Alternation of generations (also known as metagenesis or heterogenesis ) is the predominant type of life cycle in plants and algae . In plants both phases are multicellular : the haploid sexual phase – the gametophyte – alternates with a diploid asexual phase – the sporophyte .
85-408: Xenophyophorea / ˌ z ɛ n ə ˌ f aɪ ə ˈ f oʊ r iː ə / is a clade of foraminiferans . Xenophyophores are multinucleate unicellular organisms found on the ocean floor throughout the world's oceans , at depths of 500 to 10,600 metres (1,600 to 34,800 ft). They are a kind of foraminiferan that extract minerals from their surroundings and use them to form an exoskeleton known as
170-439: A gymnosperm is similar. However, flowering plants have in addition a phenomenon called ' double fertilization '. In the process of double fertilization , two sperm nuclei from a pollen grain (the microgametophyte), rather than a single sperm, enter the archegonium of the megagametophyte; one fuses with the egg nucleus to form the zygote, the other fuses with two other nuclei of the gametophyte to form ' endosperm ', which nourishes
255-408: A phylum or class of Rhizarian protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly an external shell (called a " test ") of diverse forms and materials. Tests of chitin (found in some simple genera, and Textularia in particular) are believed to be the most primitive type. Most foraminifera are marine, the majority of which live on or within
340-466: A test , or shell, which can have either one or multiple chambers, some becoming quite elaborate in structure. These shells are commonly made of calcium carbonate ( CaCO 3 ) or agglutinated sediment particles. Over 50,000 species are recognized, both living (6,700–10,000) and fossil (40,000). They are usually less than 1 mm in size, but some are much larger, the largest species reaching up to 20 cm. In modern scientific English,
425-438: A test . They were first described by Henry Bowman Brady in 1883. They are abundant on abyssal plains , and in some regions are the dominant species. Fifteen genera and 75 species have been described, varying widely in size. The largest, Syringammina fragilissima , is among the largest known coenocytes , reaching up to 20 centimetres (8 in) in diameter. The name Xenophyophora means "bearer of foreign bodies", from
510-728: A bewildering variety of life cycles. The terms used by botanists in describing these life cycles can be equally bewildering. As Bateman and Dimichele say "[...] the alternation of generations has become a terminological morass; often, one term represents several concepts or one concept is represented by several terms." Possible variations are: There are some correlations between these variations, but they are just that, correlations, and not absolute. For example, in flowering plants, microspores ultimately produce microgametes (sperm) and megaspores ultimately produce megagametes (eggs). However, in ferns and their allies there are groups with undifferentiated spores but differentiated gametophytes. For example,
595-421: A complex triphasic alternation of generations, in which there is a gametophyte phase and two distinct sporophyte phases. For further information, see Red algae: Reproduction . Land plants all have heteromorphic (anisomorphic) alternation of generations, in which the sporophyte and gametophyte are distinctly different. All bryophytes , i.e. liverworts , mosses and hornworts , have the gametophyte generation as
680-616: A few cells which grow entirely inside the sporophyte. Animals develop differently. They directly produce haploid gametes. No haploid spores capable of dividing are produced, so generally there is no multicellular haploid phase. Some insects have a sex-determining system whereby haploid males are produced from unfertilized eggs; however females produced from fertilized eggs are diploid. Life cycles of plants and algae with alternating haploid and diploid multicellular stages are referred to as diplohaplontic . The equivalent terms haplodiplontic , diplobiontic and dibiontic are also in use, as
765-514: A few millimetres to 20 centimetres across. The softness and structure of tests varies from soft and lumpy shapes to fans and complex structures. Some xenophyophores—notably Psammina —have compartmentalized tests consisting of multiple chambers. Species of this group are morphologically variable, but the general structural pattern includes a test enclosing a branching system of organic tubules together with masses of waste material. A number of unique terms are used to refer to anatomical aspects of
850-465: A haploid ( n ) generation of multicellular gametophytes. The situation is quite different from that in animals, where the fundamental process is that a multicellular diploid (2 n ) individual directly produces haploid ( n ) gametes by meiosis. In animals, spores (i.e. haploid cells which are able to undergo mitosis) are not produced, so there is no asexual multicellular generation. Some insects have haploid males that develop from unfertilized eggs, but
935-556: A host of larger organisms, including invertebrates, fish, shorebirds, and other foraminifera. It has been suggested, however, that in some cases predators may be more interested in the calcium from foram shells than in the organisms themselves. Several aquatic snail species are known to selectively feed upon foraminifera, often even preferring individual species. Certain benthic foraminifera have been found to be capable of surviving anoxic conditions for over 24 hours, indicating that they are capable of selective anaerobic respiration . This
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#17328510439761020-653: A major group within the Protozoa known as the Rhizaria . Prior to the recognition of evolutionary relationships among the members of the Rhizaria, the Foraminifera were generally grouped with other amoeboids as phylum Rhizopodea (or Sarcodina) in the class Granuloreticulosa. The Rhizaria are problematic, as they are often called a "supergroup", rather than using an established taxonomic rank such as phylum . Cavalier-Smith defines
1105-467: A nerve tube. Alcide d'Orbigny , in his 1826 work, considered them to be a group of minute cephalopods and noted their odd morphology, interpreting the pseudopodia as tentacles and noting the highly reduced (in actuality, absent) head. He named the group foraminifères , or "hole-bearers", as members of the group had holes in the divisions between compartments in their shells, in contrast to nautili or ammonites . The protozoan nature of foraminifera
1190-623: A process referred to as plasmogamy and karyogamy to form a diploid zygote. The zygote develops into a plasmodium, and the mature plasmodium produces, depending on the species, one to many fruiting bodies containing haploid spores. Alternation between a multicellular diploid and a multicellular haploid generation is never encountered in animals. In some animals, there is an alternation between parthenogenic and sexually reproductive phases ( heterogamy ), for instance in salps and doliolids (class Thaliacea ). Both phases are diploid. This has sometimes been called "alternation of generations", but
1275-420: A process which reduces the number of chromosomes to half, from two sets to one. The resulting haploid spores germinate and grow into multicellular haploid gametophytes. At maturity, a gametophyte produces gametes by mitosis , the normal process of cell division in eukaryotes, which maintains the original number of chromosomes. Two haploid gametes (originating from different organisms of the same species or from
1360-400: A single opening or through many perforations in the test. Individual pseudopods characteristically have small granules streaming in both directions. Foraminifera are unique in having granuloreticulose pseudopodia ; that is, their pseudopodia appear granular under the microscope; these pseudopodia are often elongate and may split and rejoin each other. These can be extended and retracted to suit
1445-506: A specialized group of monothalamous (single-chambered) Foraminifera . A 2013 molecular study using small subunit rDNA found Syringammina and Shinkaiya to form a monophyletic clade closely related to Rhizammina algaeformis . Further molecular evidence has confirmed the monophyly of xenophyophores. This study also suggested that many individual genera are polyphyletic, with similar body shapes convergently evolving multiple times. Historically xenophyophores have been divided into
1530-418: A spore will germinate and grow into a rather inconspicuous plant body called a prothallus . The haploid prothallus does not resemble the sporophyte, and as such ferns and their allies have a heteromorphic alternation of generations. The prothallus is short-lived, but carries out sexual reproduction, producing the diploid zygote that then grows out of the prothallus as the sporophyte. In the spermatophytes ,
1615-594: A stalked bulb (in Cerelasma ). Gametes are reportedly about 20 μm in diameter, with two flagella; after this, an amoeba-like stage seems to be present. It is also possible that the amoeboid stage represents amoeboid gametes, found in other foraminifera. These amoeboid structures are also sometimes found inside the granellare. Juveniles have occasionally been found in association with adults; in Psametta they are horseshoe-shaped and already covered in xenophyae. The location of
1700-445: A xenophyophore's test. Brittle stars (Ophiuroidea) also appear to have a relationship with xenophyophores, as they are consistently found directly underneath or on top of the protozoans. They can also function as nurseries for fish; snailfish have been found to lay eggs in the shelter of the xenophyophore test. Starfish , monoplacophorans , and molpadiid sea cucumbers have all been observed feeding on xenophyophores; specifically,
1785-449: Is also common in the group, and at least some species can take advantage of dissolved organic carbon . A few foram species are parasitic , infecting sponges, molluscs, corals, or even other foraminifera. Parasitic strategies vary; some act as ectoparasites, using their pseudopodia to steal food from the host, while others burrow through the shell or body wall of their host to feed on its soft tissue. Foraminifera are themselves eaten by
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#17328510439761870-402: Is called karyogamy , and may not occur until sporangia are formed. Karogamy produces a diploid zygote, which is a short-lived sporophyte that soon undergoes meiosis to form haploid spores. When the spores germinate, they develop into new mycelia. The life cycle of slime moulds is very similar to that of fungi. Haploid spores germinate to form swarm cells or myxamoebae . These fuse in
1955-480: Is describing such an organism as having a diphasic ontogeny . Life cycles of animals, in which there is only a diploid multicellular stage, are referred to as diplontic . Life cycles in which there is only a haploid multicellular stage are referred to as haplontic . Alternation of generations is defined as the alternation of multicellular diploid and haploid forms in the organism's life cycle, regardless of whether these forms are free-living. In some species, such as
2040-536: Is interpreted as an adaptation to survive changing oxygenic conditions near the sediment-water interface. Foraminifera are found in the deepest parts of the ocean such as the Mariana Trench , including the Challenger Deep , the deepest part known. At these depths, below the carbonate compensation depth , the calcium carbonate of the tests is soluble in water due to the extreme pressure. The Foraminifera found in
2125-557: Is known of their life history . As they occur in all the world's oceans and in great numbers, xenophyophores could be indispensable agents in the process of sediment deposition and in maintaining biological diversity in benthic ecosystems. Scientists in the submersible DSV Alvin at a depth of 3,088 metres at the Alaskan continental margin in the Gulf of Alaska collected a spatangoid urchin , Cystochinus loveni , about 5 cm diameter, which
2210-404: Is no change in nuclear phase, but the alternation of generations is maintained. The diagram above shows the fundamental elements of the alternation of generations in plants. There are many variations in different groups of plants. The processes involved are as follows: The 'alternation of generations' in the life cycle is thus between a diploid (2 n ) generation of multicellular sporophytes and
2295-426: Is no more effective in the haploid than in the diploid phases of the lifecycle of mosses and angiosperms. Some organisms currently classified in the clade Rhizaria and thus not plants in the sense used here, exhibit alternation of generations. Most Foraminifera undergo a heteromorphic alternation of generations between haploid gamont and diploid agamont forms. The diploid form is typically much larger than
2380-415: Is recognizable as being Elphidium . Early workers classified foraminifera within the genus Nautilus , noting their similarity to certain cephalopods . It was recognised by Lorenz Spengler in 1781 that foraminifera had holes in the septa, which would eventually grant the group its name. Spengler also noted that the septa of foraminifera arced the opposite way from those of nautili and that they lacked
2465-481: The Greek . This refers to the sediments, called xenophyae, which are cemented together to construct their tests . In 1883, Henry Bowman Brady classified them as primitive Foraminifera . Later they were placed within the sponges . In the beginning of the 20th century they were considered an independent class of Rhizopoda, and later as a new eukaryotic phylum of Protista . Phylogenetic studies suggest that xenophyophores are
2550-530: The green algae , red algae , golden algae , diatoms , and dinoflagellates . These mixotrophic foraminifers are particularly common in nutrient-poor oceanic waters. Some forams are kleptoplastic , retaining chloroplasts from ingested algae to conduct photosynthesis . Most foraminifera are heterotrophic, consuming smaller organisms and organic matter; some smaller species are specialised feeders on phytodetritus , while others specialise in consuming diatoms. Some benthic forams construct feeding cysts, using
2635-512: The seafloor sediment (i.e., are benthic , with different sized species playing a role within the macrobenthos , meiobenthos , and microbenthos ), while a smaller number float in the water column at various depths (i.e., are planktonic ), which belong to the suborder Globigerinina . Fewer are known from freshwater or brackish conditions, and some very few (nonaquatic) soil species have been identified through molecular analysis of small subunit ribosomal DNA . Foraminifera typically produce
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2720-514: The 5th century BCE noted them as making up the rock that forms the Great Pyramid of Giza . These are today recognized as representatives of the genus Nummulites . Strabo , in the 1st Century BCE, noted the same foraminifera, and suggested that they were the remains of lentils left by the workers who built the pyramids. Robert Hooke observed a foraminifera under the microscope, as described and illustrated in his 1665 book Micrographia : I
2805-557: The Challenger Deep thus have no carbonate test, but instead have one of organic material. Nonmarine foraminifera have traditionally been neglected in foram research, but recent studies show them to be substantially more diverse than previously known. They are known to inhabit disparate ecological niches, including mosses , rivers, lakes and ponds, wetlands, soils, peat bogs , and sand dunes. Alternation of generations A mature sporophyte produces haploid spores by meiosis ,
2890-477: The Eocene fossil Benkovacina . However, analysis of the latter found neither barite crystals nor evidence of agglutinated foraminifera in the wall. A 2011 study that examined growth and development of Palaeopascichnus concluded it was likely not a xenophyophore. A 2014 study of Pteridinum reached similar conclusions. Some researchers have suggested that the enigmatic graphoglyptids Paleodictyon , known from
2975-474: The Foraminifera has varied since Schultze in 1854, who referred to as an order, Foraminiferida. Loeblich (1987) and Tappan (1992) reranked Foraminifera as a class as it is now commonly regarded. The Foraminifera have typically been included in the Protozoa , or in the similar Protoctista or Protist kingdom . Compelling evidence, based primarily on molecular phylogenetics , exists for their belonging to
3060-505: The Rhizaria as an infra-kingdom within the kingdom Protozoa. Some taxonomies put the Foraminifera in a phylum of their own, putting them on par with the amoeboid Sarcodina in which they had been placed. Although as yet unsupported by morphological correlates, molecular data strongly suggest the Foraminifera are closely related to the Cercozoa and Radiolaria , both of which also include amoeboids with complex shells; these three groups make up
3145-524: The Rhizaria. However, the exact relationships of the forams to the other groups and to one another are still not entirely clear. Foraminifera are closely related to testate amoebae . The most striking aspect of most foraminifera are their hard shells, or tests. These may consist of one of multiple chambers, and may be composed of protein, sediment particles, calcite, aragonite, or (in one case) silica. Some foraminifera lack tests entirely. Unlike other shell-secreting organisms, such as molluscs or corals ,
3230-503: The agglutinated Psamminida and the flexible, proteinaceous Stannomida . However, cladistic analyses based on molecular data have suggested a high amount of homoplasy , and that the division between psamminids and stannomids is not well supported. Xenophyophores are unicellular, but have many nuclei . Many form delicate and elaborate agglutinated tests —shells often made of calcium carbonate (CaCO 3 ) and other foreign mineral particles glued together with organic cements—that range from
3315-458: The alga Ulva lactuca , the diploid and haploid forms are indeed both free-living independent organisms, essentially identical in appearance and therefore said to be isomorphic . In many algae, the free-swimming, haploid gametes form a diploid zygote which germinates into a multicellular diploid sporophyte. The sporophyte produces free-swimming haploid spores by meiosis that germinate into haploid gametophytes. However, in land plants , either
3400-409: The alternation between diploid and haploid nuclear phases, also called cytological alternation of nuclear phases. Although most often coinciding, morphological alternation and nuclear phases alternation are sometimes independent of one another, e.g., in many red algae , the same nuclear phase may correspond to two diverse morphological generations. In some ferns which lost sexual reproduction, there
3485-470: The barium content was later determined to be due to diagenetic alteration of the material and the morphology of the specimen instead supported an algal affinity. Local population densities may be as high as 2,000 individuals per 100 square metres (1,100 sq ft), making them dominant organisms in some areas. Xenophyophores have been found to be " ecosystem engineers ", providing habitat and serving as traps for organic particles, increasing diversity in
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3570-470: The developing embryo. It has been proposed that the basis for the emergence of the diploid phase of the life cycle (sporophyte) as the dominant phase (e.g. as in vascular plants) is that diploidy allows masking of the expression of deleterious mutations through genetic complementation . Thus if one of the parental genomes in the diploid cells contained mutations leading to defects in one or more gene products , these deficiencies could be compensated for by
3655-491: The development of trematodes in 1842, and also tunicates and cnidarians ) described the succession of differently organized generations (sexual and asexual) in animals as "alternation of generations". Later, the phenomenon in animals became known as heterogamy , while the term "alternation of generations" was restricted to the life cycles of plants, meaning specifically the alternation of haploid gametophytes and diploid sporophytes. In 1851, Wilhelm Hofmeister demonstrated
3740-428: The discovery of C 27 sterols associated with the fossils of Dickinsonia cast doubt on this identification, as these sterols are today associated only with animals. These researchers suggest that Dickinsonia and relatives are instead stem - bilaterians . Other ediacaran fossils, such as Palaeopascichnus Intrites , Yelovichnus , and Neonereites have been posited as fossil xenophyophores and linked to
3825-793: The dominant scheme of classification until Tappan and Loeblich's 1964 classification, which placed foraminifera into the general groupings still used today, based on microstructure of the test wall. These groups have been variously moved around according to different schemes of higher-level classification. Pawlowski's (2013) use of molecular systematics has generally confirmed Tappan and Loeblich's groupings, with some being found as polyphyletic or paraphyletic; this work has also helped to identify higher-level relationships among major foraminiferal groups. "Monothalamids" (paraphyletic) Lagenida "Monothalamids" Miliolida Spirillinida "Monothalamids" Xenophyophorea " Textulariida " (paraphyletic) Robertinida Rotaliida The taxonomic position of
3910-455: The early Cambrian through recent times, could represent the remains of xenophyophores, and noted the similarity of the extant xenophyophore Occultammina to the fossil. Supporting this notion is the similar abyssal habitat of living xenophyophores to the inferred habitat of fossil graphoglyptids; however, the large size (up to 0.5m) and regularity of many graphoglyptids as well as the apparent absence of xenophyae in their fossils casts doubt on
3995-490: The exact purpose of these is unclear, but they have been suggested to function as a reservoir of nitrate. Mitochondria are distributed evenly throughout the cell, though in some species they are concentrated under the pores and around the external margin of the cell. This has been hypothesised to be an adaptation to low-oxygen environments. Several species of xenophyophore have been found to have unusually high concentrations of radioactive isotopes within their cells, among
4080-487: The females are all diploid. The diagram shown above is a good representation of the life cycle of some multi-cellular algae (e.g. the genus Cladophora ) which have sporophytes and gametophytes of almost identical appearance and which do not have different kinds of spores or gametes. However, there are many possible variations on the fundamental elements of a life cycle which has alternation of generations. Each variation may occur separately or in combination, resulting in
4165-431: The fern Ceratopteris thalictrioides has spores of only one kind, which vary continuously in size. Smaller spores tend to germinate into gametophytes which produce only sperm-producing antheridia. Plant life cycles can be complex. Alternation of generations can take place in plants which are at once heteromorphic, sporophytic, oogametic, dioicous, heterosporic and dioecious, such as in a willow tree (as most species of
4250-476: The fragile tests and deepwater habitat of the group makes in vivo observation difficult. Early propositions included suspension feeding , bacterial farming, deposit feeding, and trapping particulate matter inside the test. Studies have since confirmed active uptake of food from surrounding sediments using the pseudopodia and using the test to trap particles. Analysis of lipid concentrations within xenophyophores revealed especially high concentrations of bacteria in
4335-412: The gametophyte is less well developed than the sporophyte, although their Devonian ancestors had gametophytes and sporophytes of approximately equivalent complexity. In ferns the gametophyte is a small flattened autotrophic prothallus on which the young sporophyte is briefly dependent for its nutrition. In flowering plants , the reduction of the gametophyte is much more extreme; it consists of just
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#17328510439764420-517: The genus Salix are dioecious). The processes involved are: The term "plants" is taken here to mean the Archaeplastida , i.e. the glaucophytes , red and green algae and land plants . Alternation of generations occurs in almost all multicellular red and green algae, both freshwater forms (such as Cladophora ) and seaweeds (such as Ulva ). In most, the generations are homomorphic (isomorphic) and free-living. Some species of red algae have
4505-530: The group: The protoplasm of xenophyophores contributes less than 1% of the total mass of the organism. They select certain minerals and elements from their environment that are included in its tests and cytoplasm , or concentrated in excretions. The selected minerals vary with species, but often include barite, lead and uranium. The granellare of Shinkaiya have been found to contain high concentrations of mercury . Studies have found unusually high concentrations of radioactive nuclides in xenophyophores; this
4590-452: The haploid form; these forms are known as the microsphere and megalosphere , respectively. Fungal mycelia are typically haploid. When mycelia of different mating types meet, they produce two multinucleate ball-shaped cells, which join via a "mating bridge". Nuclei move from one mycelium into the other, forming a heterokaryon (meaning "different nuclei"). This process is called plasmogamy . Actual fusion to form diploid nuclei
4675-666: The highest of any eukaryote. The purpose of this is unknown. Modern Foraminifera are primarily marine organisms, but living individuals have been found in brackish, freshwater and even terrestrial habitats. The majority of the species are benthic , and a further 50 morphospecies are planktonic . This count may, however, represent only a fraction of actual diversity, since many genetically distinct species may be morphologically indistinguishable. Benthic foraminifera are typically found in fine-grained sediments, where they actively move between layers; however, many species are found on hard rock substrates, attached to seaweeds, or sitting atop
4760-729: The idea that multiple different characters must separate taxonomic groups, and as such placed agglutinated and calcareous genera in close relation. This overall scheme of classification would remain until Cushman 's work in the late 1920s. Cushman viewed wall composition as the single most important trait in classification of foraminifera; his classification became widely accepted but also drew criticism from colleagues for being "not biologically sound". Geologist Irene Crespin undertook extensive research in this field, publishing some ninety papers—including notable work on foraminifera—as sole author as well as more than twenty in collaboration with other scientists. Cushman's scheme nevertheless remained
4845-535: The initial plasma can sometimes be pointed out in adult xenophyophores. In some species this is denoted by a sharp change in the type of xenophyae; in others, the juvenile is regular and the adult is irregular; still others flip this pattern, so that the juvenile is irregular and the adult is regular. Growth is episodic; one observational study taking place over a period of eight months saw a three-to-tenfold growth in specimens of Reticulammina labyrinthica . This growth occurred in phases lasting 2–3 days each; each phase
4930-548: The majority of algae , the sporophyte and gametophyte are separate independent organisms, which may or may not have a similar appearance. In liverworts , mosses and hornworts , the sporophyte is less well developed than the gametophyte and is largely dependent on it. Although moss and hornwort sporophytes can photosynthesise, they require additional photosynthate from the gametophyte to sustain growth and spore development and depend on it for supply of water, mineral nutrients and nitrogen. By contrast, in all modern vascular plants
5015-463: The monoplacophoran Neopilina galatheae has been proposed as a specialised predator of the group. Despite this abundance, the relatively low amount of protoplasm per unit of test means that xenophyophores often contribute little to total biomass. Xenophyophores are difficult to study due to their extreme fragility. Specimens are invariably damaged during sampling, rendering them useless for captive study or cell culture . For this reason, very little
5100-507: The morphological alternation of generations in plants, between a spore-bearing generation (sporophyte) and a gamete-bearing generation (gametophyte). By that time, a debate emerged focusing on the origin of the asexual generation of land plants (i.e., the sporophyte) and is conventionally characterized as a conflict between theories of antithetic ( Ladislav Josef Čelakovský , 1874) and homologous ( Nathanael Pringsheim , 1876) alternation of generations. In 1874, Eduard Strasburger discovered
5185-503: The most conspicuous. As an illustration, consider a monoicous moss. Antheridia and archegonia develop on the mature plant (the gametophyte). In the presence of water, the biflagellate sperm from the antheridia swim to the archegonia and fertilisation occurs, leading to the production of a diploid sporophyte. The sporophyte grows up from the archegonium. Its body comprises a long stalk topped by a capsule within which spore-producing cells undergo meiosis to form haploid spores. Most mosses rely on
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#17328510439765270-470: The needs of the cell. The pseudopods are used for locomotion, anchoring, excretion, test construction and in capturing food, which consists of small organisms such as diatoms or bacteria. Aside from the tests, foraminiferal cells are supported by a cytoskeleton of microtubules, which are loosely arranged without the structure seen in other amoeboids. Forams have evolved special cellular mechanisms to quickly assemble and disassemble microtubules, allowing for
5355-719: The original shape of the organism is regained (albeit larger); and finally, the surface texture is rebuilt. The rapid rate of growth observed suggests that xenophyophores may not be as long-lived as previously hypothesised. Xenophyophores are an important part of the deep sea-floor, as they have been found in all four major ocean basins. They are often found in areas of enhanced organic carbon flux, such as beneath productive surface waters, in sub-marine canyons, in settings with sloped topography (e.g. seamounts, abyssal hills) and on continental slopes. They are not found in areas of hypoxic waters. Xenophyophores have been found between depths of 500 and 10,600 metres. Most are epifaunal (living atop
5440-498: The other parental genome (which nevertheless may have its own defects in other genes). As the diploid phase was becoming predominant, the masking effect likely allowed genome size , and hence information content, to increase without the constraint of having to improve accuracy of DNA replication. The opportunity to increase information content at low cost was advantageous because it permitted new adaptations to be encoded. This view has been challenged, with evidence showing that selection
5525-494: The possibility. Modern examples of Paleodictyon have been discovered; however, no evidence of tests, stercomares, grannelares, or xenophyophore DNA was found, and the trace may alternately represent a burrow or a glass sponge. Certain Carboniferous fossils have been suggested to represent the remains of xenophyophores due to the concentration of barium within the fossils as well as supposed morphological similarity; however,
5610-424: The pseuodopodia to encyst themselves inside of sediment and organic particles. Certain foraminifera prey upon small animals such as copepods or cumaceans ; some forams even predate upon other forams, drilling holes into the tests of their prey. One group, the xenophyophores, has been suggested to farm bacteria within their tests, although studies have failed to find support for this hypothesis. Suspension feeding
5695-665: The rapid formation and retraction of elongated pseudopodia. In the gamont (sexual form), foraminifera generally have only a single nucleus, while the agamont (asexual form) tends to have multiple nuclei. In at least some species the nuclei are dimorphic, with the somatic nuclei containing three times as much protein and RNA than the generative nuclei. However, nuclear anatomy seems to be highly diverse. The nuclei are not necessarily confined to one chamber in multi-chambered species. Nuclei can be spherical or have many lobes. Nuclei are typically 30-50 μm in diameter. Some species of foraminifera have large, empty vacuoles within their cells;
5780-427: The same organism) fuse to produce a diploid zygote , which divides repeatedly by mitosis, developing into a multicellular diploid sporophyte. This cycle, from gametophyte to sporophyte (or equally from sporophyte to gametophyte), is the way in which all land plants and most algae undergo sexual reproduction . The relationship between the sporophyte and gametophyte phases varies among different groups of plants. In
5865-412: The seabed), but one species ( Occultammina profunda ), is known to be infaunal; it buries itself up to 6 centimetres (2.4 in) deep into the sediment. Xenophyophore densities are highest on soft sediments; however, they may still be found on rocky substrates including basalts , canyon walls, and manganese crusts. The diet and feeding ecology of xenophyophores was long the subject of speculation;
5950-562: The sediment surface. The majority of planktonic foraminifera are found in the globigerinina , a lineage within the rotaliida . However, at least one other extant rotaliid lineage, Neogallitellia , seems to have independently evolved a planktonic lifestyle. Further, it has been suggested that some Jurassic fossil foraminifera may have also independently evolved a planktonic lifestyle, and may be members of Robertinida. A number of forams, both benthic and planktonic, have unicellular algae as endosymbionts , from diverse lineages such as
6035-452: The seed plants, the sporophyte is the dominant multicellular phase; the gametophytes are strongly reduced in size and very different in morphology. The entire gametophyte generation, with the sole exception of pollen grains (microgametophytes), is contained within the sporophyte. The life cycle of a dioecious flowering plant (angiosperm), the willow, has been outlined in some detail in an earlier section ( A complex life cycle ). The life cycle of
6120-428: The sporophyte or the gametophyte is very much reduced and is incapable of free living. For example, in all bryophytes the gametophyte generation is dominant and the sporophyte is dependent on it. By contrast, in all seed plants the gametophytes are strongly reduced, although the fossil evidence indicates that they were derived from isomorphic ancestors. In seed plants , the female gametophyte develops totally within
6205-582: The sporophyte, which protects and nurtures it and the embryonic sporophyte that it produces. The pollen grains, which are the male gametophytes, are reduced to only a few cells (just three cells in many cases). Here the notion of two generations is less obvious; as Bateman & Dimichele say "sporophyte and gametophyte effectively function as a single organism". The alternative term 'alternation of phases' may then be more appropriate. Initially, Adelbert von Chamisso (studying salps , colonial marine animals between 1815 and 1818 ) and Japetus Steenstrup (studying
6290-402: The stercomata, suggesting that xenophyophores utilise bacteria growing on their waste products in order to supplement their feeding. A 2021 study that utilised isotopic labeling to examine the question of xenophyophore feeding confirmed rapid uptake of both diatoms and dissolved organic matter in the form of glucose . This study found no evidence to support a bacterial farming function for
6375-470: The substrate is sloped or near canyon walls, while more fan-shaped forms like Stannophyllum are more common in areas with quieter water and/or lower primary productivity. Foraminifera " Monothalamea " Tubothalamea Globothalamea incertae sedis Foraminifera ( / f ə ˌ r æ m ə ˈ n ɪ f ə r ə / fə- RAM -ə- NIH -fə-rə ; Latin for "hole bearers"; informally called " forams ") are single-celled organisms , members of
6460-489: The surrounding area. Research has shown that areas dominated by xenophyophores have 3–4 times the number of benthic crustaceans , echinoderms , and molluscs than equivalent areas that lack xenophyophores. The xenophyophores themselves also play commensal host to a number of organisms—such as isopods (e.g., genus Hebefustis ), sipunculan and polychaete worms, nematodes , and harpacticoid copepods —some of which may take up semi-permanent residence within
6545-408: The term foraminifera is both singular and plural (irrespective of the word's Latin derivation), and is used to describe one or more specimens or taxa: its usage as singular or plural must be determined from context. Foraminifera is frequently used informally to describe the group, and in these cases is generally lowercase. The earliest known reference to foraminifera comes from Herodotus , who in
6630-479: The test, and instead proposed that it aided to function in the collection of phytodetritus by increasing surface area. These authors argued that xenophyophores fill a major role in ocean-floor biogeochemical cycling . As of 2017, no positively-identified xenophyophore fossils had been discovered. It has been suggested that the mysterious vendozoans of the Ediacaran period represent fossil xenophyophores. However,
6715-417: The tests of foraminifera are located inside the cell membrane , within the protoplasm . The organelles of the cell are located within the compartment(s) of the test, and the hole(s) of the test allow the transfer of material from the pseudopodia to the internal cell and back. The foraminiferal cell is divided into granular endoplasm and transparent ectoplasm from which a pseudopodial net may emerge through
6800-456: The wind to disperse these spores, although Splachnum sphaericum is entomophilous , recruiting insects to disperse its spores. The life cycle of ferns and their allies, including clubmosses and horsetails , the conspicuous plant observed in the field is the diploid sporophyte. The haploid spores develop in sori on the underside of the fronds and are dispersed by the wind (or in some cases, by floating on water). If conditions are right,
6885-513: Was first recognized by Dujardin in 1835. Shortly after, in 1852, d'Orbigny produced a classification scheme, recognising 72 genera of foraminifera, which he classified based on test shape—a scheme that drew severe criticism from colleagues. H.B. Brady 's 1884 monograph described the foraminiferal finds of the Challenger expedition . Brady recognized 10 families with 29 subfamilies, with little regard to stratigraphic range; his taxonomy emphasized
6970-551: Was first reported in Occultammina but has since been found to be true of many other xenophyophore species from different parts of the ocean. Very little is known about xenophyophore reproduction. It is assumed that an alternation of generations takes place, as in other foraminifera ; however, this has not been confirmed. Gametes form in a specialised part of the granellare that may look like swollen side-branch (in Psammetta ) or
7055-420: Was separated by a resting period of approximately two months. These growth phases were approximately synchronous between specimens, but it is unclear if this is biologically or developmentally controlled; some evidence suggests the synchrony may have been due to chance. Each episode of growth occurred in three phases: first, the base becomes wider and flatter, causing the surface texture to become smoother; then,
7140-529: Was trying several small and single Magnifying Glasses, and casually viewing a parcel of white Sand, when I perceiv'd one of the grains exactly shap'd and wreath'd like a Shell[...] I view'd it every way with a better Microscope and found it on both sides, and edge-ways, to resemble the Shell of a small Water-Snail with a flat spiral Shell[...] Antonie van Leeuwenhoek described and illustrated foraminiferal tests in 1700, describing them as minute cockles; his illustration
7225-647: Was wearing a cloak consisting of over 1,000 protists and other creatures, including 245 living xenophyophores, mainly Psammina species, each 3–6 mm. The fragility of the xenophyophores suggests that the urchin either very carefully collected them, or that they settled and grew there. Among several possible explanations for the urchin's behaviour, perhaps the most likely are chemical camouflage and weighing itself down to avoid being moved in currents. Different xenophyophore ecomorphs are found in different settings; reticulated or heavily folded genera such as Reticulammina and Syringammina are more common in areas where
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