |last4=Kong |first4=Dong |last5=Royfman |first5=Rachel |last6=Cekic |first6=Anthony R. |last7=Khanal |first7=Sushil |last8=Miller |first8=Ann L. |last9=Simerly |first9=Calvin |last10=Schatten |first10=Gerald |last11=Loncarek |first11=Jadranka |last12=Mennella |first12=Vito |last13=Avidor-Reiss |first13=Tomer |title=A novel atypical sperm centriole is functional during human fertilization |journal=Nature Communications |date=December 2018 |volume=9 |issue=1 |page=2210 |doi=10.1038/s41467-018-04678-8 |pmid=29880810 |pmc=5992222 |bibcode=2018NatCo...9.2210F }}</ref> This centrosome nucleates microtubules in the shape of stars called astral microtubules. The microtubules span the whole valium of the egg, allowing the egg pronucleus to use the cables to get to the male pronucleus. As the male and female pronuclei approach each other, the single centrosome split into two centrosomes located in the interphase between the pronuclei. Then the centrosome via the astral microtubules polarises the genome inside the pronuclei.<ref>{{cite journal |last1=Cavazza |first1=Tommaso |last2=Takeda |first2=Yuko |last3=Politi |first3=Antonio Z. |last4=Aushev |first4=Magomet |last5=Aldag |first5=Patrick |last6=Baker |first6=Clara |last7=Choudhary |first7=Meenakshi |last8=Bucevičius |first8=Jonas |last9=Lukinavičius |first9=Gražvydas |last10=Elder |first10=Kay |last11=Blayney |first11=Martyn |last12=Lucas-Hahn |first12=Andrea |last13=Niemann |first13=Heiner |last14=Herbert |first14=Mary |last15=Schuh |first15=Melina |title=Parental genome unification is highly error-prone in mammalian embryos |journal=Cell |date=May 2021 |volume=184 |issue=11 |pages=2860–2877.e22 |doi=10.1016/j.cell.2021.04.013 |pmid=33964210 |pmc=8162515 }}</ref>

==Parthenogenesis==
{{main|Parthenogenesis}}
Organisms that normally reproduce sexually can also reproduce via [[parthenogenesis]], wherein an unfertilised female gamete produces viable offspring. These offspring may be clones of the mother, or in some cases genetically differ from her but inherit only part of her DNA. Parthenogenesis occurs in many plants and animals and may be induced in others through a chemical or electrical stimulus to the egg cell. In 2004, Japanese researchers led by [[Tomohiro Kono]] succeeded after 457 attempts to merge the [[ovum|ova]] of two mice by blocking certain proteins that would normally prevent the possibility; the resulting embryo normally developed into a mouse.<ref>{{cite journal |doi=10.1038/nature02402 |pmid=15103378 |title=Birth of parthenogenetic mice that can develop to adulthood |journal=Nature |volume=428 |issue=6985 |pages=860–4 |year=2004 |last1=Kono |first1=Tomohiro |last2=Obata |first2=Yayoi |last3=Wu |first3=Quiong |last4=Niwa |first4=Katsutoshi |last5=Ono |first5=Yukiko |last6=Yamamoto |first6=Yuji |last7=Park |first7=Eun Sung |last8=Seo |first8=Jeong-Sun |last9=Ogawa |first9=Hidehiko |bibcode=2004Natur.428..860K |s2cid=4353479}}
* {{cite magazine |author=Bijal P. Trivedi |date=2004-04-21 |title=The End of Males? Mouse Made to Reproduce Without Sperm |magazine=National Geographic |url=http://news.nationalgeographic.com/news/2004/04/0421_040421_whoneedsmales.html |archive-url=https://web.archive.org/web/20040423005226/http://news.nationalgeographic.com/news/2004/04/0421_040421_whoneedsmales.html |archive-date=2004-04-23}}</ref>

==Allogamy and autogamy==
[[Allogamy]], which is also known as cross-fertilisation, refers to the fertilisation of an egg cell from one individual with the male gamete of another.

Autogamy which is also known as self-fertilisation, occurs in such hermaphroditic organisms as plants and flatworms; therein, two gametes from one individual fuse.

==Other variants of bisexual reproduction==

Some relatively unusual forms of reproduction are:<ref name=Stenberg2013>{{cite journal |doi=10.1159/000351731 |pmid=23796636 |title=Meiosis and Its Deviations in Polyploid Animals |journal=Cytogenetic and Genome Research |volume=140 |issue=2–4 |pages=185–203 |year=2013 |last1=Stenberg |first1=P |last2=Saura |first2=A |s2cid=19955975 }}</ref><ref>{{cite journal |doi=10.1098/rspb.2011.1738 |pmid=21993502 |title=Simultaneous Mendelian and clonal genome transmission in a sexually reproducing, all-triploid vertebrate |journal=Proceedings of the Royal Society B: Biological Sciences |volume=279 |issue=1732 |pages=1293–1299 |year=2011 |last1=Stock |first1=M |last2=Ustinova |first2=J |last3=Betto-Colliard |first3=C |last4=Schartl |first4=M |last5=Moritz |first5=C |last6=Perrin |first6=N |pmc=3282369 }}</ref>

[[Gynogenesis]]: A sperm stimulates the egg to develop without fertilisation or syngamy. The sperm may enter the egg.

[[Hybridogenesis]]: One genome is eliminated to produce haploid eggs.

[[Canina meiosis]]: (sometimes called "permanent odd polyploidy") one genome is transmitted in the Mendelian fashion, others are transmitted clonally.

==Benefits of cross-fertilisation==
{{main|Allogamy|Heterosis}}
The major benefit of cross-fertilisation is generally thought to be the avoidance of [[inbreeding depression]]. [[Charles Darwin]], in his 1876 book ''[[The Effects of Cross and Self Fertilisation in the Vegetable Kingdom]]'' (pages 466-467) summed up his findings in the following way.<ref>{{cite book |last1=Darwin |first1=Charles |title=The effects of cross and self fertilisation in the vegetable kingdom |date=1876 |publisher=J. Murray |oclc=57556547 |pages=466–467 |url=https://www.gutenberg.org/ebooks/4346 }}</ref>
<blockquote>"It has been shown in the present volume that the offspring from the union of two distinct individuals, especially if their progenitors have been subjected to very different conditions, have an immense advantage in height, weight, constitutional vigour and fertility over the self-fertilised offspring from one of the same parents. And this fact is amply sufficient to account for the development of the sexual elements, that is, for the genesis of the two sexes."</blockquote>

In addition, it is thought by some,<ref>{{cite journal |doi=10.1042/BST0340519 |pmid=16856849 |title=Why have sex? The population genetics of sex and recombination |journal=Biochemical Society Transactions |volume=34 |issue=4 |pages=519–22 |year=2006 |last1=Otto |first1=S.P |last2=Gerstein |first2=A.C |s2cid=14542950 }}</ref> that a long-term advantage of out-crossing in nature is increased genetic variability that promotes adaptation or avoidance of extinction (see [[Genetic variability]]).

==See also==
* [[Cell fusion]]
* [[Conception cap]]
* [[Conception device]]
* [[Female sperm]]
* [[Fetal development]]
* [[In vitro fertilisation]]
* [[Kaguya (mouse)]]
* [[Parthenogenesis]], a type of reproduction that does not involve fertilisation
* [[Pollination]]
* [[Pre-embryo]]
* [[Pronucleus]]
* [[Superfecundation]]
* [[Superfetation]]
* [[Symmetry breaking and cortical rotation]]
*[[Cortical reaction]]
*[[Polyspermy]]

==References==
{{Reflist}}

==External links==
* [https://www.youtube.com/watch?v=BFrVmDgh4v4 Fertilisation (Conception)] video
*[https://worms.zoology.wisc.edu/dd2/index.html Dynamics of Development] Includes echinoderm discussion and a links page for the development of various animals.
{{Animalbirth}}
{{Sex (biology)}}
{{Embryology}}
{{Reproductive physiology}}

{{Authority control}}

[[Category:Reproduction]]
[[Category:Fertility]]
[[Category:Pollination]]