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74-612: Hyphochytrids are eukaryotic organisms in the group of Stramenopiles (Heterokonta). They are distinguished by an anterior tinsel flagellum on their zoospores . Also they have a rhizoidal or hypha-like vegetative system (hence the prefix "Hypho-"). This group may be put alternatively at the phylum, class, subclass or order level, being referred to as Hyphochytriomycota, Hyphochytriomycetes (or Hyphochytrea), Hyphochytriomycetidae (or Hyphochytridae) and Hyphochytriales, respectively. The variants Hyphochytri di omycota and Hyphochytri di omycetes are also sometimes used, presumably by analogy to
148-405: A microtubular spindle during nuclear division, in the distinctively eukaryotic process of mitosis . Eukaryotes differ from prokaryotes in multiple ways, with unique biochemical pathways such as sterane synthesis. The eukaryotic signature proteins have no homology to proteins in other domains of life, but appear to be universal among eukaryotes. They include the proteins of the cytoskeleton,
222-419: A symbiotic union of a fungus and photosynthetic algae or cyanobacteria , reproduce through fragmentation to ensure that new individuals contain both symbionts. These fragments can take the form of soredia , dust-like particles consisting of fungal hyphae wrapped around photobiont cells. Clonal Fragmentation in multicellular or colonial organisms is a form of asexual reproduction or cloning where an organism
296-513: A "symbiosis-based phylogeny", giving the description "Eukarya (symbiosis-derived nucleated organisms)". By 2014, a rough consensus started to emerge from the phylogenomic studies of the previous two decades. The majority of eukaryotes can be placed in one of two large clades dubbed Amorphea (similar in composition to the unikont hypothesis) and the Diphoda (formerly bikonts), which includes plants and most algal lineages. A third major grouping,
370-430: A body, with its cells dividing by mitosis , and at some stage produce haploid gametes through meiosis , a division that reduces the number of chromosomes and creates genetic variability . There is considerable variation in this pattern. Plants have both haploid and diploid multicellular phases . Eukaryotes have lower metabolic rates and longer generation times than prokaryotes, because they are larger and therefore have
444-479: A bundle of microtubules arising from a centriole , characteristically arranged as nine doublets surrounding two singlets. Flagella may have hairs ( mastigonemes ), as in many stramenopiles . Their interior is continuous with the cell's cytoplasm . Centrioles are often present, even in cells and groups that do not have flagella, but conifers and flowering plants have neither. They generally occur in groups that give rise to various microtubular roots. These form
518-497: A chemical cue accumulates and induces the transition to sexual reproduction. Many protists and fungi alternate between sexual and asexual reproduction. A few species of amphibians, reptiles, and birds have a similar ability. The slime mold Dictyostelium undergoes binary fission (mitosis) as single-celled amoebae under favorable conditions. However, when conditions turn unfavorable, the cells aggregate and follow one of two different developmental pathways, depending on conditions. In
592-414: A fertilization event. These haploid individuals produce gametes through mitosis . Meiosis and gamete formation therefore occur in separate multicellular generations or "phases" of the life cycle, referred to as alternation of generations . Since sexual reproduction is often more narrowly defined as the fusion of gametes ( fertilization ), spore formation in plant sporophytes and algae might be considered
666-418: A form of asexual reproduction (agamogenesis) despite being the result of meiosis and undergoing a reduction in ploidy . However, both events (spore formation and fertilization) are necessary to complete sexual reproduction in the plant life cycle. Fungi and some algae can also utilize true asexual spore formation, which involves mitosis giving rise to reproductive cells called mitospores that develop into
740-567: A functionally similar manner by mitosis ; most of these are also capable of sexual reproduction. Multiple fission at the cellular level occurs in many protists , e.g. sporozoans and algae . The nucleus of the parent cell divides several times by mitosis , producing several nuclei. The cytoplasm then separates, creating multiple daughter cells . In apicomplexans , multiple fission, or schizogony appears either as merogony , sporogony or gametogony . Merogony results in merozoites , which are multiple daughter cells, that originate within
814-564: A male gamete. Examples are parthenogenesis and apomixis . Parthenogenesis is a form of agamogenesis in which an unfertilized egg develops into a new individual. It has been documented in over 2,000 species. Parthenogenesis occurs in the wild in many invertebrates (e.g. water fleas, rotifers , aphids, stick insects , some ants, bees and parasitic wasps) and vertebrates (mostly reptiles, amphibians, and fish). It has also been documented in domestic birds and in genetically altered lab mice. Plants can engage in parthenogenesis as well through
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#1732844028307888-415: A mate becomes difficult. For example, female zebra sharks will reproduce asexually if they are unable to find a mate in their ocean habitats. Parthenogenesis was previously believed to rarely occur in vertebrates, and only be possible in very small animals. However, it has been discovered in many more species in recent years. Today, the largest species that has been documented reproducing parthenogenically
962-429: A new organism after dispersal. This method of reproduction is found for example in conidial fungi and the red algae Polysiphonia , and involves sporogenesis without meiosis. Thus the chromosome number of the spore cell is the same as that of the parent producing the spores. However, mitotic sporogenesis is an exception and most spores, such as those of plants and many algae, are produced by meiosis . Fragmentation
1036-402: A phenomenon known as "egg parasitism." This method of reproduction has been found in several species of the clam genus Corbicula , many plants like, Cupressus dupreziana , Lomatia tasmanica , Pando and recently in the fish Squalius alburnoides . Other species where androgenesis has been observed naturally are the stick insects Bacillus rossius and Bassillus Grandii ,
1110-406: A primary component of the cytoskeleton, and are often assembled over the course of several cell divisions, with one flagellum retained from the parent and the other derived from it. Centrioles produce the spindle during nuclear division. The cells of plants, algae, fungi and most chromalveolates , but not animals, are surrounded by a cell wall. This is a layer outside the cell membrane , providing
1184-538: A process called apomixis . However this process is considered by many to not be an independent reproduction method, but instead a breakdown of the mechanisms behind sexual reproduction. Parthenogenetic organisms can be split into two main categories: facultative and obligate. In facultative parthenogenesis, females can reproduce both sexually and asexually. Because of the many advantages of sexual reproduction, most facultative parthenotes only reproduce asexually when forced to. This typically occurs in instances when finding
1258-402: A process called thelytoky . The freshwater crustacean Daphnia reproduces by parthenogenesis in the spring to rapidly populate ponds, then switches to sexual reproduction as the intensity of competition and predation increases. Monogonont rotifers of the genus Brachionus reproduce via cyclical parthenogenesis: at low population densities females produce asexually and at higher densities
1332-510: A smaller surface area to volume ratio. The evolution of sexual reproduction may be a primordial characteristic of eukaryotes. Based on a phylogenetic analysis, Dacks and Roger have proposed that facultative sex was present in the group's common ancestor. A core set of genes that function in meiosis is present in both Trichomonas vaginalis and Giardia intestinalis , two organisms previously thought to be asexual. Since these two species are descendants of lineages that diverged early from
1406-516: A sperm cell is used to initiate reproduction. However, the sperm's genes never get incorporated into the egg cell. The best known example of this is the Amazon molly . Because they are obligate parthenotes, there are no males in their species so they depend on males from a closely related species (the Sailfin molly ) for sperm. Apomixis in plants is the formation of a new sporophyte without fertilization. It
1480-417: A system of domains rather than kingdoms as top level rank being put forward by Carl Woese , Otto Kandler , and Mark Wheelis in 1990, uniting all the eukaryote kingdoms in the domain "Eucarya", stating, however, that " 'eukaryotes' will continue to be an acceptable common synonym". In 1996, the evolutionary biologist Lynn Margulis proposed to replace kingdoms and domains with "inclusive" names to create
1554-400: A year. This switch is triggered by environmental changes in the fall and causes females to develop eggs instead of embryos. This dynamic reproductive cycle allows them to produce specialized offspring with polyphenism , a type of polymorphism where different phenotypes have evolved to carry out specific tasks. The cape bee Apis mellifera subsp. capensis can reproduce asexually through
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#17328440283071628-548: Is a form of asexual reproduction where a new organism grows from a fragment of the parent. Each fragment develops into a mature, fully grown individual. Fragmentation is seen in many organisms. Animals that reproduce asexually include planarians , many annelid worms including polychaetes and some oligochaetes , turbellarians and sea stars . Many fungi and plants reproduce asexually. Some plants have specialized structures for reproduction via fragmentation, such as gemmae in mosses and liverworts . Most lichens , which are
1702-414: Is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes . The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the full set of genes of their single parent and thus the newly created individual is genetically and physically similar to the parent or an exact clone of the parent. Asexual reproduction
1776-559: Is a type of asexual reproduction found in plants where new individuals are formed without the production of seeds or spores and thus without syngamy or meiosis . Examples of vegetative reproduction include the formation of miniaturized plants called plantlets on specialized leaves, for example in kalanchoe ( Bryophyllum daigremontianum ) and many produce new plants from rhizomes or stolon (for example in strawberry ). Some plants reproduce by forming bulbs or tubers , for example tulip bulbs and Dahlia tubers. In these examples, all
1850-471: Is a widespread form of asexual reproduction in animals, whereby the fertilized egg or a later stage of embryonic development splits to form genetically identical clones. Within animals, this phenomenon has been best studied in the parasitic Hymenoptera . In the nine-banded armadillos , this process is obligatory and usually gives rise to genetically identical quadruplets. In other mammals, monozygotic twinning has no apparent genetic basis, though its occurrence
1924-610: Is closer in structure to bacterial RNA than to eukaryote RNA. Some eukaryotes, such as the metamonads Giardia and Trichomonas , and the amoebozoan Pelomyxa , appear to lack mitochondria, but all contain mitochondrion-derived organelles, like hydrogenosomes or mitosomes , having lost their mitochondria secondarily. They obtain energy by enzymatic action in the cytoplasm. Plants and various groups of algae have plastids as well as mitochondria. Plastids, like mitochondria, have their own DNA and are developed from endosymbionts , in this case cyanobacteria . They usually take
1998-402: Is common. There are at least 10 million identical human twins and triplets in the world today. Bdelloid rotifers reproduce exclusively asexually, and all individuals in the class Bdelloidea are females. Asexuality evolved in these animals millions of years ago and has persisted since. There is evidence to suggest that asexual reproduction has allowed the animals to evolve new proteins through
2072-418: Is derived entirely from pollen . Androgenesis occurs when a zygote is produced with only paternal nuclear genes . During standard sexual reproduction , one female and one male parent each produce haploid gametes (such as a sperm or egg cell, each containing only a single set of chromosomes ), which recombine to create offspring with genetic material from both parents. However, in androgenesis, there
2146-414: Is important in ferns and in flowering plants, but is very rare in other seed plants. In flowering plants, the term "apomixis" is now most often used for agamospermy , the formation of seeds without fertilization, but was once used to include vegetative reproduction . An example of an apomictic plant would be the triploid European dandelion . Apomixis mainly occurs in two forms: In gametophytic apomixis,
2220-618: Is much larger than that of prokaryotes. The eukaryotes seemingly emerged within the Asgard archaea , and are closely related to the Heimdallarchaeia . This implies that there are only two domains of life , Bacteria and Archaea, with eukaryotes incorporated among the Archaea. Eukaryotes first emerged during the Paleoproterozoic , likely as flagellated cells. The leading evolutionary theory
2294-766: Is no recombination of maternal and paternal chromosomes, and only the paternal chromosomes are passed down to the offspring (the inverse of this is gynogenesis , where only the maternal chromosomes are inherited, which is more common than androgenesis). The offspring produced in androgenesis will still have maternally inherited mitochondria , as is the case with most sexually reproducing species. Androgenesis occurs in nature in many invertebrates (for example, clams, stick insects, some ants, bees, flies and parasitic wasps ) and vertebrates (mainly amphibians and fish ). The androgenesis has also been seen in genetically modified laboratory mice. One of two things can occur to produce offspring with exclusively paternal genetic material:
Hyphochytriomycetes - Misplaced Pages Continue
2368-667: Is relatively rare among multicellular organisms , particularly animals . It is not entirely understood why the ability to reproduce sexually is so common among them. Current hypotheses suggest that asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, while sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments. Developmental constraints may underlie why few animals have relinquished sexual reproduction completely in their life-cycles. Almost all asexual modes of reproduction maintain meiosis either in
2442-441: Is split into fragments. Each of these fragments develop into mature, fully grown individuals that are clones of the original organism. In echinoderms , this method of reproduction is usually known as fissiparity . Due to many environmental and epigenetic differences, clones originating from the same ancestor might actually be genetically and epigenetically different. Agamogenesis is any form of reproduction that does not involve
2516-532: Is that their cells have nuclei . This gives them their name, from the Greek εὖ ( eu , "well" or "good") and κάρυον ( karyon , "nut" or "kernel", here meaning "nucleus"). Eukaryotic cells have a variety of internal membrane-bound structures, called organelles , and a cytoskeleton which defines the cell's organization and shape. The nucleus stores the cell's DNA , which is divided into linear bundles called chromosomes ; these are separated into two matching sets by
2590-521: Is the Komodo dragon at 10 feet long and over 300 pounds. Heterogony is a form of facultative parthenogenesis where females alternate between sexual and asexual reproduction at regular intervals (see Alternation between sexual and asexual reproduction ). Aphids are one group of organism that engages in this type of reproduction. They use asexual reproduction to reproduce quickly and create winged offspring that can colonize new plants and reproduce sexually in
2664-527: Is the primary form of reproduction for single-celled organisms such as archaea and bacteria . Many eukaryotic organisms including plants , animals , and fungi can also reproduce asexually. In vertebrates , the most common form of asexual reproduction is parthenogenesis , which is typically used as an alternative to sexual reproduction in times when reproductive opportunities are limited. Some monitor lizards , including Komodo dragons , can reproduce asexually. While all prokaryotes reproduce without
2738-864: Is they were created by symbiogenesis between an anaerobic Asgard archaean and an aerobic proteobacterium , which formed the mitochondria . A second episode of symbiogenesis with a cyanobacterium created the plants, with chloroplasts . Eukaryotic cells contain membrane-bound organelles such as the nucleus , the endoplasmic reticulum , and the Golgi apparatus . Eukaryotes may be either unicellular or multicellular . In comparison, prokaryotes are typically unicellular. Unicellular eukaryotes are sometimes called protists . Eukaryotes can reproduce both asexually through mitosis and sexually through meiosis and gamete fusion ( fertilization ). Eukaryotes are organisms that range from microscopic single cells , such as picozoans under 3 micrometres across, to animals like
2812-1197: The Excavata , has been abandoned as a formal group as it is paraphyletic . The proposed phylogeny below includes only one group of excavates ( Discoba ), and incorporates the 2021 proposal that picozoans are close relatives of rhodophytes. The Provora are a group of microbial predators discovered in 2022. Ancyromonadida [REDACTED] Malawimonada [REDACTED] CRuMs [REDACTED] Amoebozoa [REDACTED] Breviatea [REDACTED] Apusomonadida [REDACTED] Holomycota (inc. fungi) [REDACTED] Holozoa (inc. animals) [REDACTED] ? Metamonada [REDACTED] Discoba [REDACTED] Cryptista [REDACTED] Rhodophyta (red algae) [REDACTED] Picozoa [REDACTED] Glaucophyta [REDACTED] Viridiplantae (plants) [REDACTED] Hemimastigophora [REDACTED] Provora [REDACTED] Haptista [REDACTED] Telonemia [REDACTED] Rhizaria [REDACTED] Alveolata [REDACTED] Stramenopiles [REDACTED] [REDACTED] Asexual reproduction Asexual reproduction
2886-661: The Meselson effect that have allowed them to survive better in periods of dehydration. Bdelloid rotifers are extraordinarily resistant to damage from ionizing radiation due to the same DNA-preserving adaptations used to survive dormancy. These adaptations include an extremely efficient mechanism for repairing DNA double-strand breaks. This repair mechanism was studied in two Bdelloidea species, Adineta vaga , and Philodina roseola . and appears to involve mitotic recombination between homologous DNA regions within each species. Molecular evidence strongly suggests that several species of
2960-518: The archaea —having a volume of around 10,000 times greater. Eukaryotes represent a small minority of the number of organisms , but, as many of them are much larger, their collective global biomass (468 gigatons) is far larger than that of prokaryotes (77 gigatons), with plants alone accounting for over 81% of the total biomass of Earth . The eukaryotes are a diverse lineage, consisting mainly of microscopic organisms . Multicellularity in some form has evolved independently at least 25 times within
3034-583: The blue whale , weighing up to 190 tonnes and measuring up to 33.6 metres (110 ft) long, or plants like the coast redwood , up to 120 metres (390 ft) tall. Many eukaryotes are unicellular; the informal grouping called protists includes many of these, with some multicellular forms like the giant kelp up to 200 feet (61 m) long. The multicellular eukaryotes include the animals, plants, and fungi , but again, these groups too contain many unicellular species . Eukaryotic cells are typically much larger than those of prokaryotes —the bacteria and
Hyphochytriomycetes - Misplaced Pages Continue
3108-548: The domain of Eukaryota or Eukarya , organisms whose cells have a membrane-bound nucleus . All animals , plants , fungi , and many unicellular organisms are eukaryotes. They constitute a major group of life forms alongside the two groups of prokaryotes : the Bacteria and the Archaea . Eukaryotes represent a small minority of the number of organisms, but given their generally much larger size, their collective global biomass
3182-488: The hammerhead shark and the blacktip shark . In both cases, the sharks had reached sexual maturity in captivity in the absence of males, and in both cases the offspring were shown to be genetically identical to the mothers. The New Mexico whiptail is another example. Some reptiles use the ZW sex-determination system , which produces either males (with ZZ sex chromosomes) or females (with ZW or WW sex chromosomes). Until 2010, it
3256-431: The rotifer Brachionus calyciflorus asexual reproduction (obligate parthenogenesis ) can be inherited by a recessive allele, which leads to loss of sexual reproduction in homozygous offspring. Inheritance of asexual reproduction by a single recessive locus has also been found in the parasitoid wasp Lysiphlebus fabarum . Asexual reproduction is found in nearly half of the animal phyla. Parthenogenesis occurs in
3330-457: The stick insect genus Timema have used only asexual (parthenogenetic) reproduction for millions of years, the longest period known for any insect. Similar findings suggest that the mite species Oppiella nova may have reproduced entirely asexually for millions of years. In the grass thrips genus Aptinothrips there have been several transitions to asexuality, likely due to different causes. A complete lack of sexual reproduction
3404-510: The taxonomic rank of kingdom by Linnaeus in the 18th century. Though he included the fungi with plants with some reservations, it was later realized that they are quite distinct and warrant a separate kingdom. The various single-cell eukaryotes were originally placed with plants or animals when they became known. In 1818, the German biologist Georg A. Goldfuss coined the word Protozoa to refer to organisms such as ciliates , and this group
3478-495: The Chytridiomycetes, or due to the perpetuation of a typographical error. However, the stem is Hyphochytri- (from Hyphochytrium ) and not Hyphochytridi- (from Chytridium ). In the past the classes Hyphochytridiomycetes, Oomycetes and Chytridiomycetes were grouped together in the now obsolete taxon Mastigomycotina as fungi with flagellate spores or gametes. Now the Chytridiomycetes are still considered true fungi, but
3552-709: The cell to move, change shape, or transport materials. The motor structures are microfilaments of actin and actin-binding proteins , including α- actinin , fimbrin , and filamin are present in submembranous cortical layers and bundles. Motor proteins of microtubules, dynein and kinesin , and myosin of actin filaments, provide dynamic character of the network. Many eukaryotes have long slender motile cytoplasmic projections, called flagella , or multiple shorter structures called cilia . These organelles are variously involved in movement, feeding, and sensation. They are composed mainly of tubulin , and are entirely distinct from prokaryotic flagella. They are supported by
3626-404: The cell with structural support, protection, and a filtering mechanism. The cell wall also prevents over-expansion when water enters the cell. The major polysaccharides making up the primary cell wall of land plants are cellulose , hemicellulose , and pectin . The cellulose microfibrils are linked together with hemicellulose, embedded in a pectin matrix. The most common hemicellulose in
3700-427: The complex transcription machinery, the membrane-sorting systems, the nuclear pore , and some enzymes in the biochemical pathways. Eukaryote cells include a variety of membrane-bound structures, together forming the endomembrane system. Simple compartments, called vesicles and vacuoles , can form by budding off other membranes. Many cells ingest food and other materials through a process of endocytosis , where
3774-622: The cytoplasm. Mitochondria are organelles in eukaryotic cells. The mitochondrion is commonly called "the powerhouse of the cell", for its function providing energy by oxidising sugars or fats to produce the energy-storing molecule ATP . Mitochondria have two surrounding membranes , each a phospholipid bilayer , the inner of which is folded into invaginations called cristae where aerobic respiration takes place. Mitochondria contain their own DNA , which has close structural similarities to bacterial DNA , from which it originated, and which encodes rRNA and tRNA genes that produce RNA which
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#17328440283073848-476: The embryo arises from an unfertilized egg within a diploid embryo sac that was formed without completing meiosis. In nucellar embryony , the embryo is formed from the diploid nucellus tissue surrounding the embryo sac. Nucellar embryony occurs in some citrus seeds. Male apomixis can occur in rare cases, such as in the Saharan Cypress Cupressus dupreziana , where the genetic material of the embryo
3922-406: The eukaryotes. Complex multicellular organisms, not counting the aggregation of amoebae to form slime molds , have evolved within only six eukaryotic lineages: animals , symbiomycotan fungi , brown algae , red algae , green algae , and land plants . Eukaryotes are grouped by genomic similarities, so that groups often lack visible shared characteristics. The defining feature of eukaryotes
3996-484: The eukaryotic evolutionary tree, core meiotic genes, and hence sex, were likely present in the common ancestor of eukaryotes. Species once thought to be asexual, such as Leishmania parasites, have a sexual cycle. Amoebae, previously regarded as asexual, may be anciently sexual; while present-day asexual groups could have arisen recently. In antiquity , the two lineages of animals and plants were recognized by Aristotle and Theophrastus . The lineages were given
4070-438: The fall to lay eggs for the next season. However, some aphid species are obligate parthenotes. In obligate parthenogenesis, females only reproduce asexually. One example of this is the desert grassland whiptail lizard , a hybrid of two other species. Typically hybrids are infertile but through parthenogenesis this species has been able to develop stable populations. Gynogenesis is a form of obligate parthenogenesis where
4144-646: The form of chloroplasts which, like cyanobacteria, contain chlorophyll and produce organic compounds (such as glucose ) through photosynthesis . Others are involved in storing food. Although plastids probably had a single origin, not all plastid-containing groups are closely related. Instead, some eukaryotes have obtained them from others through secondary endosymbiosis or ingestion. The capture and sequestering of photosynthetic cells and chloroplasts, kleptoplasty , occurs in many types of modern eukaryotic organisms. The cytoskeleton provides stiffening structure and points of attachment for motor structures that enable
4218-495: The formation and fusion of gametes, mechanisms for lateral gene transfer such as conjugation , transformation and transduction can be likened to sexual reproduction in the sense of genetic recombination in meiosis . Prokaryotes ( Archaea and Bacteria ) reproduce asexually through binary fission , in which the parent organism divides in two to produce two genetically identical daughter organisms. Eukaryotes (such as protists and unicellular fungi ) may reproduce in
4292-527: The individuals are clones, and the clonal population may cover a large area. Many multicellular organisms produce spores during their biological life cycle in a process called sporogenesis . Exceptions are animals and some protists, which undergo meiosis immediately followed by fertilization. Plants and many algae on the other hand undergo sporic meiosis where meiosis leads to the formation of haploid spores rather than gametes. These spores grow into multicellular individuals called gametophytes , without
4366-502: The little fire ant Wasmannia auropunctata , Vollenhovia emeryi , Paratrechina longicornis , occasionally in Apis mellifera , the Hypseleotris carp gudgeons, the parasitoid Venturia canescens , and occasionally in fruit flies Drosophila melanogaster carrying a specific mutant allele. It has also been induced in many crops and fish via irradiation of an egg cell to destroy
4440-404: The little fire ant Wasmannia auropunctata , Vollenhovia emeryi , Paratrechina longicornis , occasionally in Apis mellifera , the Hypseleotris carp gudgeons, the parasitoid Venturia canescens , and occasionally in fruit flies Drosophila melanogaster carrying a specific mutant allele. It has also been induced in many crops and fish via irradiation of an egg cell to destroy
4514-412: The maternal nuclear genome can be eliminated from the zygote, or the female can produce an egg with no nucleus , resulting in an embryo developing with only the genome of the male gamete. Other type of androgenesis is the male apomixis or paternal apomixis is a reproductive process in which a plant develops from a sperm cell (male gamete) without the participation of a female cell (ovum). In this process,
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#17328440283074588-415: The maternal nuclear genome. Obligate androgenesis is the process in which males are capable of producing both eggs and sperm, however, the eggs have no genetic contribution and the offspring come only from the sperm, which allows these individuals to self-fertilize and produce clonal offspring without the need for females. They are also capable of interbreeding with sexual and other androgenetic lineages in
4662-591: The maternal nuclear genome. Some species can alternate between sexual and asexual strategies, an ability known as heterogamy , depending on many conditions. Alternation is observed in several rotifer species (cyclical parthenogenesis e.g. in Brachionus species) and a few types of insects. One example of this is aphids which can engage in heterogony. In this system, females are born pregnant and produce only female offspring. This cycle allows them to reproduce very quickly. However, most species reproduce sexually once
4736-428: The more common apomixis, where development occurs without fertilization, but with genetic material only from the mother. There are also clonal species that reproduce through vegetative reproduction like Lomatia tasmanica and Pando , where the genetic material is exclusively male. Other species where androgenesis has been observed naturally are the stick insects Bacillus rossius and Bassillus Grandii ,
4810-591: The other two sub-groups are classified in the kingdom Protista , or in the group Stramenopiles . Hyphochytriomycetes are closely related to Oomycetes . Order Hyphochytriales Bessey 1950 ex Sparrow 1960 This is a relatively small group, composed of about 16 known species, which may be due in part, to sampling methods of scientists. This heterokont -related article is a stub . You can help Misplaced Pages by expanding it . Eukaryote The eukaryotes ( / j uː ˈ k ær i oʊ t s , - ə t s / yoo- KARR -ee-ohts, -əts ) constitute
4884-449: The outer membrane invaginates and then pinches off to form a vesicle. Some cell products can leave in a vesicle through exocytosis . The nucleus is surrounded by a double membrane known as the nuclear envelope , with nuclear pores that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form the endoplasmic reticulum , which is involved in protein transport and maturation. It includes
4958-557: The parent organism. Internal budding is a process of asexual reproduction, favoured by parasites such as Toxoplasma gondii . It involves an unusual process in which two ( endodyogeny ) or more ( endopolygeny ) daughter cells are produced inside a mother cell, which is then consumed by the offspring prior to their separation. Also, budding (external or internal) occurs in some worms like Taenia or Echinococcus ; these worms produce cysts and then produce (invaginated or evaginated) protoscolex with budding . Vegetative propagation
5032-417: The primary cell wall is xyloglucan . Eukaryotes have a life cycle that involves sexual reproduction , alternating between a haploid phase, where only one copy of each chromosome is present in each cell, and a diploid phase, with two copies of each chromosome in each cell. The diploid phase is formed by fusion of two haploid gametes, such as eggs and spermatozoa , to form a zygote ; this may grow into
5106-498: The rough endoplasmic reticulum, covered in ribosomes which synthesize proteins; these enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the smooth endoplasmic reticulum. In most eukaryotes, these protein-carrying vesicles are released and further modified in stacks of flattened vesicles ( cisternae ), the Golgi apparatus . Vesicles may be specialized; for instance, lysosomes contain digestive enzymes that break down biomolecules in
5180-446: The same cell membrane, sporogony results in sporozoites , and gametogony results in micro gametes . Some cells divide by budding (for example baker's yeast ), resulting in a "mother" and a "daughter" cell that is initially smaller than the parent. Budding is also known on a multicellular level; an animal example is the hydra , which reproduces by budding. The buds grow into fully matured individuals which eventually break away from
5254-740: The social pathway, they form a multi-cellular slug which then forms a fruiting body with asexually generated spores. In the sexual pathway, two cells fuse to form a giant cell that develops into a large cyst. When this macrocyst germinates, it releases hundreds of amoebic cells that are the product of meiotic recombination between the original two cells. The hyphae of the common mold ( Rhizopus ) are capable of producing both mitotic as well as meiotic spores. Many algae similarly switch between sexual and asexual reproduction. A number of plants use both sexual and asexual means to produce new plants, some species alter their primary modes of reproduction from sexual to asexual under varying environmental conditions. In
5328-489: The zygote is formed solely with genetic material from the father, resulting in offspring genetically identical to the male organism. This has been noted in many plants like Nicotiana , Capsicum frutescens , Cicer arietinum , Poa arachnifera , Solanum verrucosum , Phaeophyceae , Pripsacum dactyloides , Zea mays , and occurs as the regular reproductive method in Cupressus dupreziana . This contrasts with
5402-498: Was expanded until Ernst Haeckel made it a kingdom encompassing all single-celled eukaryotes, the Protista , in 1866. The eukaryotes thus came to be seen as four kingdoms: The protists were at that time thought to be "primitive forms", and thus an evolutionary grade , united by their primitive unicellular nature. Understanding of the oldest branchings in the tree of life only developed substantially with DNA sequencing , leading to
5476-475: Was thought that the ZW chromosome system used by reptiles was incapable of producing viable WW offspring, but a (ZW) female boa constrictor was discovered to have produced viable female offspring with WW chromosomes. The female boa could have chosen any number of male partners (and had successfully in the past) but on this occasion she reproduced asexually, creating 22 female babies with WW sex-chromosomes. Polyembryony
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