An embryo is the early stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm cell. The resulting fusion of these two cells produces a single-celled zygote that undergoes many cell divisions that produce cells known as blastomeres. The blastomeres are arranged as a solid ball that when reaching a certain size, called a morula, takes in fluid to create a cavity called a blastocoel. The structure is then termed a blastula, or a blastocystin mammals.
The mammalian blastocyst hatches before implantating into the endometrial lining of the womb. Once implanted the embryo will continue its development through the next stages of gastrulation, neurulation, and organogenesis. Gastrulation is the formation of the three germ layers that will form all of the different parts of the body. Neurulation forms the nervous system, and organogenesis is the development of all the various tissues and organs of the body.
A newly developing human is typically referred to as an embryo until the ninth week after conception, when it is then referred to as a fetus. In other multicellular organisms, the word "embryo" can be used more broadly to any early developmental or life cycle stage prior to birth or hatching.
| Embryo | |
|---|---|
 | |
| Identifiers | |
| TE | E1.0.2.6.4.0.5 |
| Anatomical terminology | |
https://en.wikipedia.org/wiki/Embryo
Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layersformed from gastrulation (the ectoderm, endoderm, and mesoderm) form the internal organs of the organism.[1]
The cells of each of the three germ layers undergo differentiation, a process where less-specialized cells become more-specialized through the expression of a specific set of genes. Cell differentiation is driven by cell signaling cascades.[2] Differentiation is influenced by extracellular signals such as growth factors that are exchanged to adjacent cells which is called juxtracrine signaling or to neighboring cells over short distances which is called paracrine signaling.[3] Intracellular signals consist of a cell signaling itself (autocrine signaling), also play a role in organ formation. These signaling pathways allow for cell rearrangement and ensure that organs form at specific sites within the organism.[1] The organogenesis process can be studied using embryos and organoids.[4]
https://en.wikipedia.org/wiki/Organogenesis
Neurulation refers to the folding process in vertebrate embryos, which includes the transformation of the neural plate into the neural tube.[1] The embryo at this stage is termed the neurula.
The process begins when the notochord induces the formation of the central nervous system(CNS) by signaling the ectoderm germ layer above it to form the thick and flat neural plate. The neural plate folds in upon itself to form the neural tube, which will later differentiate into the spinal cord and the brain, eventually forming the central nervous system.[2] Computer simulations found that cell wedging and differential proliferation are sufficient for mammalian neurulation.[3]
Different portions of the neural tube form by two different processes, called primary and secondary neurulation, in different species.[citation needed]
- In primary neurulation, the neural plate creases inward until the edges come in contact and fuse.
- In secondary neurulation, the tube forms by hollowing out of the interior of a solid precursor.
| Neurulation | |
|---|---|
 Transverse sections that show the progression of the neural plate to the neural groove from bottom to top | |
| Identifiers | |
| MeSH | D054261 |
| Anatomical terminology | |
Primary neural induction[edit]
The concept of induction originated in work by Pandor in 1817.[4] The first experiments proving induction were attributed by Viktor Hamburger[5] to independent discoveries of both Hans Spemann of Germany in 1901[6] and Warren Lewis of the USA in 1904.[7] It was Hans Spemann who first popularized the term “primary neural induction” in reference to the first differentiation of ectoderm into neural tissue during neurulation.[8][9] It was called "primary" because it was thought to be the first induction event in embryogenesis. The Nobel prize-winning experiment was done by his student Hilda Mangold.[8] Ectoderm from the region of the dorsal lip of the blastopore of a developing salamander embryo was transplanted into another embryo and this "organizer" tissue “induced” the formation of a full secondary axis changing surrounding tissue in the original embryo from ectodermal to neural tissue. The tissue from the donor embryo was therefore referred to as the inducer because it induced the change.[8] It is important to note that while the organizer is the dorsal lip the blastopore, this is not one set of cells but rather is a constantly changing group of cells that are migrating over the dorsal lip of the blastopore by forming apically constricted bottle cells. At any given time during gastrulation there will be different cells that make up the organizer.[10]
Subsequent work on inducers by scientists over the 20th Century demonstrated that not only could the dorsal lip of the blastopore act as an inducer but so could a huge number of other seemingly unrelated items. This began when boiled ectoderm was found to still be able to induce by Johannes Holtfreter.[11] Items as diverse as low pH, cyclic AMP, even floor dust could act as inducers leading to considerable consternation.[12] Even tissue which could not induce when living could induce when boiled.[13] Other items such as lard, wax, banana peels and coagulated frog’s blood did not induce.[14] The hunt for a chemically based inducer molecule was taken up by developmental molecular biologists and a vast literature of items shown to have inducer abilities continued to grow.[15][16] More recently the inducer molecule has been attributed to genes and in 1995 there was a call for all the genes involved in primary neural induction and all their interactions to be catalogued in an effort to determine “the molecular nature of Spemann’s organizer”.[17] Several other proteins and growth factors have also been invoked as inducers including soluble growth factors such as bone morphogenetic protein, and a requirement for “inhibitory signals” such as noggin and follistatin.
Even before the term induction was popularized several authors, beginning with Hans Driesch in 1894,[18] suggested that primary neural induction might be mechanical in nature. A mechanochemical-based model for primary neural induction was proposed in 1985 by G.W. Brodland and R. Gordon.[19] An actual physical wave of contraction has been shown to originate from the precise location of the Spemann organizer which then traverses the presumptive neural epithelium[20] and a full working model of how primary neural inductions was proposed in 2006.[21][22] There has long been a general reluctance in the field to consider the possibility that primary neural induction might be initiated by mechanical effects.[23] A full explanation for primary neural induction remains yet to be found.
Shape change[edit]
As neurulation proceeds after induction the cells of the neural plate become high-columnar and can be identified through microscopy as different from the surrounding presumptive epithelial ectoderm (epiblastic endoderm in amniotes). The cells move laterally and away from the central axis and change into a truncated pyramid shape. This pyramid shape is achieved through tubulin and actin in the apical portion of the cell which constricts as they move. The variation in cell shapes is partially determined by the location of the nucleus within the cell, causing bulging in areas of the cells forcing the height and shape of the cell to change. This process is known as apical constriction.[24][25] The result is a flattening of the differentiating neural plate which is particularly obvious in salamanders when the previously round gastrula becomes a rounded ball with a flat top.[26] See Neural plate
Folding[edit]
The process of the flat neural plate folding into the cylindrical neural tube is termed primary neurulation. As a result of the cellular shape changes, the neural plate forms the medial hinge point (MHP). The expanding epidermis puts pressure on the MHP and causes the neural plate to fold resulting in neural folds and the creation of the neural groove. The neural folds form dorsolateral hinge points (DLHP) and pressure on this hinge cause the neural folds to meet and fuse at the midline. The fusion requires the regulation of cell adhesion molecules. The neural plate switches from E-cadherin expression to N-cadherin and N-CAM expression to recognize each other as the same tissue and close the tube. This change in expression stops the binding of the neural tube to the epidermis. Neural plate folding is a complicated step.[citation needed]
The notochord plays an integral role in the development of the neural tube. Prior to neurulation, during the migration of epiblastic endoderm cells towards the hypoblastic endoderm, the notochordal process opens into an arch termed the notochordal plate and attaches overlying neuroepithelium of the neural plate. The notochordal plate then serves as an anchor for the neural plate and pushes the two edges of the plate upwards while keeping the middle section anchored. Some of the notochodral cells become incorporated into the center section neural plate to later form the floor plate of the neural tube. The notochord plate separates and forms the solid notochord.[citation needed]
The folding of the neural tube to form an actual tube does not occur all at once. Instead, it begins approximately at the level of the fourth somite at Carnegie stage 9 (around Embryonic day 20 in humans). The lateral edges of the neural plate touch in the midline and join together. This continues both cranially (toward the head) and caudally (toward the tail). The openings that are formed at the cranial and caudal regions are termed the cranial and caudal neuropores. In human embryos, the cranial neuropore closes approximately on day 24 and the caudal neuropore on day 28.[27] Failure of the cranial (superior) and caudal (inferior) neuropore closure results in conditions called anencephaly and spina bifida, respectively. Additionally, failure of the neural tube to close throughout the length of the body results in a condition called rachischisis.[28]
Patterning[edit]
According to the French Flag model where stages of development are directed by gene product gradients, several genes are considered important for inducing patterns in the open neural plate, especially for the development of neurogenic placodes. These placodes first become evident histologically in the open neural plate. After sonic hedgehog (SHH) signalling from the notochord induces its formation, the floor plate of the incipient neural tube also secretes SHH. After closure, the neural tube forms a basal or floor plate and a roof or alar plate in response to the combined effects of SHH and factors including BMP4secreted by the roof plate. The basal plate forms most of the ventral portion of the nervous system, including the motor portion of the spinal cord and brain stem; the alar plate forms the dorsal portions, devoted mostly to sensory processing.[29]
The dorsal epidermis expresses BMP4 and BMP7. The roof plate of the neural tube responds to those signals by expressing more BMP4 and other transforming growth factor beta (TGF-β) signals to form a dorsal/ventral gradient among the neural tube. The notochord expresses SHH. The floor plate responds to SHH by producing its own SHH and forming a gradient. These gradients allow for the differential expression of transcription factors.[29]
Complexities of the model[edit]
Neural tube closure is not entirely understood. Closure of the neural tube varies by species. In mammals closure occurs by meeting at multiple points which then close up and down. In birds neural tube closure begins at one point of the midbrain and moves anteriorly and posteriorly.[30][31]
Secondary neurulation[edit]
Primary neurulation develops into secondary neurulation when the caudal neuropore undergoes final closure. The cavity of the spinal cord extends into the neural cord.[32] In secondary neurulation, the neural ectoderm and some cells from the endoderm form the medullary cord. The medullary cord condenses, separates and then forms cavities.[33] These cavities then merge to form a single tube. Secondary neurulation occurs in the posterior section of most animals but it is better expressed in birds. Tubes from both primary and secondary neurulation eventually connect at around the sixth week of development.[34]
In humans, the mechanisms of secondary neurulation plays an important role given its impact on the proper formation of the human posterior spinal cord. Errors at any point in the process can yield problems. For example, retained medullary cord occurs due to a partial or complete arrest of secondary neurulation that creates a non-functional portion on the vestigial end.[35]
Early brain development[edit]
The anterior portion of the neural tube forms the three main parts of the brain: the forebrain (prosencephalon), midbrain (mesencephalon), and the hindbrain(rhombencephalon).[36] These structures initially appear just after neural tube closure as bulges called brain vesicles in a pattern specified by anterior-posterior patterning genes, including Hox genes, other transcription factors such as Emx, Otx, and Pax genes, and secreted signaling factors such as fibroblast growth factors(FGFs) and Wnts.[37] These brain vesicles further divide into subregions. The prosencephalon gives rise to the telencephalon and diencephalon, and the rhombencephalon generates the metencephalon and myelencephalon. The hindbrain, which is the evolutionarily most ancient part of the chordate brain, also divides into different segments called rhombomeres. The rhombomeres generate many of the most essential neural circuits needed for life, including those that control respiration and heart rate, and produce most of the cranial nerves.[36] Neural crest cells form ganglia above each rhombomere. The early neural tube is primarily composed of the germinal neuroepithelium, later called the ventricular zone, which contains primary neural stem cells called radial glial cells and serves as the main source of neurons produced during brain development through the process of neurogenesis.[38][39]
Non-neural ectoderm tissue[edit]
Paraxial mesoderm surrounding the notochord at the sides will develop into the somites (future muscles, bones, and contributes to the formation of limbs of the vertebrate ).[40]
Neural crest cells[edit]
Masses of tissue called the neural crest that are located at the very edges of the lateral plates of the folding neural tube separate from the neural tube and migrate to become a variety of different but important cells.[citation needed]
Neural crest cells will migrate through the embryo and will give rise to several cell populations, including pigment cells and the cells of the peripheral nervous system.[citation needed]
Neural tube defects[edit]
Failure of neurulation, especially failure of closure of the neural tube are among the most common and disabling birth defects in humans, occurring in roughly 1 in every 500 live births.[41] Failure of the rostral end of the neural tube to close results in anencephaly, or lack of brain development, and is most often fatal.[42] Failure of the caudal end of the neural tube to close causes a condition known as spina bifida, in which the spinal cord fails to close.[43]
See also[edit]
A germ layer is a primary layer of cells that forms during embryonic development.[1] The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans (animals that are sister taxa to the sponges) produce two or three primary germ layers. Some animals, like cnidarians, produce two germ layers (the ectoderm and endoderm) making them diploblastic. Other animals such as bilaterians produce a third layer (the mesoderm) between these two layers, making them triploblastic. Germ layers eventually give rise to all of an animal’s tissues and organs through the process of organogenesis.
https://en.wikipedia.org/wiki/Germ_layer
A germ cell is any biological cell that gives rise to the gametes of an organism that reproduces sexually. In many animals, the germ cells originate in the primitive streak and migrate via the gut of an embryo to the developing gonads. There, they undergo meiosis, followed by cellular differentiation into mature gametes, either eggs or sperm. Unlike animals, plants do not have germ cells designated in early development. Instead, germ cells can arise from somatic cells in the adult, such as the floral meristem of flowering plants.[1][2][3]
https://en.wikipedia.org/wiki/Germ_cell
A gamete (/ˈɡæmiːt/; from Ancient Greek γαμετή (gametḗ) 'wife', ultimately from Ancient Greek γάμος(gámos) 'marriage') is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually.[1] Gametes are an organism's reproductive cells, also referred to as sex cells.[2] In species that produce two morphologically distinct types of gametes, and in which each individual produces only one type, a female is any individual that produces the larger type of gamete—called an ovum— and a male produces the smaller type—called a sperm. Sperm cells or spermatozoa are small and motile due to the flagellum, a tail-shaped structure that allows the cell to propel and move. In contrast, each egg cell or ovum is relatively large and non-motile.[2] In short a gamete is an egg cell (female gamete) or a sperm (male gamete). In animals, ova mature in the ovaries of females and sperm develop in the testes of males. During fertilization, a spermatozoon and ovum unite to form a new diploid organism.[2] Gametes carry half the genetic information of an individual, one ploidy of each type, and are created through meiosis, in which a germ cell undergoes two fissions, resulting in the production of four gametes.[1] In biology, the type of gamete an organism produces determines the classification of its sex.[3]
This is an example of anisogamy or heterogamy, the condition in which females and males produce gametes of different sizes (this is the case in humans; the human ovum has approximately 100,000 times the volume of a single human sperm cell). In contrast, isogamy is the state of gametes from both sexes being the same size and shape, and given arbitrary designators for mating type. The name gamete was introduced by the German cytologist Eduard Strasburger. Male and female gametes set the basis for the sexual roles and sexual selection.[4]
Oogenesis is the process of female gamete formation in animals. This process involves meiosis (including meiotic recombination) occurring in the diploid primary oocyte to produce the haploid ovum. Spermatogenesis is the process of male gamete formation in animals. This process also involves meiosis occurring in the diploid primary spermatocyte to produce the haploid spermatozoon.
https://en.wikipedia.org/wiki/Gamete
A spermatozoon (pronounced /ˌspɜːrmætəˈzoʊən/, alternate spelling spermatozoön; plural spermatozoa; from Ancient Greek: σπέρμα ("seed") and Ancient Greek: ζῷον ("living being")) is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote. (A zygote is a single cell, with a complete set of chromosomes, that normally develops into an embryo.)
Sperm cells contribute approximately half of the nuclear genetic information to the diploid offspring (excluding, in most cases, mitochondrial DNA). In mammals, the sex of the offspring is determined by the sperm cell: a spermatozoon bearing an X chromosome will lead to a female (XX) offspring, while one bearing a Y chromosomewill lead to a male (XY) offspring. Sperm cells were first observed in Antonie van Leeuwenhoek's laboratory in 1677.[1]
https://en.wikipedia.org/wiki/Spermatozoon
Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testis. This process starts with the mitotic division of the stem cells located close to the basement membrane of the tubules.[1] These cells are called spermatogonial stem cells. The mitotic division of these produces two types of cells. Type A cells replenish the stem cells, and type B cells differentiate into primary spermatocytes. The primary spermatocyte divides meiotically (Meiosis I) into two secondary spermatocytes; each secondary spermatocyte divides into two equal haploid spermatids by Meiosis II. The spermatids are transformed into spermatozoa (sperm) by the process of spermiogenesis. These develop into mature spermatozoa, also known as sperm cells.[2] Thus, the primary spermatocyte gives rise to two cells, the secondary spermatocytes, and the two secondary spermatocytes by their subdivision produce four spermatozoa and four haploid cells.[3]
Spermatozoa are the mature male gametes in many sexually reproducing organisms. Thus, spermatogenesis is the male version of gametogenesis, of which the female equivalent is oogenesis. In mammals it occurs in the seminiferous tubulesof the male testes in a stepwise fashion. Spermatogenesis is highly dependent upon optimal conditions for the process to occur correctly, and is essential for sexual reproduction. DNA methylation and histone modification have been implicated in the regulation of this process.[4] It starts at puberty and usually continues uninterrupted until death, although a slight decrease can be discerned in the quantity of produced sperm with increase in age (see Male infertility).
Spermatogenesis starts in the bottom part of seminiferous tubes and, progressively, cells go deeper into tubes and moving along it until mature spermatozoa reaches the lumen, where mature spermatozoa are deposited. The division happens asynchronically; if the tube is cut transversally one could observe different maturation states. A group of cells with different maturation states that are being generated at the same time is called a spermatogenic wave.[5]
https://en.wikipedia.org/wiki/Spermatogenesis
Development[edit]
Animal embryos[edit]
In animals, fertilization begins the process of embryonic development with the creation of a zygote, a single cell resulting from the fusion of gametes (e.g. egg and sperm).[5] The development of a zygote into a multicellular embryo proceeds through a series of recognizable stages, often divided into cleavage, blastula, gastrulation, and organogenesis.[6]
Cleavage is the period of rapid mitotic cell divisions that occur after fertilization. During cleavage, the overall size of the embryo does not change, but the size of individual cells decrease rapidly as they divide to increase the total number of cells.[7] Cleavage results in a blastula.[6]
Depending on the species, a blastula or blastocyst stage embryo can appear as a ball of cells on top of yolk, or as a hollow sphere of cells surrounding a middle cavity.[8] The embryo's cells continue to divide and increase in number, while molecules within the cells such as RNAs and proteins actively promote key developmental processes such as gene expression, cell fate specification, and polarity.[9] Before implanting into the uterine wall the embryo is sometimes known as the pre-implantation embryo or pre-implantation conceptus.[10] Sometimes this is called the pre-embryo a term employed to differentiate from an embryo proper in relation to embryonic stem cell discourses.[11]
Gastrulation is the next phase of embryonic development, and involves the development of two or more layers of cells (germinal layers). Animals that form two layers (such as Cnidaria) are called diploblastic, and those that form three (most other animals, from flatworms to humans) are called triploblastic. During gastrulation of triploblastic animals, the three germinal layers that form are called the ectoderm, mesoderm, and endoderm.[8] All tissues and organs of a mature animal can trace their origin back to one of these layers.[12] For example, the ectoderm will give rise to the skin epidermis and the nervous system,[13] the mesoderm will give rise to the vascular system, muscles, bone, and connective tissues,[14] and the endoderm will give rise to organs of the digestive system and epithelium of the digestive system and respiratory system.[15][16] Many visible changes in embryonic structure happen throughout gastrulation as the cells that make up the different germ layers migrate and cause the previously round embryo to fold or invaginate into a cup-like appearance.[8]
Past gastrulation, an embryo continues to develop into a mature multicellular organism by forming structures necessary for life outside of the womb or egg. As the name suggests, organogenesis is the stage of embryonic development when organs form. During organogenesis, molecular and cellular interactions prompt certain populations of cells from the different germ layers to differentiate into organ-specific cell types.[17] For example, in neurogenesis, a subpopulation of cells from the ectoderm segregate from other cells and further specialize to become the brain, spinal cord, or peripheral nerves.[18]
The embryonic period varies from species to species. In human development, the term fetus is used instead of embryo after the ninth week after conception,[19] whereas in zebrafish, embryonic development is considered finished when a bone called the cleithrum becomes visible.[20] In animals that hatch from an egg, such as birds, a young animal is typically no longer referred to as an embryo once it has hatched. In viviparous animals (animals whose offspring spend at least some time developing within a parent's body), the offspring is typically referred to as an embryo while inside of the parent, and is no longer considered an embryo after birth or exit from the parent. However, the extent of development and growth accomplished while inside of an egg or parent varies significantly from species to species, so much so that the processes that take place after hatching or birth in one species may take place well before those events in another. Therefore, according to one textbook, it is common for scientists interpret the scope of embryology broadly as the study of the development of animals.[8]
Plant embryos[edit]
Flowering plants (angiosperms) create embryos after the fertilization of a haploid ovule by pollen. The DNA from the ovule and pollen combine to form a diploid, single-cell zygote that will develop into an embryo.[21] The zygote, which will divide multiple times as it progresses throughout embryonic development, is one part of a seed. Other seed components include the endosperm, which is tissue rich in nutrients that will help support the growing plant embryo, and the seed coat, which is a protective outer covering. The first cell division of a zygote is asymmetric, resulting in an embryo with one small cell (the apical cell) and one large cell (the basal cell).[22] The small, apical cell will eventually give rise to most of the structures of the mature plant, such as the stem, leaves, and roots.[23] The larger basal cell will give rise to the suspensor, which connects the embryo to the endosperm so that nutrients can pass between them.[22] The plant embryo cells continue to divide and progress through developmental stages named for their general appearance: globular, heart, and torpedo. In the globular stage, three basic tissue types (dermal, ground, and vascular) can be recognized.[22] The dermal tissue will give rise to the epidermis or outer covering of a plant,[24] ground tissue will give rise to inner plant material that functions in photosynthesis, resource storage, and physical support,[25] and vascular tissue will give rise to connective tissue like the xylem and phloem that transport fluid, nutrients, and minerals throughout the plant.[26] In heart stage, one or two cotyledons (embryonic leaves) will form. Meristems(centers of stem cell activity) develop during the torpedo stage, and will eventually produce many of the mature tissues of the adult plant throughout its life.[22] At the end of embryonic growth, the seed will usually go dormant until germination.[27] Once the embryo begins to germinate (grow out from the seed) and forms its first true leaf, it is called a seedling or plantlet.[28]
Plants that produce spores instead of seeds, like bryophytes and ferns, also produce embryos. In these plants, the embryo begins its existence attached to the inside of the archegonium on a parental gametophyte from which the egg cell was generated.[29] The inner wall of the archegonium lies in close contact with the "foot" of the developing embryo; this "foot" consists of a bulbous mass of cells at the base of the embryo which may receive nutrition from its parent gametophyte.[30] The structure and development of the rest of the embryo varies by group of plants.[31]
Since all land plants create embryos, they are collectively referred to as embryophytes (or by their scientific name, Embryophyta). This, along with other characteristics, distinguishes land plants from other types of plants, such as algae, which do not produce embryos.[32]
Research and technology[edit]
Biological processes[edit]
Embryos from numerous plant and animal species are studied in biological research laboratories across the world to learn about topics such as stem cells,[33] evolution and development,[34] cell division,[35] and gene expression.[36] Examples of scientific discoveries made while studying embryos that were awarded the Nobel Prize in Physiology or Medicine include the Spemann-Mangold organizer, a group of cells originally discovered in amphibian embryos that give rise to neural tissues,[37] and genes that give rise to body segments discovered in Drosophila fly embryos by Christiane Nüsslein-Volhard and Eric Wieschaus.[38]
Assisted reproductive technology[edit]
Creating and/or manipulating embryos via assisted reproductive technology (ART) is used for addressing fertility concerns in humans and other animals, and for selective breeding in agricultural species. Between the years 1987 and 2015, ART techniques including in vitro fertilization (IVF) were responsible for an estimated 1 million human births in the United States alone.[39] Other clinical technologies include preimplantation genetic diagnosis (PGD), which can identify certain serious genetic abnormalities, such as aneuploidy, prior to selecting embryos for use in IVF.[40] Some have proposed (or even attempted - see He Jiankui affair) genetic editingof human embryos via CRISPR-Cas9 as a potential avenue for preventing disease;[41] however, this has been met with widespread condemnation from the scientific community.[42][43]
ART techniques are also used to improve the profitability of agricultural animal species such as cows and pigs by enabling selective breeding for desired traits and/or to increase numbers of offspring.[44] For example, when allowed to breed naturally, cows typically produce one calf per year, whereas IVF increases offspring yield to 9-12 calves per year.[45] IVF and other ART techniques, including cloning via interspecies somatic cell nuclear transfer (iSCNT),[46] are also used in attempts to increase the numbers of endangered or vulnerable species, such as Northern white rhinos,[47] cheetahs,[48] and sturgeons.[49]
Cryoconservation of plant and animal biodiversity[edit]
Cryoconservation of genetic resources involves collecting and storing the reproductive materials, such as embryos, seeds, or gametes, from animal or plant species at low temperatures in order to preserve them for future use.[50] Some large-scale animal species cryoconservation efforts include "frozen zoos" in various places around the world, including in the UK's Frozen Ark,[51] the Breeding Centre for Endangered Arabian Wildlife (BCEAW) in the United Arab Emirates,[52] and the San Diego ZooInstitute for Conservation in the United States.[53][54] As of 2018, there were approximately 1,700 seed banks used to store and protect plant biodiversity, particularly in the event of mass extinction or other global emergencies.[55] The Svalbard Global Seed Vault in Norway maintains the largest collection of plant reproductive tissue, with more than a million samples stored at −18 °C (0 °F).[56]
Fossilized embryos[edit]
Fossilized animal embryos are known from the Precambrian, and are found in great numbers during the Cambrian period. Even fossilized dinosaur embryos have been discovered.[57]
See also[edit]
In developmental biology, animal embryonic development, also known as embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm cell, (spermatozoon).[1]Once fertilized, the ovum becomes a single diploid cell known as a zygote. The zygote undergoes mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of a multicellular embryo[2]after passing through an organizational checkpoint during mid-embryogenesis.[3] In mammals, the term refers chiefly to the early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.[2][4]
The main stages of animal embryonic development are as follows:
- The zygote undergoes a series of cell divisions (called cleavage) to form a structure called a morula.
- The morula develops into a structure called a blastula through a process called blastulation.
- The blastula develops into a structure called a gastrula through a process called gastrulation.
- The gastrula then undergoes further development, including the formation of organs (organogenesis).
The embryo then transforms into the next stage of development, the nature of which varies between different animal species (examples of possible next stages include a fetus and a larva).
https://en.wikipedia.org/wiki/Animal_embryonic_development
Plant embryonic development, also plant embryogenesis is a process that occurs after the fertilization of an ovule to produce a fully developed plant embryo. This is a pertinent stage in the plant life cycle that is followed by dormancy and germination.[1] The zygote produced after fertilization must undergo various cellular divisions and differentiations to become a mature embryo.[1] An end stage embryo has five major components including the shoot apical meristem, hypocotyl, root meristem, root cap, and cotyledons.[1] Unlike the embryonic development in animals, and specifically in humans, plant embryonic development results in an immature form of the plant, lacking most structures like leaves, stems, and reproductive structures.[2] However, both plants and animals including humans, pass through a phylotypic stage that evolved independently[3] and that causes a developmental constraint limiting morphological diversification.[4][5][6][7]
https://en.wikipedia.org/wiki/Plant_embryonic_development
A larva (/ˈlɑːrvə/; plural larvae /ˈlɑːrviː/) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
The larva's appearance is generally very different from the adult form (e.g. caterpillars and butterflies) including different unique structures and organs that do not occur in the adult form. Their diet may also be considerably different.
Larvae are frequently adapted to different environments than adults. For example, some larvae such as tadpoles live almost exclusively in aquatic environments, but can live outside water as adult frogs. By living in a distinct environment, larvae may be given shelter from predators and reduce competition for resources with the adult population.
Animals in the larval stage will consume food to fuel their transition into the adult form. In some organisms like polychaetes and barnacles, adults are immobile but their larvae are mobile, and use their mobile larval form to distribute themselves.[1][2]
Some larvae are dependent on adults to feed them. In many eusocial Hymenoptera species, the larvae are fed by female workers. In Ropalidia marginata (a paper wasp) the males are also capable of feeding larvae but they are much less efficient, spending more time and getting less food to the larvae.[3]
The larvae of some organisms (for example, some newts) can become pubescent and do not develop further into the adult form. This is a type of neoteny.[4]
It is a misunderstanding that the larval form always reflects the group's evolutionary history. This could be the case, but often the larval stage has evolved secondarily, as in insects.[5][6] In these cases the larval form may differ more than the adult form from the group's common origin.[7]
https://en.wikipedia.org/wiki/Larva
Dormancy is a period in an organism's life cycle when growth, development, and (in animals) physical activity are temporarily stopped. This minimizes metabolic activity and therefore helps an organism to conserve energy. Dormancy tends to be closely associated with environmental conditions. Organisms can synchronize entry to a dormant phase with their environment through predictive or consequential means. Predictive dormancy occurs when an organism enters a dormant phase before the onset of adverse conditions. For example, photoperiod and decreasing temperature are used by many plants to predict the onset of winter. Consequential dormancy occurs when organisms enter a dormant phase afteradverse conditions have arisen. This is commonly found in areas with an unpredictable climate. While very sudden changes in conditions may lead to a high mortality rate among animals relying on consequential dormancy, its use can be advantageous, as organisms remain active longer and are therefore able to make greater use of available resources.
https://en.wikipedia.org/wiki/Dormancy
A cotyledon (/ˌkɒtɪˈliːdən/; lit. 'seed leaf'; from Latin cotyledon;[1] from κοτυληδών (kotulēdṓn), gen. κοτυληδόνος(kotulēdónos), from κοτύλη (kotýlē) 'cup, bowl') is a significant part of the embryo within the seed of a plant, and is defined as "the embryonic leaf in seed-bearing plants, one or more of which are the first to appear from a germinating seed."[2] The number of cotyledons present is one characteristic used by botanists to classify the flowering plants (angiosperms). Species with one cotyledon are called monocotyledonous ("monocots"). Plants with two embryonic leaves are termed dicotyledonous("dicots").
In the case of dicot seedlings whose cotyledons are photosynthetic, the cotyledons are functionally similar to leaves. However, true leaves and cotyledons are developmentally distinct. Cotyledons are formed during embryogenesis, along with the root and shoot meristems, and are therefore present in the seed prior to germination. True leaves, however, are formed post-embryonically (i.e. after germination) from the shoot apical meristem, which is responsible for generating subsequent aerial portions of the plant.
The cotyledon of grasses and many other monocotyledons is a highly modified leaf composed of a scutellum and a coleoptile. The scutellum is a tissue within the seed that is specialized to absorb stored food from the adjacent endosperm. The coleoptile is a protective cap that covers the plumule (precursor to the stem and leaves of the plant).
Gymnosperm seedlings also have cotyledons. Gnetophytes, cycads, and ginkgos all have 2, whereas in conifers they are often variable in number (multicotyledonous), with 2-24 cotyledons forming a whorl at the top of the hypocotyl (the embryonic stem) surrounding the plumule. Within each species, there is often still some variation in cotyledon numbers, e.g. Monterey pine(Pinus radiata) seedlings have 5–9, and Jeffrey pine (Pinus jeffreyi) 7–13 (Mirov 1967), but other species are more fixed, with e.g. Mediterranean cypress always having just two cotyledons. The highest number reported is for big-cone pinyon (Pinus maximartinezii), with 24 (Farjon & Styles 1997).
Cotyledons may be ephemeral, lasting only days after emergence, or persistent, enduring at least a year on the plant. The cotyledons contain (or in the case of gymnosperms and monocotyledons, have access to) the stored food reserves of the seed. As these reserves are used up, the cotyledons may turn green and begin photosynthesis, or may wither as the first true leaves take over food production for the seedling.[3]
https://en.wikipedia.org/wiki/Cotyledon
In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the integument, forming its outer layer, the nucellus (or remnant of the megasporangium), and the female gametophyte (formed from a haploid megaspore) in its center. The female gametophyte — specifically termed a megagametophyte— is also called the embryo sac in angiosperms. The megagametophyte produces an egg cell for the purpose of fertilization. The ovule is a small structure present in the ovary. It is attached to the placenta by a stalk called a funicle. The funicle provides nourishment to the ovule.
https://en.wikipedia.org/wiki/Ovule
A zygote (from Ancient Greek ζυγωτός (zygōtós) 'joined, yoked', from ζυγοῦν (zygoun) 'to join, to yoke')[1] is a eukaryotic cellformed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism.
In multicellular organisms, the zygote is the earliest developmental stage. In humans and most other anisogamous organisms, a zygote is formed when an egg cell and sperm cell come together to create a new unique organism. In single-celled organisms, the zygote can divide asexually by mitosis to produce identical offspring.
German zoologists Oscar and Richard Hertwig made some of the first discoveries on animal zygote formation in the late 19th century.
https://en.wikipedia.org/wiki/Zygote
A spermatogonial stem cell (SSC), also known as a type A spermatogonium, is a spermatogonium that does not differentiate into a spermatocyte, a precursor of sperm cells. Instead, they continue dividing into other spermatogonia or remain dormant to maintain a reserve of spermatogonia. Type B spermatogonia, on the other hand, differentiate into spermatocytes, which in turn undergo meiosis to eventually form mature sperm cells.
In mice[edit]
A Single (As) spermatogonia are capable of creating 2 separate daughter SSCs when they divide or the daughter cells can join and form A Paired (Apr) spermatogonia.
Both As and Apr spermatogonia are undifferentiated. Chains of these cells form and are referred to as A Aligned (Aal). Aal spermatogonia differentiate and thus are no longer classed as stem cells. They go on to divide 6 times eventually forming B type spermatogonia.
https://en.wikipedia.org/wiki/Spermatogonial_stem_cell
A peritubular myoid (PTM) cell is one of the smooth muscle cells which surround the seminiferous tubules in the testis.[1][2] These cells are present in all mammals but their organization and abundance varies between species.[2] The exact role of PTM cells is still somewhat uncertain and further work into this is needed. However, a number of functions of these cells have been established. They are contractile cells which contain actin filaments and are primarily involved in transport of spermatozoa through the tubules.[2] They provide structural integrity to the tubules through their involvement in laying down the basement membrane.[3] This has also been shown to affect Sertoli cell function and PTM cells also communicate with Sertoli cells through the secretion of growth factors and ECM (extra-cellular matrix) components.[3][2]Studies have shown PTM cells to be critical in achieving normal spermatogenesis.[3] Overall, PTM cells have a role in both maintaining the structure of the tubules and regulating spermatogenesis through cellular interaction.[2][1]
| Peritubular myoid cell | |
|---|---|
 Peritubular myoid cells in the adult mouse testis | |
| Details | |
| System | Reproductive, muscular |
| Location | Testis |
| Function | Contraction and transport of spermatoza through the tubules of the testis |
| Anatomical terms of microanatomy | |
https://en.wikipedia.org/wiki/Peritubular_myoid_cell
Anisogamy is a form of sexual reproduction that involves the union or fusion of two gametes that differ in size and/or form. The smaller gamete is male, a sperm cell, whereas the larger gamete is female, typically an egg cell. Anisogamy is predominant among multicellular organisms.[1] In both plants and animals gamete size difference is the fundamental difference between females and males.[2]
Anisogamy most likely evolved from isogamy.[3] Since the biological definition of male and female is based on gamete size, the evolution of anisogamy is viewed as the evolutionary origin of male and female sexes.[4] [5] Anisogamy is the prerequisite to sexual selection,[6] and led the sexes to different primary and secondary sex characteristics[7] including sex differences in behavior.[8]
Geoff Parker, Robin Baker and Vic Smith were the first to provide a mathematical model for the evolution of anisogamy that was consistent with modern evolutionary theory.[4] Their theory was widely accepted but there are alternative hypotheses about the evolution of anisogamy.[9][1]
https://en.wikipedia.org/wiki/Anisogamy
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.
https://en.wikipedia.org/wiki/Autogamy
Protists[edit]
Paramecium aurelia[edit]
Paramecium aurelia is the most commonly studied protozoan for autogamy. Similar to other unicellular organisms, Paramecium aurelia typically reproduce asexually via binary fission or sexually via cross-fertilization. However, studies have shown that when put under nutritional stress, Paramecium aurelia will undergo meiosis and subsequent fusion of gametic-like nuclei.[1] This process, defined as hemixis, a chromosomal rearrangement process, takes place in a number of steps. First, the two micronuclei of P. aurelia enlarge and divide two times to form eight nuclei. Some of these daughter nuclei will continue to divide to create potential future gametic nuclei. Of these potential gametic nuclei, one will divide two more times. Of the four daughter nuclei arising from this step, two of them become anlagen, or cells that will form part of the new organism. The other two daughter nuclei become the gametic micronuclei that will undergo autogamous self-fertilization.[2] These nuclear divisions are observed mainly when the P. aurelia is put under nutritional stress. Research shows that P. aurelia undergo autogamy synchronously with other individuals of the same species.
Clonal aging and rejuvenation[edit]
In Paramecium tetraurelia, vitality declines over the course of successive asexual cell divisions by binary fission. Clonal aging is associated with a dramatic increase in DNA damage.[3][4][5] When paramecia that have experienced clonal aging undergo meiosis, either during conjugation or automixis, the old macronucleus disintegrates and a new macronucleus is formed by replication of the micronuclear DNA that had just experienced meiosis followed by syngamy. These paramecia are rejuvenated in the sense of having a restored clonal lifespan. Thus it appears that clonal aging is due in large part to the progressive accumulation of DNA damage, and that rejuvenation is due to repair of DNA damage during meiosis that occurs in the micronucleus during conjugation or automixis and reestablishment of the macronucleus by replication of the newly repaired micronuclear DNA.
Tetrahymena rostrata[edit]
Similar to Paramecium aurelia, the parasitic ciliate Tetrahymena rostrata has also been shown to engage in meiosis, autogamy and development of new macronuclei when placed under nutritional stress.[6] Due to the degeneration and remodeling of genetic information that occurs in autogamy, genetic variability arises and possibly increases an offspring's chances of survival in stressful environments.
Allogromia laticollaris[edit]
Allogromia laticollaris is perhaps the best-studied foraminiferan amoeboid for autogamy. A. laticollaris can alternate between sexual reproduction via cross-fertilization and asexual reproduction via binary fission. The details of the life cycle of A. laticollaris are unknown, but similar to Paramecium aurelia, A. laticollaris is also shown to sometimes defer to autogamous behavior when placed in nutritional stress. As seen in Paramecium, there is some nuclear dimorphism observed in A. laticollaris. There are often observations of macronuclei and chromosomal fragments coexisting in A. laticollaris. This is indicative of nuclear and chromosomal degeneration, a process similar to the subdivisions observed in P. aurelia. Multiple generations of haploid A. laticollaris individuals can exist before autogamy actually takes place.[7] The autogamous behavior in A. laticollaris has the added consequence of giving rise to daughter cells that are substantially smaller than those rising from binary fission.[8] It is hypothesized that this is a survival mechanism employed when the cell is in stressful environments, and thus not able to allocate all resources to creating offspring. If a cell was under nutritional stress and not able to function regularly, there would be a strong possibility of its offspring's fitness being sub-par.
Self-pollination in flowering plants[edit]
About 10–15% of flowering plants are predominantly self-fertilizing.[9] Self-pollination is an example of autogamy that occurs in flowering plants. Self-pollination occurs when the sperm in the pollen from the stamen of a plant goes to the carpels of that same plant and fertilizes the egg cell present. Self-pollination can either be done completely autogamously or geitonogamously. In the former, the egg and sperm cells that united came from the same flower. In the latter, the sperm and egg cells can come from a different flower on the same plant. While the latter method does blur the lines between autogamous self-fertilization and normal sexual reproduction, it is still considered autogamous self-fertilization.[10]
Self-pollination can lead to inbreeding depression due to expression of deleterious recessive mutations.[11] Meiosis followed by self-pollination results in little genetic variation, raising the question of how meiosis in self-pollinating plants is adaptively maintained over an extended period in preference to a less complicated and less costly asexual ameiotic process for producing progeny. For instance, Arabidopsis thaliana is a predominantly self-pollinating plant that has an outcrossing rate in the wild estimated at less than 0.3%,[12] and self-pollination appears to have evolved roughly a million years ago or more.[13] An adaptive benefit of meiosis that may explain its long-term maintenance in self-pollinating plants is efficient recombinational repair of DNA damage.[14]
Fungi[edit]
There are basically two distinct types of sexual reproduction among fungi. The first is outcrossing (in heterothallic fungi). In this case, mating occurs between two different haploid individuals to form a diploid zygote, that can then undergo meiosis. The second type is self-fertilization or selfing (in homothallic fungi). In this case, two haploid nuclei derived from the same individual fuse to form a zygote than can then undergo meiosis. Examples of homothallic fungi that undergo selfing include species with an aspergillus-like asexual stage (anamorphs) occurring in many different genera,[15] several species of the ascomycete genus Cochliobolus,[16] and the ascomycete Pneumocystis jirovecii[17] (for other examples, see Homothallism). A review of evidence on the evolution of sexual reproduction in the fungi led to the concept that the original mode of sexual reproduction in the last eukaryotic common ancestor was homothallic or self-fertile unisexual reproduction.[18]
https://en.wikipedia.org/wiki/Autogamy#Clonal_aging_and_rejuvenation
Seminiferous tubules are located within the testes, and are the specific location of meiosis, and the subsequent creation of male gametes, namely spermatozoa.
https://en.wikipedia.org/wiki/Seminiferous_tubule
A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testosterone. Testosterone release is controlled by the anterior pituitary luteinizing hormone, whereas sperm production is controlled both by the anterior pituitary follicle-stimulating hormone and gonadal testosterone.
https://en.wikipedia.org/wiki/Testicle
The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary (from Latin ovarium 'egg, nut') found on each side of the body. The ovaries also secrete hormones that play a role in the menstrual cycle and fertility. The ovary progresses through many stages beginning in the prenatal period through menopause. It is also an endocrine glandbecause of the various hormones that it secretes.[1]
https://en.wikipedia.org/wiki/Ovary
In animals, a gland is a group of cells[1] in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland).
Structure[edit]
Development[edit]
Every gland is formed by an ingrowth from an epithelial surface. This ingrowth may in the beginning possess a tubular structure, but in other instances glands may start as a solid column of cells which subsequently becomes tubulated.[2]
As growth proceeds, the column of cells may split or give off offshoots, in which case a compound gland is formed. In many glands, the number of branches is limited, in others (salivary, pancreas) a very large structure is finally formed by repeated growth and sub-division. As a rule, the branches do not unite with one another, but in one instance, the liver, this does occur when a reticulated compound gland is produced. In compound glands the more typical or secretory epithelium is found forming the terminal portion of each branch, and the uniting portions form ducts and are lined with a less modified type of epithelial cell.[2]
Glands are classified according to their shape.
- If the gland retains its shape as a tube throughout it is termed a tubular gland.
- In the second main variety of gland the secretory portion is enlarged and the lumens variously increased in size. These are termed alveolar or saccular glands.[2]
Types of glands[edit]
Glands are divided based on their function into two groups:
Endocrine glands[edit]
Endocrine glands secrete substances that circulate through the blood stream. The glands secrete their products through basal lamina into the blood stream. Basal lamina typically can be seen as a layer around the glands to which a million, maybe more, tiny blood vessels are attached. These glands often secrete hormones which play an important role in maintaining homeostasis. The pineal gland, thymus gland, pituitary gland, thyroid gland, and the two adrenal glands are all endocrine glands.
Exocrine glands[edit]
Exocrine glands secrete their products through a duct onto an outer or inner surface of the body, such as the skin or the gastrointestinal tract. Secretion is directly onto the apical surface. The glands in this group can be divided into three groups:
- Apocrine glands – a portion of the secreting cell's body is lost during secretion. 'Apocrine glands' is often used to refer to the apocrine sweat glands, however it is thought that apocrine sweat glands may not be true apocrine glands as they may not use the apocrine method of secretion, e.g. mammary gland, sweat gland of arm pit, pubic region, skin around anus, lips and nipples.
- Holocrine glands – the entire cell disintegrates to secrete its substances, e.g. sebaceous glands: meibomian and zeis glands.
- Merocrine glands – cells secrete their substances by exocytosis, e.g. mucous and serous glands; also called "eccrine", e.g. max sweat gland of humans, goblet cells, salivary gland, tear gland and intestinal glands.
The type of secretory product of exocrine glands may also be one of three categories:
- Serous glands secrete a watery, often protein-rich, fluid-like product, e.g. sweat glands.
- Mucous glands secrete a viscous product, rich in carbohydrates (such as glycoproteins), e.g. goblet cells.
- Sebaceous glands secrete a lipid product. These glands are also known as oil glands, e.g. Fordyce spots and meibomian glands.
Clinical significance[edit]
![[icon]](https://en.wikipedia.org/wiki/File:Wiki_letter_w_cropped.svg){kind=small}
Adenosis is any disease of a gland. The diseased gland has abnormal formation or development of glandular tissue which is sometimes tumorous.[3]
https://en.wikipedia.org/wiki/Gland
Disadvantages of autogamy[edit]
In flowering plants, autogamy has the disadvantage of producing low genetic diversity in the species that use it as the predominant mode of reproduction. This leaves those species particularly susceptible to pathogens and viruses that can harm it. In addition, the foraminiferans that use autogamy have shown to produce substantially smaller progeny as a result.[20] This indicates that since it is generally an emergency survival mechanism for unicellular species, the mechanism does not have the nutritional resources that would be provided by the organism if it were undergoing binary fission.
Genetic consequences of self-fertilization[edit]
Self-fertilization results in the loss of genetic variation within an individual (offspring), because many of the genetic loci that were heterozygous become homozygous. This can result in the expression of harmful recessive alleles, which can have serious consequences for the individual. The effects are most extreme when self-fertilization occurs in organisms that are usually out-crossing.[21] In plants, selfing can occur as autogamous or geitonogamous pollinations and can have varying fitness affects that show up as autogamy depression. After several generations, inbreeding depression is likely to purge the deleterious alleles from the population because the individuals carrying them have mostly died or failed to reproduce.
If no other effects interfere, the proportion of heterozygous loci is halved in each successive generation, as shown in the following table.
- Parental : x (100%), and in
- 1 generation gives: : : , which means that the frequency of heterozygotes now is 50% of the starting value.
- By the 10 generation, heterozygotes have almost completely disappeared, and the population is polarized, with almost exclusively homozygous individuals (and )
Illustration model of the decrease in genetic variation in a population of self-fertilized organisms derived from a heterozygous individual, assuming equal fitness
| Generation | AA (%) | Aa (%) | aa (%) |
| P | – | 100 | – |
| F1 | 25 | 50 | 25 |
| F2 | 37.5 | 25 | 37.5 |
| F3 | 43.75 | 12.5 | 43.75 |
| F4 | 46.875 | 6.25 | 46.875 |
| F5 | 48.4375 | 3.125 | 48.4375 |
| F6 | 49.21875 | 1.5625 | 49.21875 |
| F7 | 49.609375 | 0.78125 | 49.609375 |
| F8 | 49.8046875 | 0.390625 | 49.8046875 |
| F9 | 49.90234375 | 0.1953125 | 49.90234375 |
| F10 | 49.995117187 ≈ 50.0 | 0.09765626 ≈ 0.0 | 49.995117187 ≈ 50.0 |
Evolution of autogamy[edit]
The evolutionary shift from outcrossing to self-fertilization is one of the most frequent evolutionary transitions in plants. Since autogamy in flowering plants and autogamy in unicellular species is fundamentally different, and plants and protists are not related, it is likely that both instances evolved separately. However, due to the little overall genetic variation that arises in progeny, it is not fully understood how autogamy has been maintained in the tree of life.
See also[edit]
- Effective selfing model
- Parthenogenesis
- Inbreeding
- Outcrossing
- Inbreeding depression
- Outbreeding depression
- Sequential hermaphroditism; the organism spends part of its life as a female and part as a male; self-fertilization is not possible.
Parthenogenesis (/ˌpɑːrθɪnoʊˈdʒɛnɪsɪs, -θɪnə-/;[1][2] from the Greek παρθένος, parthénos, 'virgin' + γένεσις, génesis, 'creation'[3]) is a natural form of asexual reproduction in which growth and development of embryos occur without fertilization by sperm. In animals, parthenogenesis means development of an embryo from an unfertilized egg cell. In plants parthenogenesis is a component process of apomixis.
Parthenogenesis occurs naturally in some plants, some invertebrate animal species (including nematodes, some tardigrades, water fleas, some scorpions, aphids, some mites, some bees, some Phasmatodea and parasitic wasps) and a few vertebrates (such as some fish,[4] amphibians, reptiles[5][6] and very rarely birds[7][8]). This type of reproduction has been induced artificially in a few species including fish, amphibians, and mice.[9][10]
Normal egg cells form in the process of meiosis and are haploid, with half as many chromosomes as their mother's body cells. Haploid individuals, however, are usually non-viable, and parthenogenetic offspring usually have the diploid chromosome number. Depending on the mechanism involved in restoring the diploid number of chromosomes, parthenogenetic offspring may have anywhere between all and half of the mother's alleles. The offspring having all of the mother's genetic material are called full clones and those having only half are called half clones. Full clones are usually formed without meiosis. If meiosis occurs, the offspring will get only a fraction of the mother's alleles since crossing over of DNA takes place during meiosis, creating variation.
Parthenogenetic offspring in species that use either the XY or the X0 sex-determination system have two X chromosomes and are female. In species that use the ZW sex-determination system, they have either two Z chromosomes (male) or two W chromosomes (mostly non-viable but rarely a female), or they could have one Z and one W chromosome (female).
Parthenogenesis does not apply to isogamous or monogamous species.[11]
https://en.wikipedia.org/wiki/Parthenogenesis
The nematodes (/ˈnɛmətoʊdz/ NEM-ə-tohdz or [2] NEEM- Greek: Νηματώδη; Latin: Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes),[3][4] with plant-parasitic nematodes also known as eelworms.[5] They are a diverse animal phylum inhabiting a broad range of environments. Taxonomically, they are classified along with insects and other moulting animals in the clade Ecdysozoa, and unlike flatworms, have tubular digestive systems with openings at both ends. Like tardigrades, they have a reduced number of Hox genes, but their sister phylum Nematomorpha has kept the ancestral protostome Hox genotype, which shows that the reduction has occurred within the nematode phylum.[6]
https://en.wikipedia.org/wiki/Nematode
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