fertilization
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Fertilisation or fertilization (see
spelling differences Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthography is the system of writing convention ...
), also known as generative fertilisation, syngamy and impregnation, is the fusion of
gametes A gamete (Help:IPA/English, /ˈɡæmiːt/; from Ancient Greek γαμετή ''gamete'' from gamein "to marry") is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reprodu ...
to give rise to a new individual organism or offspring and initiate its development. Processes such as
insemination Insemination is the introduction of sperm Sperm is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, "female" reproductive cell and a smaller, "male" one). Anim ...
or
pollination Pollination is the transfer of pollen from an anther (male part) of a plant to the stigma (female part) of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by Anemophily, wind. Pollinating agents are a ...

pollination
which happen before the fusion of gametes are also sometimes informally called fertilisation. The cycle of fertilisation and development of new individuals is called
sexual reproduction Sexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismo ...
. During
double fertilisation Double fertilization is a complex fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual orga ...
in
angiosperms The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of land plants, with 64 orders, 416 families, approximately 13,000 known genera and 300,000 known species In biology, a species is the ...
the
haploid Ploidy () is the number of complete sets of chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by ...
male gamete combines with two haploid
polar nuclei Double fertilization is a complex fertilization mechanism of flowering plants (angiosperms The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of land plants, with 64 orders, 416 families, a ...
to form a
triploid Polyploidy is a condition in which the cells of an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. ...
primary
endosperm 350px, right The endosperm is a tissue produced inside the seeds of most of the flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Embryophyte, land plants, with 64 Order(biolo ...
nucleus by the process of vegetative fertilisation.


History

In Antiquity,
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy. The term ''philosopher'' comes from the grc, φιλόσοφος, , translit ...

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
epigenetic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...
. In 1784,
Spallanzani Lazzaro Spallanzani (; 12 January 1729 – 11 February 1799) was an Italian Catholic The Catholic Church, often referred to as the Roman Catholic Church, is the List of Christian denominations by number of members, largest Christian chur ...

Spallanzani
established the need of interaction between the female's ovum and male's sperm to form a zygote in frogs. In 1827, von Baer observed a
theria Theria (; Greek language, Greek: , wild beast) is a Scientific classification, subclass of Mammal, mammals amongst the Theriiformes (the sister taxon to Yinotheria). Theria includes the Eutheria, eutherians (including the placental mammals) and ...
n mammalian egg for the first time.
Oscar Hertwig Illustration from O. Hertwig's book ''Lehrbuch der Entwicklungsgeschichte des Menschen und der Wirbeltiere'' (Textbook of developmental history of humans and vertebrates), 1906. Oscar Hertwig (21 April 1849 in Friedberg – 25 October 1922 in B ...
(1876), in Germany, described the fusion of nuclei of spermatozoa and of ova from
sea urchin Sea urchins (), are typically spiny, globular animals, echinoderm An echinoderm is any member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is ...

sea urchin
.


Evolution

The evolution of fertilisation is related to the origin of meiosis, as both are part of
sexual reproduction Sexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismo ...
, originated in
eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus enclosed within a nuclear envelope. Eukaryotes belong to the Domain (biology), domain Eukaryota or Eukarya; their name comes from the Greek language, Greek wi ...

eukaryote
s. There are two conflicting theories on how the couple meiosis–fertilisation arose. One is that it evolved from prokaryotic sex (
bacterial recombinationBacterial recombination is a type of genetic recombination Genetic recombination (also known as genetic reshuffling) is the exchange of DNA, genetic material between different organisms which leads to production of offspring with combinations of tra ...
) as eukaryotes evolved from prokaryotes. The other is that
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proces ...

mitosis
originated meiosis.


Fertilisation in plants

The gametes that participate in fertilisation of plants are the sperm (male) and the egg (female) cell. Various families of plants have differing methods by which the gametes produced by the male and female
gametophyte A gametophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one ...
s come together and are fertilised. In
Bryophyte Bryophytes are an informal group consisting of three divisions of non-vascular land plants (embryophyte The Embryophyta () or land plants are the most familiar group of green plants Plants are mainly multicellular organisms, predominantl ...
land plants, fertilisation of the sperm and egg takes place within the
archegonium 240px, Diagram of archegonium anatomy An archegonium (pl: archegonia), from the ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 3 ...
. In
seed plant The spermatophytes, also known as phanerogams (taxon Phanerogamae) or phaenogams (taxon Phaenogamae), comprise those plants that produce seeds, hence the alternative name seed plants. They are a subset of the embryophytes or land plants. The t ...
s, the male gametophyte is called a
pollen File:Pollen Tube.svg, Pollen Tube Diagram Pollen is a powdery substance consisting of pollen grains which are Sporophyte, microsporophytes of spermatophyta, seed plants, which produce male gametes (sperm cells). Pollen grains have a hard coat ma ...

pollen
grain. After
pollination Pollination is the transfer of pollen from an anther (male part) of a plant to the stigma (female part) of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by Anemophily, wind. Pollinating agents are a ...

pollination
, the
pollen File:Pollen Tube.svg, Pollen Tube Diagram Pollen is a powdery substance consisting of pollen grains which are Sporophyte, microsporophytes of spermatophyta, seed plants, which produce male gametes (sperm cells). Pollen grains have a hard coat ma ...

pollen
grain germinates, and a
pollen tube A pollen tube is a tubular structure produced by the male gametophyte of seed plants when it germinates. Pollen tube elongation is an integral stage in the plant life cycle. The pollen tube acts as a conduit to transport the male gamete cells from ...
grows and penetrates the
ovule In seed plant The spermatophytes, also known as phanerogams (taxon Phanerogamae) or phaenogams (taxon Phaenogamae), comprise those plants that produce seeds, hence the alternative name seed plants. They are a subset of the embryophytes or l ...

ovule
through a tiny pore called a
micropyleMicropyle may refer to: * Micropyle (botany) a minute opening in the integument of an ovule of a seed plant. * Micropyle (zoology), a differentiated area of surface in an egg, through which a sperm enters {{Disambiguation ...

micropyle
. The sperm are transferred from the pollen through the pollen tube to the ovule where the egg is fertilised. In
flowering plant Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλά ...

flowering plant
s, two sperm cells are released from the pollen grain, and a second fertilisation event involving the second sperm cell and the central cell of the ovule, which is a second female gamete.


Pollen tube growth

Unlike animal sperm which is motile, plant sperm is immotile and relies on the pollen tube to carry it to the ovule where the sperm is released. The pollen tube penetrates the
stigma Stigma or plural stigmata, stigmas may refer to: * Social stigma, the disapproval of a person based on physical or behavioral characteristics that distinguish them from others Symbolism * Stigmata, bodily marks or wounds resembling the crucifix ...
and elongates through the extracellular matrix of the style before reaching the ovary. Then near the receptacle, it breaks through the
ovule In seed plant The spermatophytes, also known as phanerogams (taxon Phanerogamae) or phaenogams (taxon Phaenogamae), comprise those plants that produce seeds, hence the alternative name seed plants. They are a subset of the embryophytes or l ...

ovule
through the micropyle (an opening in the ovule wall) and the pollen tube "bursts" into the embryo sac, releasing sperm. 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 revealed a family of
glycoprotein Glycoproteins are protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme ...
s called TTS proteins that enhanced growth of pollen tubes. 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. TTS proteins were also placed on various locations of semi in vevo pollinated pistils, and pollen tubes were observed to immediately extend toward the proteins. Transgenic plants lacking the ability to produce TTS proteins exhibited slower pollen tube growth and reduced fertility.


Rupture of pollen tube

The rupture of the pollen tube to release sperm in ''
Arabidopsis ''Arabidopsis'' (rockcress) is a genus in the family Brassicaceae Brassicaceae () or Cruciferae () is a medium-sized and economically important Family (biology), family of flowering plants commonly known as the mustards, the crucifers, or the c ...

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 Reactive oxygen species (ROS) are highly reactive Reactive may refer to: *Generally, capable of having a reaction (disambiguation) *An adjective abbreviation denoting a Bowling ball#Coverstock technology, bowling ball coverstock made of reactive ...
(ROS). ROS levels have been shown via 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. 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. Pistil feeding assays in which plants were fed diphenyl iodonium chloride (DPI) suppressed ROS concentrations in ''Arabidopsis'', which in turn prevented pollen tube rupture.


Bryophytes

Bryophyte is a traditional name used to refer to all embryophytes (land plants) that do not have true vascular tissue and are therefore called "non-vascular plants". Some bryophytes do have specialised tissues for the transport of water; however, since these do not contain lignin, they are not considered true vascular tissue.


Ferns

A fern is a member of a group of roughly 12,000 species of vascular plants that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular (i.e. having water-conducting vessels). They have stems and leaves, like other vascular plants. Most ferns have what are called fiddleheads that expand into fronds, which are each delicately divided.


Gymnosperms

The gymnosperms are a group of seed producing plants that includes conifers,
Cycad :''For the insect, see Cicada.'' Cycads are seed plants that typically have a stout and woody (ligneous) trunk (botany), trunk with a crown (botany), crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious ...

Cycad
s,
Ginkgo ''Ginkgo'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. ...

Ginkgo
, and
Gnetales ''Gnetum'' is a genus of gymnosperms, the sole genus in the family Gnetaceae and order Gnetales. They are tropical evergreen trees, shrub A shrub (or bush, but this is more of a gardening term) is a small- to medium-sized perennial woody ...
. The term "gymnosperm" comes from the Greek composite word γυμνόσπερμος (γυμνός gymnos, "naked" and σπέρμα sperma, "seed"), meaning "naked seeds", after the unenclosed condition of their seeds (called ovules in their unfertilised state). Their naked condition stands in contrast to the seeds and ovules of flowering plants (angiosperms), which are enclosed within an ovary. Gymnosperm seeds develop either on the surface of scales or leaves, often modified to form cones, or at the end of short stalks as in Ginkgo.


Flowering plants

After being fertilised, the ovary starts to swell and develop into the
fruit In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the ...

fruit
. 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 Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates int ...
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) – a growing pollen tube therefore contains three separate nuclei, two sperm and one tube. 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".
Double fertilisation Double fertilization is a complex fertilization mechanism of flowering plants (angiosperms). This process involves the joining of a female gametophyte (Gametophyte#Heteromorphy, megagametophyte, also called the embryo sac) with two male gametes ...
is the process in
angiosperms The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of land plants, with 64 orders, 416 families, approximately 13,000 known genera and 300,000 known species In biology, a species is the ...
(flowering plants) in which two
sperm Sperm is the male Male (♂) is the sex of an organism that produces the gamete known as sperm. A male gamete can fuse with a larger female gamete, or ovum, in the process of fertilization. A male cannot sexual reproduction, reproduce sexuall ...

sperm
from each pollen tube fertilise two
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
in a female
gametophyte A gametophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one ...
(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 Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mate ...
(2n)
zygote A zygote (from Greek ζυγωτός ''zygōtos'' "joined" or "yoked", from ζυγοῦν ''zygoun'' "to join" or "to yoke") is a eukaryotic cell formed by a fertilization event between two gamete A gamete ( /ˈɡæmiːt/; from Ancient Gree ...

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 Polyploidy is a condition in which the cells of an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. ...
(3n). This triploid cell divides through
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proces ...

mitosis
and forms the
endosperm 350px, right The endosperm is a tissue produced inside the seeds of most of the flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Embryophyte, land plants, with 64 Order(biolo ...
, a
nutrient A nutrient is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a definite chemical composition * Matter, anything that has mass and ta ...
-rich
tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubitata'', a species of geometer mot ...
, inside the
seed A seed is an embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double album, it was released to generally positiv ...

seed
. 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 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 linages, 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. In many plants, the development of the flesh of the fruit is proportional to the percentage of fertilised ovules. For example, with
watermelon Watermelon (''Citrullus lanatus'') is a flowering plant Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek Greek may refer to: Greece Any ...

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. Cross-fertilisation and self-fertilisation represent different strategies with differing benefits and costs. An estimated 48.7% of plant species are either dioecious or self-incompatible obligate out-crossers. It is also estimated that about 42% of flowering plants exhibit a mixed mating system in nature. In the most common kind of mixed mating system, individual plants produce a single type of flower and fruits may contain self-fertilised, out-crossed 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. About 10-15% of flowering plants are predominantly self-fertilising.


Self-pollination

Under circumstances where
pollinator A pollinator is an animal that moves pollen File:Pollen Tube.svg, Pollen Tube Diagram Pollen is a powdery substance consisting of pollen grains which are Sporophyte, microsporophytes of spermatophyta, seed plants, which produce male gametes ...

pollinator
s or mates are rare, self-fertilisation offers the advantage of reproductive assurance. Self-fertilisation can therefore result in improved colonisation ability. In some species, self-fertilisation has persisted over many generations. ''
Capsella rubella ''Capsella rubella'', the pink shepherd's-purse, is a plant species in the genus '' Capsella'', a very close relative of ''Arabidopsis thaliana'' and a member of the mustard family, Brassicaceae. It has a very similar appearance to ''Capsella burs ...
'' is a self-fertilisating species that became self-compatible 50,000 to 100,000 years ago. ''
Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Embryophyte, land plants, with 64 Orde ...

Arabidopsis thaliana
'' is a predominantly self-fertilising plant with an out-crossing rate in the wild of less than 0.3%; a study suggested that self-fertilisation evolved roughly a million years ago or more in ''A. thaliana''. In long-established self-fertilising plants, the masking of deleterious
mutation A red tulip exhibiting a partially yellow petal due to a mutation in its genes In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processe ...
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 recombinational repair of DNA damage during formation of germ cells provided by meiosis at each generation.


Fertilisation in animals

The mechanics behind fertilisation has been studied extensively in sea urchins and mice. This research addresses the question of how the
sperm Sperm is the male Male (♂) is the sex of an organism that produces the gamete known as sperm. A male gamete can fuse with a larger female gamete, or ovum, in the process of fertilization. A male cannot sexual reproduction, reproduce sexuall ...
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 Internal may refer to: *InternalityAn internality is the long-term benefit or cost to an individual that they do not consider when making the decision to consume a good or service. One way this is related to behavioral economics is by means of the c ...
or
external fertilisationExternal fertilization is a mode of reproduction in which a male organism's sperm fertilizes a female organism's egg outside of the female's body. It is contrasted with internal fertilization, in which sperm are introduced via insemination and then c ...
is often dependent on the method of birth.
Oviparous Oviparous animals are female animals that lay their egg Diagram of a chicken egg in its 9th day. Membranes: allantois, chorion, amnion, and vitellus/ yolk. An egg is the organic vessel containing the zygote in which an embryo develops un ...
animals laying eggs with thick calcium shells, such as
chickens The chicken (''Gallus gallus domesticus''), a subspecies of the red junglefowl, is a type of domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influen ...
, 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.
Ovoviviparous Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is an outmoded term used as a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparous reproduction. Ovoviviparous animals have the embryos develop ...
and
viviparous Among animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aero ...
animals also use internal fertilisation. It is important to note that although some organisms reproduce via
amplexus Amplexus (Latin "embrace") is a type of mating behavior exhibited by some externally fertilizing species (chiefly amphibian Amphibians are ectothermic, tetrapod Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals c ...
, they may still use internal fertilisation, as with some salamanders. Advantages to internal fertilisation include: minimal waste of gametes; greater chance of individual egg fertilisation, relatively "longer" time 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. 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. Advantages to external fertilisation include: minimal contact and transmission of bodily fluids; decreasing the risk of disease transmission, and greater genetic variation (especially during broadcast spawning external fertilisation methods).


Sea urchins

Sperm find the eggs via
chemotaxis Chemotaxis (from '' chemo-'' + '' taxis'') is the movement of an organism in response to a chemical stimulus. Somatic cells, bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They ...

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. After finding the egg, the sperm penetrates the 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. 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 fertilization cone, fertilisation cone.


Mammals

Mammals internally fertilise through copulation (zoology), copulation. After a male male ejaculation, ejaculates, 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. When sperm are deposited into the anterior vagina, they are not capable of fertilisation (i.e., non-capacitated) and are characterized by slow linear motility patterns. This motility, combined with muscular contractions enables sperm transport towards the uterus and fallopian tubes. 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 fallopian tubes (approximately 8). 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. Spermatozoa respond (see Sperm thermotaxis) to the temperature gradient of ~2 °C between the oviduct and the ampulla, 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. 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. 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 mice. 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. 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. Fusion between the oocyte plasma membranes 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. The protein CD9 likely mediates this fusion in mice (the binding homolog). 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. This process ultimately leads to the formation of a
diploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mate ...
cell called a
zygote A zygote (from Greek ζυγωτός ''zygōtos'' "joined" or "yoked", from ζυγοῦν ''zygoun'' "to join" or "to yoke") is a eukaryotic cell formed by a fertilization event between two gamete A gamete ( /ˈɡæmiːt/; from Ancient Gree ...

zygote
. The zygote divides to form a blastocyst and, upon entering the uterus, Implantation (human embryo), implants 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

Fertilisation in humans is the union of a human ovum, egg and
sperm Sperm is the male Male (♂) is the sex of an organism that produces the gamete known as sperm. A male gamete can fuse with a larger female gamete, or ovum, in the process of fertilization. A male cannot sexual reproduction, reproduce sexuall ...
, usually occurring in the ampulla of the fallopian tube, producing a
zygote A zygote (from Greek ζυγωτός ''zygōtos'' "joined" or "yoked", from ζυγοῦν ''zygoun'' "to join" or "to yoke") is a eukaryotic cell formed by a fertilization event between two gamete A gamete ( /ˈɡæmiːt/; from Ancient Gree ...

zygote
cell, or fertilised egg, initiating prenatal development. Scientists discovered the dynamics of human fertilisation in the nineteenth century. The term ''conception'' commonly refers to "the process of becoming pregnant involving fertilization or implantation or both". 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. 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. The term "conception" is not usually used in scientific literature because of its variable definition and connotation.


Insects

Insects in different groups, including the Odonata (dragonflies and damselflies) and the Hymenoptera (ants, bees, and wasps) 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. 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.


Fertilisation in fungi

In many fungi (except chytrids), 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: * gametogamy; * autogamy; * gamontogamy.


Fertilisation in algae

Fertilisation in algae occurs by binary fission. The pseudopodia is first withdrawn and the nucleus starts dividing. When the cytoplasm is divided, the cytoplasm is also divided into two equal parts for each daughter cell. Two daughter cells are produced by one parent cell. It involves the process of
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proces ...

mitosis
. 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 in fungi-like protists

Fertilisation in fungi. In many fungi (except chytrids), as in some protists, fertilisation is a two step process. ... In chytrid fungi, fertilisation occurs in a single step with the fusion of gametes, as in animals and plants.


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 chromosomes combine. In humans, (2²²)² = 17.6x1012 chromosomally different
zygote A zygote (from Greek ζυγωτός ''zygōtos'' "joined" or "yoked", from ζυγοῦν ''zygoun'' "to join" or "to yoke") is a eukaryotic cell formed by a fertilization event between two gamete A gamete ( /ˈɡæmiːt/; from Ancient Gree ...

zygote
s are possible for the non-sex chromosomes, even assuming no chromosomal crossover. If crossover occurs once, then on average (4²²)² = 309x1024 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 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 centrioles form the embryo first centrosome and microtubule Aster (cell biology), aster. The sperm centriole, found near the male pronucleus, recruit egg Pericentriolar material proteins forming the zygote first centrosome. 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 polarizes the genome inside the pronuclei.


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.


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: 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

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.
“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.”
In addition, it is thought by some, 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 * Proembryo * Pronucleus * Superfecundation * Superfetation * Symmetry breaking and cortical rotation


References


External links


Fertilisation (Conception)
video {{Authority control Reproduction Fertility Pollination