Fertilisation

Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, insemination, pollination,[1] fecundation, syngamy and impregnation,[2] is the fusion of gametes to initiate the development of a new individual organism or offspring.[3] This 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.

Sperm-egg
Sperm and ovum fusing

History

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 epigenetic.[4]

In 1784, Spallanzani established the need of interaction between the female's ovum and male's sperm to form a zygote in frogs.[5] In 1827, von Baer observed a therian mammalian egg for the first time.[4] Oscar Hertwig (1876), in Germany, described the fusion of nuclei of spermatozoa and of ova from sea urchin.[5]

Evolution

The evolution of fertilisation is related to the origin of meiosis, as both are part of sexual reproduction, originated in eukaryotes. There are two conflicting theories on how the couple meiosis–fertilisation arose. One is that it evolved from prokaryotic sex (bacterial recombination) as eukaryotes evolved from prokaryotes.[6] The other is that mitosis originated meiosis.[7]

Fertilisation in plants

DEV035048A
In the Bryophyte land plants, fertilisation takes place within the archegonium. This moss has been genetically modified so that the unfertilised egg within the archegonium produces a blue colour.

The gametes that participate in fertilisation of plants are the pollen (male), and the egg (female) cell. Various families of plants have differing methods by which the female gametophyte is fertilized. In Bryophyte land plants, fertilisation takes place within the archegonium. In flowering plants a second fertilisation event involves another sperm cell and the central cell which is a second female gamete. In flowering plants there are two sperm from each pollen grain.

In seed plants, after pollination, a pollen grain germinates, and a pollen tube grows and penetrates the ovule through a tiny pore called a micropyle.The sperm are transferred from the pollen through the pollen tube to the ovule.

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.[8] The pollen tube penetrates the 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.[9] 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 tobacco plants revealed a family of glycoproteins called TTS proteins that enhanced growth of pollen tubes.[9] 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.[9] 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.[9]

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).[10] 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 fertilization.[8] 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.[8] Pistil feeding assays in which plants were fed diphenyl iodonium chloride (DPI) suppressed ROS concentrations in Arabidopsis, which in turn prevented pollen tube rupture.[8]

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, Cycads, Ginkgo, and Gnetales. 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.[11] 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)[12] – a growing pollen tube therefore contains three separate nuclei, two sperm and one tube.[13] 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".[14]

Double fertilisation is the process in angiosperms (flowering plants) in which two sperm from each pollen tube fertilise two 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, inside the 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.[15]

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.

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.[16] It is also estimated that about 42% of flowering plants exhibit a mixed mating system in nature.[17]

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.[18] About 10-15% of flowering plants are predominantly self-fertilising.[18]

Self-pollination

Under circumstances where pollinators and/or mates are rare, self-fertilisation offers the advantage of reproductive assurance.[18] Self-fertilisation can therefore result in improved colonisation ability. In some species, self-fertilisation has persisted over many generations. Capsella rubella is a self-fertilisating species that became self-compatible 50,000 to 100,000 years ago.[19] Arabidopsis thaliana is a predominantly self-fertilising plant with an out-crossing rate in the wild of less than 0.3%;[20] a study suggested that self-fertilisation evolved roughly a million years ago or more in A. thaliana.[21] In long-established self-fertilising plants, the masking of deleterious mutations 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.[22]

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 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:

  1. Chemotaxis
  2. Sperm activation/acrosomal reaction
  3. Sperm/egg adhesion

Internal vs. external

Consideration as to whether an animal (more specifically a vertebrate) uses internal or external fertilisation is often dependent on the method of birth. 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. Ovoviviparous and viviparous animals also use internal fertilisation. It is important to note that although some organisms reproduce via amplexus, 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. 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

Acrosome reaction diagram en
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.

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 fertilisation cone.

Mammals

Mammals internally fertilise through copulation. After a male 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 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 cells are deposited into the anterior vagina, they are not capable of fertilization (i.e., non-capacitated) and are characterized by slow linear motility patters. This motility pattern, combined with muscular contractions enables sperm transport towards the uterus and fallopian tubes.[23] There is a pH gradient within the microenvironment of the female reproductive tract such that the pH near the vaginal opening is lower (approximately 5.0) than the fallopian tubes (approximately 8.0).[24] 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 the sperm approach the oocyte. The capacitated spermatozoon and the oocyte meet and interact in the ampulla of the fallopian tube. Rheotaxis, thermotaixs and chemotaxis are known mechanisms in guiding sperm towards the egg during the final stage of sperm migration.[25] Spermatozoa respond (see Sperm thermotaxis) to the temperature gradient of ~2 °C between the oviduct and the ampulla,[26] and chemotactic gradients of progesterone have been confirmed as the signal emanating from the cumulus oophorus cells surrounding rabbit and human oocytes.[27] 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.[28]

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 with 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.[29] Fusion between the oocyte plasma membranes and sperm follows and allows the sperm nucleus, the typical centriole, and atypical centriole that is attached to the flagellum, but not the mitochondria, to enter the oocyte.[30] The protein CD9 likely mediates this fusion in mice (the binding homolog). The egg "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.[31]

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, implants in the endometrium, beginning pregnancy. Embryonic implantation not in the 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

Human Fertilization
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.

Fertilisation in humans is the union of a human egg and sperm, usually occurring in the ampulla of the fallopian tube, producing a zygote cell, or fertilized egg, initiating prenatal development. Scientists discovered the dynamics of human fertilization in the nineteenth century.

The term conception commonly refers to "the process of becoming pregnant involving fertilization or implantation or both".[32] Its use makes it a subject of 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.[33] The term "conception" is not usually used in scientific literature because of its variable definition and connotation.

Insects

Voo nupcial detail
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.

Insects in different groups, including the Odonata (dragonflies and damselflies) and the Hymenoptera (ants, bees, and wasps) practise delayed fertilisation. Anong 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.[34] 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 drones. She then stores the sperm for the rest of her life, perhaps for five years or more.[35][36]

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 dikaryotic or 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:[37]

  • gametogamy;
  • autogamy;[38][39]
  • 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.

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 zygotes 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.

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 ova of two mice by blocking certain proteins that would normally prevent the possibility; the resulting embryo normally developed into a mouse.[40]

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:[41][42]

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.[43]

“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,[44] that a long-term advantage of out-crossing in nature is increased genetic variability that promotes adaptation and/or avoidance of extinction (see Genetic variability).

See also

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  40. ^ Kono, Tomohiro; Obata, Yayoi; Wu, Quiong; Niwa, Katsutoshi; Ono, Yukiko; Yamamoto, Yuji; Park, Eun Sung; Seo, Jeong-Sun; Ogawa, Hidehiko (2004). "Birth of parthenogenetic mice that can develop to adulthood". Nature. 428 (6985): 860–4. doi:10.1038/nature02402. PMID 15103378. Lay summaryNational Geographic (2004-04-21).
  41. ^ Stenberg, P; Saura, A (2013). "Meiosis and Its Deviations in Polyploid Animals". Cytogenetic and Genome Research. 140 (2–4): 185. doi:10.1159/000351731. PMID 23796636.
  42. ^ Stock, M; Ustinova, J; Betto-Colliard, C; Schartl, M; Moritz, C; Perrin, N (2011). "Simultaneous Mendelian and clonal genome transmission in a sexually reproducing, all-triploid vertebrate". Proceedings of the Royal Society B: Biological Sciences. 279 (1732): 1293. doi:10.1098/rspb.2011.1738. PMC 3282369.
  43. ^ Darwin CR (1876). The effects of cross and self fertilisation in the vegetable kingdom. London: John Murray. http://darwin-online.org.uk/converted/published/1881-Worms-CrossandSelfFertilisation-F1249/1876-F1249.html see page 466-467
  44. ^ Otto, S.P; Gerstein, A.C (2006). "Why have sex? The population genetics of sex and recombination". Biochemical Society Transactions. 34 (4): 519–22. doi:10.1042/BST0340519. PMID 16856849.

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Autogamy

Autogamy, or self-fertilization, refers to the fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a reproductive mechanism employed by many flowering plants. However, species of protists have also been observed using autogamy as a means of reproduction. Flowering plants engage in autogamy regularly, while the protists that engage in autogamy only do so in stressful environments.

Double fertilization

Double fertilization is a complex fertilization mechanism of flowering plants (angiosperms). This process involves the joining of a female gametophyte (megagametophyte, also called the embryo sac) with two male gametes (sperm). It begins when a pollen grain adheres to the stigma of the carpel, the female reproductive structure of a flower. The pollen grain then takes in moisture and begins to germinate, forming a pollen tube that extends down toward the ovary through the style. The tip of the pollen tube then enters the ovary and penetrates through the micropyle opening in the ovule. The pollen tube proceeds to release the two sperm in the megagametophyte.

The cells of an unfertilized ovule are 8 in number and arranged in the form of 3+2+3 (from top to bottom) i.e 3 antipodal cells, 2 polar central cells, 2 synergids & 1 egg cell. One sperm fertilizes the egg cell and the other sperm combines with the two polar nuclei of the large central cell of the megagametophyte. The haploid sperm and haploid egg combine to form a diploid zygote,the process being called syngamy, while the other sperm and the two haploid polar nuclei of the large central cell of the megagametophyte form a triploid nucleus (triple fusion). Some plants may form polyploid nuclei. The large cell of the gametophyte will then develop into the endosperm, a nutrient-rich tissue which provides nourishment to the developing embryo. The ovary, surrounding the ovules, develops into the fruit, which protects the seeds and may function to disperse them.The two central cell maternal nuclei (polar nuclei) that contribute to the endosperm, arise by mitosis from the same single meiotic product that gave rise to the egg. The maternal contribution to the genetic constitution of the triploid endosperm is double that of the embryo.

In a study conducted in 2008 of the plant Arabidopsis thaliana, the migration of male nuclei inside the female gamete, in fusion with the female nuclei, has been documented for the first time using in vivo imaging. Some of the genes involved in the migration and fusion process have also been determined.Evidence of double fertilization in Gnetales, which are non-flowering seed plants, has been reported.

External fertilization

External fertilization is a male organism's sperm fertilizing a female organism's egg outside of the female's body.Internal fertilization, on the other hand, is the occurrence of internal insemination as the mode of combining sperm and egg. External fertilization occurs in water or a moist area because it gives the sperm external mobility to get to the egg. While in the water, the sperm and ova can shed simultaneously to fertilize the egg. The release of eggs and sperm into the water is known as spawning. When females spawn, they release a batch of eggs into a spot of their choice or just into the water, as in bottom dwelling or sessile species and all of the males start to release sperm that are in close proximity. Within vertebrates, it is the amphibians and fish that use external fertilization. When it comes to invertebrates, most are benthic, sessile, or benthic sessile combined animals such as coral, sea anemones, and tube dwelling polychaetes. The benthic zone is the lowest level of the ocean where organisms called benthos reside. An organism that is sessile does not have the ability to move or be mobile. Benthic marine plants such as algae also go through external fertilization to reproduce. Overall, environmental factors and the timing have a heavy influence over the success of external fertilization.In external fertilization some of the eggs laid by the animals gets wasted due to huge rainfall, floods etc.

Fertilisation of Orchids

Fertilisation of Orchids is a book by English naturalist Charles Darwin published on 15 May 1862 under the full explanatory title On the Various Contrivances by Which British and Foreign Orchids Are Fertilised by Insects, and On the Good Effects of Intercrossing. Darwin's previous book, On the Origin of Species, had briefly mentioned evolutionary interactions between insects and the plants they fertilised, and this new idea was explored in detail. Field studies and practical scientific investigations that were initially a recreation for Darwin—a relief from the drudgery of writing—developed into enjoyable and challenging experiments. Aided in his work by his family, friends, and a wide circle of correspondents across Britain and worldwide, Darwin tapped into the contemporary vogue for growing exotic orchids.

The book was his first detailed demonstration of the power of natural selection, and explained how complex ecological relationships resulted in the coevolution of orchids and insects. The view has been expressed that the book led directly or indirectly to all modern work on coevolution and the evolution of extreme specialisation. It influenced botanists, and revived interest in the neglected idea that insects played a part in pollinating flowers. It opened up the new study areas of pollination research and reproductive ecology, directly related to Darwin's ideas on evolution, and supported his view that natural selection led to a variety of forms through the important benefits achieved by cross-fertilisation. Although the general public showed less interest and sales of the book were low, it established Darwin as a leading botanist. Orchids was the first in a series of books on his innovative investigations into plants.

The book describes how the relationship between insects and plants resulted in the beautiful and complex forms which natural theology attributed to a grand designer. By showing how practical adaptations develop from cumulative minor variations of parts of the flowers to suit new purposes, Darwin countered the prevailing view that beautiful organisms were the handiwork of a Creator. Darwin's painstaking observations, experiments, and detailed dissection of the flowers explained previously unknown features such as the puzzle of Catasetum, which had been thought to have three completely different species of flowers on the same plant. In addition, they produced testable predictions including his then-controversial proposal that the long nectary of Angraecum sesquipedale meant that there must be a moth with an equally long proboscis. This was confirmed in 1903 when Xanthopan morganii praedicta was found in Madagascar.

Fertilizer

A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin (other than liming materials) that is applied to soils or to plant tissues to supply one or more plant nutrients essential to the growth of plants. Many sources of fertilizer exist, both natural and industrially produced.

Human Fertilisation and Embryology Act 1990

The Human Fertilisation and Embryology Act 1990 is an Act of the Parliament of the United Kingdom. It created the Human Fertilisation and Embryology Authority which is in charge of human embryo research, along with monitoring and licensing fertility clinics in the United Kingdom.The Authority is composed of a chairman, a deputy chairman, and however many members are appointed by the UK Secretary of State. They are in charge of reviewing information about human embryos and subsequent development, provision of treatment services, and activities governed by the Act of 1990. The Authority also offers information and advice to people seeking treatment, and to those who have donated gametes or embryos for purposes or activities covered in the Act of 1990. Some of the subjects under the Human Fertilisation and Embryology Act of 1990 are prohibitions in connection with gametes, embryos, and germ cells. It also addresses licensing conditions, code of practice, and procedure of approval involving human embryos. This only concerns human embryos which have reached the two cell zygote stage, at which they are considered “fertilised” in the act. It also governs the keeping and using of human embryos, but only outside the woman’s body. The act contains amendments to UK law regarding termination of pregnancy, surrogacy and parental rights.

Human Fertilisation and Embryology Authority

The Human Fertilisation and Embryology Authority (HFEA) is an executive non-departmental public body of the Department of Health in the United Kingdom. It is a statutory body that regulates and inspects all clinics in the United Kingdom providing in vitro fertilisation (IVF), artificial insemination and the storage of human eggs, sperm or embryos. It also regulates human embryo research.

Human fertilization

Human fertilization is the union of a human egg and sperm, usually occurring in the ampulla of the fallopian tube. The result of this union is the production of a zygote cell, or fertilized egg, initiating prenatal development. Scientists discovered the dynamics of human fertilization in the nineteenth century.The process of fertilization involves a sperm fusing with an ovum. The most common sequence begins with ejaculation during copulation, follows with ovulation, and finishes with fertilization. Various exceptions to this sequence are possible, including artificial insemination, in vitro fertilization, external ejaculation without copulation, or copulation shortly after ovulation. Upon encountering the secondary oocyte, the acrosome of the sperm produces enzymes which allow it to burrow through the outer jelly coat of the egg. The sperm plasma, then fuses with the egg's plasma membrane, the sperm head disconnects from its flagellum and the egg travels down the Fallopian tube to reach the uterus.

In vitro fertilization (IVF) is a process by which egg cells are fertilized by sperm outside the womb, in vitro.

In vitro fertilisation

In vitro fertilisation (IVF) is a process of fertilisation where an egg is combined with sperm outside the body, in vitro ("in glass"). The process involves monitoring and stimulating a woman's ovulatory process, removing an ovum or ova (egg or eggs) from the woman's ovaries and letting sperm fertilise them in a liquid in a laboratory. After the fertilised egg (zygote) undergoes embryo culture for 2–6 days, it is implanted in the same or another woman's uterus, with the intention of establishing a successful pregnancy.

IVF is a type of assisted reproductive technology used for infertility treatment and gestational surrogacy. A fertilised egg may be implanted into a surrogate's uterus, and the resulting child is genetically unrelated to the surrogate. Some countries banned or otherwise regulate the availability of IVF treatment, giving rise to fertility tourism. Restrictions on the availability of IVF include costs and age, in order for a woman to carry a healthy pregnancy to term. IVF is generally not used until less invasive or expensive options have failed or been determined unlikely to work.

In 1978 Louise Brown was the first child successfully born after her mother received IVF treatment. Brown was born as a result of natural-cycle IVF, where no stimulation was made. The procedure took place at Dr Kershaw's Cottage Hospital (now Dr Kershaw's Hospice) in Royton, Oldham, England. Robert G. Edwards was awarded the Nobel Prize in Physiology or Medicine in 2010. The physiologist co-developed the treatment together with Patrick Steptoe and embryologist Jean Purdy but the latter two were not eligible for consideration as they had died and the Nobel Prize is not awarded posthumously.

With egg donation and IVF, women who are past their reproductive years, have infertile male partners, have idiopathic female-fertility issues, or have reached menopause, can still become pregnant. Adriana Iliescu held the record as the oldest woman to give birth using IVF and donated egg, when she gave birth in 2004 at the age of 66, a record passed in 2006. After the IVF treatment, some couples get pregnant without any fertility treatments. In 2018 it was estimated that eight million children had been born worldwide using IVF and other assisted reproduction techniques.

Insemination

Insemination is the introduction of sperm into a female animal or plant for the purpose of impregnating or fertilizing the female for sexual reproduction. The sperm is introduced into the uterus of a mammal or the oviduct of an oviparous (egg-laying) animal.

In mammals, insemination normally occurs during sexual intercourse, but insemination can take place in other ways, such as artificial insemination. Each form of insemination has legal, moral and interpersonal implications. Whether insemination takes place naturally or by artificial means, however, the pregnancy and the progress of it will be the same.

Insemination may be called in vivo fertilisation (from in vivo meaning "within the living") because an egg is fertilized inside the body, this is in contrast with in vitro fertilisation.

In plants, the process of insemination is referred to as pollination.

Internal fertilization

Internal fertilization is the union of an egg cell with a sperm during sexual reproduction inside the body of a parent. For this to happen there needs to be a method for the male to introduce the sperm into the female's reproductive tract. In mammals, reptiles, some birds, some fish and certain other groups of animals, this is done by copulation, the penis or other intromittent organ being introduced into the vagina or cloaca. In most birds, the cloacal kiss is used, the two animals pressing their cloacas together while transferring sperm. Salamanders, spiders, some insects and some molluscs undertake internal fertilization by transferring a spermatophore, a bundle of sperm, from the male to the female. Following fertilization, the embryos are laid as eggs in oviparous organisms, or in viviparous organisms, continue to develop inside the reproductive tract of the mother to be born later as live young. In some animals like in sponges fertilization is internal

Iron fertilization

Iron fertilization is the intentional introduction of iron to iron-poor areas of the ocean surface to stimulate phytoplankton production. This is intended to enhance biological productivity and/or accelerate carbon dioxide (CO2) sequestration from the atmosphere.

Iron is a trace element necessary for photosynthesis in plants. It is highly insoluble in sea water and in a variety of locations is the limiting nutrient for phytoplankton growth. Large algal blooms can be created by supplying iron to iron-deficient ocean waters. These blooms can nourish other organisms.

Multiple ocean labs, scientists and businesses have explored fertilization. Beginning in 1993, thirteen research teams completed ocean trials demonstrating that phytoplankton blooms can be stimulated by iron augmentation. Controversy remains over the effectiveness of atmospheric CO2 sequestration and ecological effects. The most recent open ocean trials of ocean iron fertilization were in 2009 (January to March) in the South Atlantic by project Lohafex, and in July 2012 in the North Pacific off the coast of British Columbia, Canada, by the Haida Salmon Restoration Corporation (HSRC).Fertilization occurs naturally when upwellings bring nutrient-rich water to the surface, as occurs when ocean currents meet an ocean bank or a sea mount. This form of fertilization produces the world's largest marine habitats. Fertilization can also occur when weather carries wind blown dust long distances over the ocean, or iron-rich minerals are carried into the ocean by glaciers, rivers and icebergs.

Louise Brown

Louise Joy Brown (born 25 July 1978) is an English woman known for being the first human to have been born after conception by in vitro fertilisation, or IVF.

Maratheftiko

Maratheftiko is an ancient grape variety indigenous to Cyprus. It is also known locally as Vambakadha (Βαμβακάδα), Vambakina (Βαμβακίνα), Pampakia (Παμπακιά), Mavrospourtiko (Μαυροσπούρτικο), Aloupostaphylo (Αλουποστάφυλο).

It is grown in sparse quantities around the island but mostly in the Pitsilia region. In the 1980s, with the revival of small boutique wineries in Cyprus this variety was rediscovered and its cultivation is slowly on the increase again, as it offers a distinctive character to local wines. Keo, the largest winery on the island has been one of the companies to encourage its growth. Maratheftiko does not have hermaphrodite flowers like many cultivated grape varieties and requires co-planting with other varieties in order to achieve fertilisation and fruit development. As a result of poor fertilisation, bunches are often greatly affected by Millerandage.

2004 statistics reveal that Maratheftiko cultivation covers 125 hectares which represents less than 1% of cultivated vineyards on the island.

Mary Warnock, Baroness Warnock

Helen Mary Warnock, Baroness Warnock, (née Wilson; 14 April 1924 – 20 March 2019) was an English philosopher of morality, education, and mind, and a writer on existentialism. She is best known for chairing an inquiry whose report formed the basis of the Human Fertilisation and Embryology Act 1990. She served as Mistress of Girton College, Cambridge from 1984 to 1991.

Outcrossing

Out-crossing or out-breeding means the crossing between different breeds. This is the practice of introducing unrelated genetic material into a breeding line. It increases genetic diversity, thus reducing the probability of an individual being subject to disease or genetic abnormalities.

Outcrossing is now the norm of most purposeful animal breeding, contrary to what is commonly believed. The outcrossing breeder intends to remove the traits by using "new blood". With dominant traits, one can still see the expression of the traits and can remove those traits whether one outcrosses, line breeds or inbreds. With recessive traits, outcrossing allows for the recessive traits to migrate across a population. The outcrossing breeder then may have individuals that have many deleterious genes that may be expressed by subsequent inbreeding. There is now a gamut of deleterious genes within each individual in many dog breeds.Increasing the variation of genes or alleles within the gene pool may protect against extinction by stressors from the environment. For example, in this context, a recent veterinary medicine study tried to determine the genetic diversity within cat breeds.Outcrossing is believed to be the "norm" in the wild. Outcrossing in plants is usually enforced by self-incompatibility.

Breeders inbreed within their genetic pool, attempting to maintain desirable traits and to cull those traits that are undesirable. When undesirable traits begin to appear, mates are selected to determine if a trait is recessive or dominant. Removal of the trait is accomplished by breeding two individuals known not to carry it.Gregor Mendel used outcrossing in his experiments with flowers. He then used the resulting offspring to chart inheritance patterns, using the crossing of siblings, and backcrossing to parents to determine how inheritance functioned.Charles Darwin, in his book The Effects of Cross and Self-Fertilization in the Vegetable Kingdom, came to clear and definite conclusions concerning the adaptive benefit of outcrossing. For example, he stated (on page 462) 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 vigor and fertility over the self-fertilizing offspring from either one of the same parents". He thought that this observation was amply sufficient to account for outcrossing sexual reproduction. The disadvantages of self-fertilized offspring (inbreeding depression) are now thought to be largely due to the homozygous expression of deleterious recessive mutations; and the fitness advantages of outcrossed offspring are thought to be largely due to the heterozygous masking of such deleterious mutations.

Oviparity

Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, reptiles, all birds, and the monotremes.

In traditional usage, most insects, molluscs, and arachnids are also described as oviparous.

Robert Edwards (physiologist)

Sir Robert Geoffrey Edwards, (27 September 1925 – 10 April 2013) was an English physiologist and pioneer in reproductive medicine, and in-vitro fertilisation (IVF) in particular. Along with the surgeon Patrick Steptoe, Edwards successfully pioneered conception through IVF, which led to the birth of Louise Brown on 25 July 1978. They founded the first IVF programme for infertile patients and trained other scientists in their techniques. Edwards was the founding editor-in-chief of Human Reproduction in 1986. In 2010, he was awarded the Nobel Prize in Physiology or Medicine "for the development of in vitro fertilization".

Rolene Strauss

Rolene Strauss (born 22 April 1992) is a South African doctor and beauty pageant titleholder who won Miss South Africa 2014 and was later crowned Miss World 2014. She is the third South African woman to be crowned Miss World, after Penelope Anne Coelen in 1958 and Anneline Kriel in 1974.

Strauss is currently the chairperson of the non-profit organization The Strauss Foundation.

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Biological terms
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Human embryogenesis in the first three weeks
Week 1
Week 2
(Bilaminar)
Week 3
(Trilaminar)
Menstrual cycle
Gametogenesis
Human sexual activity
Development of the reproductive system
Egg
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