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. In , Spallanzani established the need of interaction between the female's ovum and male's sperm to form a zygote in frogs. 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. 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. 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.
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. Work done on tobacco plants revealed a family of glycoproteins called TTS proteins that enhanced growth of pollen tubes.
However, in the TTS medium, the tubes grew at a rate 3x that of the sugar-free medium. Transgenic plants lacking the ability to produce TTS proteins exhibited slower pollen tube growth and reduced fertility. 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.
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This increased uptake of calcium causes the pollen tube to rupture, and release its sperm into the ovule. 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.
A fern is a member of a group of roughly 12, species of vascular plants that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular i. 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. The gymnosperms are a group of seed producing plants that includes conifers, Cycads , Ginkgo , and Gnetales. 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. After being fertilised, the ovary starts to swell and develop into the fruit. 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  — a growing pollen tube therefore contains three separate nuclei, two sperm and one tube.
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. In many plants, the development of the flesh of the fruit is proportional to the percentage of fertilised ovules.
Fertilization in Protozoa and Metazoan Animals : Cellular and Molecular Aspects - pingbatipac.tk
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 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.
Formation of triploid plants via possible polyspermy
In some species, self-fertilisation has persisted over many generations. Capsella rubella is a self-fertilisating species that became self-compatible 50, to , years ago. 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.
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.
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There are three steps to fertilisation that ensure species-specificity:. 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.
Resact is a 14 amino acid peptide purified from the jelly coat of A. After finding the egg, the sperm penetrates the jelly coat through a process called sperm activation. 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.
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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 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.
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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.
The biology and evolution of polyspermy: insights from cellular and functional studies of sperm and centrosomal behavior in the fertilized egg.
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