Editing Fertilisation (section)

==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 |pages=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}}
{{anchor#Animals}}
{{anchor|Fertilisation in animals}}

===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 |pages=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 |pages=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}}