Diploid Definition Example Essay

This article describes alternation of generations in plants; for a similar phenomenon in animals see Heterogamy

Alternation of generations (also known as metagenesis) is the type of life cycle that occurs in those plants and algae in the Archaeplastida and the Heterokontophyta that have distinct sexual haploid and asexual diploid stages. In these groups, a multicellular gametophyte, which is haploid with nchromosomes, alternates with a multicellular sporophyte, which is diploid with 2n chromosomes, made up of n pairs. A mature sporophyte produces spores by meiosis, a process which reduces the number of chromosomes to half, from 2n to n.

The haploid spores germinate and grow into a haploid gametophyte. At maturity, the gametophyte produces gametes by mitosis, which does not alter the number of chromosomes. Two gametes (originating from different organisms of the same species or from the same organism) fuse to produce a zygote, which develops into a diploid sporophyte. This cycle, from gametophyte to gametophyte (or equally from sporophyte to sporophyte), is the way in which all land plants and many algae undergo sexual reproduction.

The relationship between the sporophyte and gametophyte varies among different groups of plants. In those algae which have alternation of generations, 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.[1][2] By contrast, in all modern vascular plants the gametophyte is less well developed than the sporophyte, although their Devonian ancestors had gametophytes and sporophytes of approximately equivalent complexity.[3] In ferns the gametophyte is a small flattened autotrophicprothallus 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 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 they do not have a haploid gametophyte alternating with a diploid sporophyte. (Some insects have a sex-determining system whereby haploid males are produced from unfertilized eggs; however the females 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 or dibiontic are also in use). Life cycles, such as those 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.

Definition[edit]

Alternation of generations is defined as the alternation of multicellular diploid and haploid forms in the organism's life cycle, regardless of whether or not these forms are free-living.[4] In some species, such as 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. 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.[5]

However, in some other groups, either 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 modern vascular land plants the gametophytes are strongly reduced, although the fossil evidence indicates that they were derived from isomorphic ancestors.[3] In seed plants, the female gametophyte develops totally within the sporophyte which protects and nurtures it and the embryo 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 "[s]porophyte and gametophyte effectively function as a single organism".[6] The alternative term 'alternation of phases' may then be more appropriate.[7]

History[edit]

Debates about alternation of generations in the early twentieth century can be confusing because various ways of classifying "generations" co-exist (sexual vs. asexual, gametophyte vs. sporophyte, haploid vs. diploid, etc.).[8]

Initially, Chamisso and Steenstrup described the succession of differently organized generations (sexual and asexual) in animals as "alternation of generations", while studying the development of tunicates, cnidarians and trematode animals.[8] This phenomenon is also known as heterogamy. Presently, the term "alternation of generations" is almost exclusively associated with the life cycles of plants, specifically with the alternation of haploid gametophytes and diploid sporophytes.[8]

Wilhelm Hofmeister demonstrated the morphological alternation of generations in plants,[9] between a spore-bearing generation (sporophyte) and a gamete-bearing generation (gametophyte).[10][11] 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 (Čelakovský, 1874) and homologous (Pringsheim, 1876) alternation of generations.[8] Čelakovský coined the words sporophyte and gametophyte.[citation needed]

Eduard Strasburger (1874) discovered the alternation between diploid and haploid nuclear phases,[8] also called cytological alternation of nuclear phases.[12] 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.[12] In some ferns which lost sexual reproduction, there is no change in nuclear phase, but the alternation of generations is maintained.[13]

Alternation of generations in plants[edit]

Fundamental elements[edit]

The diagram above shows the fundamental elements of the alternation of generations in plants. The many variations found in different groups of plants are described by use of these concepts later in the article. Starting from the right of the diagram, the processes involved are as follows:[14]

  • Two single-celled haploid gametes, each containing n unpaired chromosomes, fuse to form a single-celled diploid zygote, which now contains n pairs of chromosomes, i.e. 2n chromosomes in total.
  • The single-celled diploid zygote germinates, dividing by the normal process (mitosis), which maintains the number of chromosomes at 2n. The result is a multi-cellular diploid organism, called the sporophyte (because at maturity it produces spores).
  • When it reaches maturity, the sporophyte produces one or more sporangia (singular: sporangium) which are the organs that produce diploid spore mother cells (sporocytes). These divide by a special process (meiosis) that reduces the number of chromosomes by a half. This initially results in four single-celled haploid spores, each containing n unpaired chromosomes.
  • The single-celled haploid spore germinates, dividing by the normal process (mitosis), which maintains the number of chromosomes at n. The result is a multi-cellular haploid organism, called the gametophyte (because it produces gametes at maturity).
  • When it reaches maturity, the gametophyte produces one or more gametangia (singular: gametangium) which are the organs that produce haploid gametes. At least one kind of gamete possesses some mechanism for reaching another gamete in order to fuse with it.

The 'alternation of generations' in the life cycle is thus between a diploid (2n) generation of sporophytes and a haploid (n) generation of gametophytes.

The situation is quite different from that in animals, where the fundamental process is that a diploid (2n) individual directly produces haploid (n) gametes by meiosis. Spores (i.e. haploid cells which are able to undergo mitosis) are not produced, so neither is an asexual multi-cellular generation that alternates with a sexual multi-cellular generation. (Some insects have haploid males that develop from unfertilized eggs, but the females are all diploid.)

Variations[edit]

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.[15]

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 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."[16]

Possible variations are:

  • Relative importance of the sporophyte and the gametophyte.
    • Equal (homomorphy or isomorphy).
      Filamentous algae of the genus Cladophora, which are predominantly found in fresh water, have diploid sporophytes and haploid gametophytes which are externally indistinguishable.[17] No living land plant has equally dominant sporophytes and gametophytes, although some theories of the evolution of alternation of generations suggest that ancestral land plants did.
    • Unequal (heteromorphy or anisomorphy).
      • Dominant gametophyte (gametophytic).
        In liverworts, mosses and hornworts, the dominant form is the haploid gametophyte. The diploid sporophyte is not capable of an independent existence, gaining most of its nutrition from the parent gametophyte, and having no chlorophyll when mature.[18]
      • Dominant sporophyte (sporophytic).
        In ferns, both the sporophyte and the gametophyte are capable of living independently, but the dominant form is the diploid sporophyte. The haploid gametophyte is much smaller and simpler in structure. In seed plants, the gametophyte is even more reduced (at the minimum to only three cells), gaining all its nutrition from the sporophyte. The extreme reduction in the size of the gametophyte and its retention within the sporophyte means that when applied to seed plants the term 'alternation of generations' is somewhat misleading: "[s]porophyte and gametophyte effectively function as a single organism".[6] Some authors have preferred the term 'alternation of phases'.[7]
  • Differentiation of the gametes.
    • Both gametes the same (isogamy).
      Like other species of Cladophora, C. callicoma has flagellated gametes which are identical in appearance and ability to move.[17]
    • Gametes of two distinct sizes (anisogamy).
      • Both of similar motility.
        Species of Ulva, the sea lettuce, have gametes which all have two flagella and so are motile. However they are of two sizes: larger 'female' gametes and smaller 'male' gametes.[19]
      • One large and sessile, one small and motile (oogamy). The larger sessile megagametes are eggs (ova), and smaller motile microgametes are sperm (spermatozoa, spermatozoids). The degree of motility of the sperm may be very limited (as in the case of flowering plants) but all are able to move towards the sessile eggs. When (as is almost always the case) the sperm and eggs are produced in different kinds of gametangia, the sperm-producing ones are called antheridia (singular antheridium) and the egg-producing ones archegonia (singular archegonium).
        • Antheridia and archegonia occur on the same gametophyte, which is then called monoicous. (Many sources, including those concerned with bryophytes, use the term 'monoecious' for this situation and 'dioecious' for the opposite.[20][21] Here 'monoecious' and 'dioecious' are used only for sporophytes.)
          The liverwort Pellia epiphylla has the gametophyte as the dominant generation. It is monoicous: the small reddish sperm-producing antheridia are scattered along the midrib while the egg-producing archegonia grow nearer the tips of divisions of the plant.[22]
        • Antheridia and archegonia occur on different gametophytes, which are then called dioicous.
          The moss Mnium hornum has the gametophyte as the dominant generation. It is dioicous: male plants produce only antheridia in terminal rosettes, female plants produce only archegonia in the form of stalked capsules.[23] Seed plant gametophytes are also dioicous. However, the parent sporophyte may be monoecious, producing both male and female gametophytes or dioecious, producing gametophytes of one gender only. Seed plant gametophytes are extremely reduced in size; the archegonium consists only of a small number of cells, and the entire male gametophyte may be represented by only two cells.
        Gametophyte of Pellia epiphylla with sporophytes growing from the remains of archegonia.
  • Differentiation of the spores.
    • All spores the same size (homospory or isospory).
      Horsetails (species of Equisetum) have spores which are all of the same size.[25]
    • Spores of two distinct sizes (heterospory or anisospory): larger megaspores and smaller microspores. When the two kinds of spore are produced in different kinds of sporangia, these are called megasporangia and microsporangia. A megaspore often (but not always) develops at the expense of the other three cells resulting from meiosis, which abort.
      • Megasporangia and microsporangia occur on the same sporophyte, which is then called monoecious.
        Most flowering plants fall into this category. Thus the flower of a lily contains six stamens (the microsporangia) which produce microspores which develop into pollen grains (the microgametophytes), and three fused carpels which produce integumented megasporangia (ovules) each of which produces a megaspore which develops inside the megasporangium to produce the megagametophyte. In other plants, such as hazel, some flowers have only stamens, others only carpels, but the same plant (i.e. sporophyte) has both kinds of flower and so is monoecious.
        Flowers of European Holly, a dioecious species: male above, female below (leaves cut to show flowers more clearly)
      • Megasporangia and microsporangia occur on different sporophytes, which are then called dioecious.
        An individual tree of the European holly (Ilex aquifolium) produces either 'male' flowers which have only functional stamens (microsporangia) producing microspores which develop into pollen grains (microgametophytes) or 'female' flowers which have only functional carpels producing integumented megasporangia (ovules) that contain a megaspore that develops into a multicellular megagametophyte.

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, 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.[25]

A complex life cycle[edit]

The diagram shows the alternation of generations in a species which is heteromorphic, sporophytic, oogametic, dioicous, heterosporic and dioecious. A seed plant example might be a willow tree (most species of the genus Salix are dioecious).[26] Starting in the centre of the diagram, the processes involved are:

  • An immobile egg, contained in the archegonium, fuses with a mobile sperm, released from an antheridium. The resulting zygote is either 'male' or 'female'.
    • A 'male' zygote develops by mitosis into a microsporophyte, which at maturity produces one or more microsporangia. Microspores develop within the microsporangium by meiosis.
      In a willow (like all seed plants) the zygote first develops into an embryo microsporophyte within the ovule (a megasporangium enclosed in one or more protective layers of tissue known as integument). At maturity, these structures become the seed. Later the seed is shed, germinates and grows into a mature tree. A 'male' willow tree (a microsporophyte) produces flowers with only stamens, the anthers of which are the microsporangia.
    • Microspores germinate producing microgametophytes; at maturity one or more antheridia are produced. Sperm develop within the antheridia.
      In a willow, microspores are not liberated from the anther (the microsporangium), but develop into pollen grains (microgametophytes) within it. The whole pollen grain is moved (e.g. by an insect or by the wind) to an ovule (megagametophyte), where a sperm is produced which moves down a pollen tube to reach the egg.
    • A 'female' zygote develops by mitosis into a megasporophyte, which at maturity produces one or more megasporangia. Megaspores develop within the megasporangium; typically one of the four spores produced by meiosis gains bulk at the expense of the remaining three, which disappear.
      'Female' willow trees (megasporophytes) produce flowers with only carpels (modified leaves that bear the megasporangia).
    • Megaspores germinate producing megagametophytes; at maturity one or more archegonia are produced. Eggs develop within the archegonia.
      The carpels of a willow produce ovules, megasporangia enclosed in integuments. Within each ovule, a megaspore develops by mitosis into a megagametophyte. An archegonium develops within the megagametophyte and produces an egg. The whole of the gametophytic 'generation' remains within the protection of the sporophyte except for pollen grains (which have been reduced to just three cells contained within the microspore wall).

Life cycles of different plant groups[edit]

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 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 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 the wind to disperse these spores, although Splachnum sphaericum is entomophilous, recruiting insects to disperse its spores. For further information, see Liverwort: Life cycle, Moss: Life cycle, Hornwort: Life cycle.

  • Diagram of alternation of generations in liverworts.

  • Hornwort life cycle diagram

In 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, 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. For further information, see Fern: Life cycle.

  • Diagram of alternation of generations in ferns.

  • A gametophyte (prothallus) of Dicksonia sp.

  • A sporophyte of Dicksonia antarctica.

  • The underside of a Dicksonia antarctica frond showing the sori, or spore-producing structures.

In the spermatophytes, 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 a gymnosperm is similar. However, flowering plants have in addition a phenomenon called '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 the developing embryo. For further information, see Double fertilization.

Evolutionary emergence of the dominant diploid phase[edit]

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.[27][28] 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 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 is no more effective in the haploid than in the diploid phases of the lifecycle of mosses and angiosperms.[29]

  • Tip of tulip stamen showing pollen (microgametophytes)

  • Plant ovules (megagametophytes): Gymnosperm ovule on left, angiosperm ovule (inside ovary) on right

Similar processes in other organisms[edit]

Rhizaria[edit]

Some organisms currently classified in the clade Rhizaria and thus not plants in the sense used here, exhibit alternation of generations. Foraminifera undergo a heteromorphic alternation of generations between haploid gamont and diploid agamont forms. The single-celled haploid organism is typically much larger than the diploid organism.

Fungi[edit]

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 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.

Slime moulds[edit]

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 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.

Animals[edit]

Alternation between a multicellular diploid and a multicellular haploid generation is never encountered in animals.[30] In some animals, there is an alternation between parthenogenic and sexually reproductive phases (heterogamy). Both phases are diploid. This has sometimes been called "alternation of generations",[31] but is quite different. In some other animals, such as hymenopterans, males are haploid and females diploid, but this is always the case rather than there being an alternation between distinct generations.

See also[edit]

Notes and references[edit]

  1. ^Thomas, R.J.; Stanton, D.S.; Longendorfer, D.H. & Farr, M.E. (1978), "Physiological evaluation of the nutritional autonomy of a hornwort sporophyte", Botanical Gazette, 139 (3): 306–311, doi:10.1086/337006 
  2. ^Glime, J.M. (2007), Bryophyte Ecology: Vol. 1 Physiological Ecology(PDF), Michigan Technological University and the International Association of Bryologists, retrieved 2013-03-04 
  3. ^ abKerp, H.; Trewin, N.H. & Hass, H. (2003), "New gametophytes from the Lower Devonian Rhynie Chert", Transactions of the Royal Society of Edinburgh: Earth Sciences, 94 (4): 411–428, doi:10.1017/S026359330000078X 
  4. ^Taylor, Kerp & Hass 2005
  5. ^""Plant Science 4 U". Retrieved 5 July 2016. 
  6. ^ abBateman & Dimichele 1994, p. 403
  7. ^ abStewart & Rothwell 1993
  8. ^ abcdeHaig, David (2008), "Homologous versus antithetic alternation of generations and the origin of sporophytes"(PDF), The Botanical Review, 74 (3): 395–418, doi:10.1007/s12229-008-9012-x, retrieved 2014-08-17 
  9. ^Svedelius, Nils (1927), "Alternation of Generations in Relation to Reduction Division", Botanical Gazette, 83 (4): 362–384, doi:10.1086/333745, JSTOR 2470766 
  10. ^Hofmeister, W. (1851), Vergleichende Untersuchungen der Keimung, Entfaltung und Fruchtbildildiung höherer Kryptogamen (Moose, Farne, Equisetaceen, Rhizocarpeen und Lycopodiaceen) und der Samenbildung der Coniferen (in German), Leipzig: F. Hofmeister, retrieved 2014-08-17 . Translated as Currey, Frederick (1862), On the germination, development, and fructification of the higher Cryptogamia, and on the fructification of the Coniferæ, London: Robert Hardwicke, retrieved 2014-08-17 
  11. ^Feldmann, J. & Feldmann, G. (1942), "Recherches sur les Bonnemaisoniacées et leur alternance de generations"(PDF), Ann. Sci. Natl. Bot., ser. 11 (in French), 3: 75–175 , p. 157
  12. ^ abFeldmann, J. (1972), "Les problèmes actuels de l'alternance de génerations chez les Algues", Bulletin de la Société Botanique de France (in French), 119: 7–38, doi:10.1080/00378941.1972.10839073 
  13. ^Schopfer, P.; Mohr, H. (1995). "Physiology of Development". Plant physiology. Berlin: Springer. pp. 288–291. ISBN 3-540-58016-6. 
  14. ^Unless otherwise indicated, the material in the whole of this section is based on Foster & Gifford 1974, Sporne 1974a and Sporne 1974b.
  15. ^Guiry & Guiry 2008
  16. ^Bateman & Dimichele 1994, p. 347
  17. ^ abShyam 1980
  18. ^Watson 1981, p. 2
  19. ^Kirby 2001
  20. ^Watson 1981, p. 33
  21. ^Bell & Hemsley 2000, p. 104
  22. ^Watson 1981, pp. 425–6
  23. ^Watson 1981, pp. 287–8
  24. ^ abBateman & Dimichele 1994, pp. 350–1
  25. ^"Willows", Encyclopædia Britannica, XIX (11th ed.), New York: Encyclopædia Britannica, 1911, retrieved 2011-01-01 
  26. ^Bernstein, H.; Byers, G.S. & Michod, R.E. (1981), "Evolution of sexual reproduction: Importance of DNA repair, complementation, and variation", The American Naturalist, 117 (4): 537–549, doi:10.1086/283734 
  27. ^Michod, R.E. & Gayley, T.W. (1992), "Masking of mutations and the evolution of sex", The American Naturalist, 139 (4): 706–734, doi:10.1086/285354 
  28. ^Szövényi, Péter; Ricca, Mariana; Hock, Zsófia; Shaw, Jonathan A.; Shimizu, Kentaro K. & Wagner, Andreas (2013), "Selection is no more efficient in haploid than in diploid life stages of an angiosperm and a moss", Molecular Biology and Evolution, 30 (8): 1929, doi:10.1093/molbev/mst095, PMID 23686659 
  29. ^Barnes et al. 2001, p. 321
  30. ^Scott 1996, p. 35

Bibliography[edit]

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  • Bateman, R.M. & Dimichele, W.A. (1994), "Heterospory – the most iterative key innovation in the evolutionary history of the plant kingdom"(PDF), Biological Reviews of the Cambridge Philosophical Society, 69: 345–417, doi:10.1111/j.1469-185x.1994.tb01276.x, retrieved 2010-12-30 
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  • Taylor, T.N.; Kerp, H. & Hass, H. (2005), "Life history biology of early land plants: Deciphering the gametophyte phase", Proceedings of the National Academy of Sciences of the United States of America, 102 (16): 5892–5897, doi:10.1073/pnas.0501985102, PMC 556298, PMID 15809414 
Diagram showing the alternation of generations between a diploid sporophyte (bottom) and a haploid gametophyte (top)
Gametophyte of the fern Onoclea sensibilis (the flat thallus at the bottom of the picture) with a descendant sporophyte beginning to grow from it (the small frond at the top of the picture).
Graphic referred in text.

Not to be confused with "polypoid", resembling a polyp.

Polyploidcells and organisms are those containing more than two paired (homologous) sets of chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two sets of chromosomes—one set inherited from each parent. However, polyploidy is found in some organisms and is especially common in plants. In addition, polyploidy occurs in some tissues of animals that are otherwise diploid, such as human muscle tissues.[1] This is known as endopolyploidy. Species whose cells do not have nuclei, that is, prokaryotes, may be polyploid, as seen in the large bacteriumEpulopiscium fishelsoni[1]. Hence ploidy is defined with respect to a cell. Most eukaryotes have diploid somatic cells, but produce haploid gametes (eggs and sperm) by meiosis. A monoploid has only one set of chromosomes, and the term is usually only applied to cells or organisms that are normally diploid. Male bees and other Hymenoptera, for example, are monoploid. Unlike animals, plants and multicellular algae have life cycles with two alternating multicellular generations. The gametophyte generation is haploid, and produces gametes by mitosis, the sporophyte generation is diploid and produces spores by meiosis.

Polyploidy refers to a numerical change in a whole set of chromosomes. Organisms in which a particular chromosome, or chromosome segment, is under- or overrepresented are said to be aneuploid (from the Greek words meaning "not", "good", and "fold"). Aneuploidy refers to a numerical change in part of the chromosome set, whereas polyploidy refers to a numerical change in the whole set of chromosomes.[2]

Polyploidy may occur due to abnormal cell division, either during mitosis, or commonly during metaphase I in meiosis. In addition, it can be induced in plants and cell cultures by some chemicals: the best known is colchicine, which can result in chromosome doubling, though its use may have other less obvious consequences as well. Oryzalin will also double the existing chromosome content.

Polyploidy occurs in highly differentiated human tissues in the liver, heart muscle and bone marrow. It occurs in the somatic cells of some animals, such as goldfish,[3]salmon, and salamanders, but is especially common among ferns and flowering plants (see Hibiscus rosa-sinensis), including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat. Many agriculturally important plants of the genus Brassica are also tetraploids.

Polyploidization is a mechanism of sympatric speciation because polyploids are usually unable to interbreed with their diploid ancestors. An example is the plant Erythranthe peregrina. Sequencing confirmed that this species originated from E. x robertsii, a sterile triploid hybrid between E. guttata and E. lutea, both of which have been introduced and naturalised in the United Kingdom. New populations of E. peregrina arose on the Scottish mainland and the Orkney Islands via genome duplication from local populations of E. x robertsii.[4] Because of a rare genetic mutation, E. peregrina is not sterile.[5]

Types[edit]

Polyploid types are labeled according to the number of chromosome sets in the nucleus. The letter x is used to represent the number of chromosomes in a single set.

  • triploid (three sets; 3x), for example seedless watermelons, common in the phylumTardigrada[6]
  • tetraploid (four sets; 4x), for example Salmonidae fish,[7] the cotton Gossypium hirsutum[8]
  • pentaploid (five sets; 5x), for example Kenai Birch (Betula papyrifera var. kenaica)
  • hexaploid (six sets; 6x), for example wheat, kiwifruit[9]
  • heptaploid or septaploid (seven sets; 7x)
  • octaploid or octoploid, (eight sets; 8x), for example Acipenser (genus of sturgeon fish), dahlias
  • decaploid (ten sets; 10x), for example certain strawberries
  • dodecaploid (twelve sets; 12x), for example the plants Celosia argentea and Spartina anglica[10] or the amphibian Xenopus ruwenzoriensis.

Animals[edit]

Examples in animals are more common in non-vertebrates[11] such as flatworms, leeches, and brine shrimp. Within vertebrates, examples of stable polyploidy include the salmonids and many cyprinids (i.e. carp).[12] Some fish have as many as 400 chromosomes.[12] Polyploidy also occurs commonly in amphibians; for example the biomedically-important Xenopus genus contains many different species with as many as 12 sets of chromosomes (dodecaploid).[13] Polyploid lizards are also quite common, but are sterile and must reproduce by parthenogenesis.[citation needed] Polyploid mole salamanders (mostly triploids) are all female and reproduce by kleptogenesis,[14] "stealing" spermatophores from diploid males of related species to trigger egg development but not incorporating the males' DNA into the offspring. While mammalian liver cells are polyploid, rare instances of polyploid mammals are known, but most often result in prenatal death.

An octodontidrodent of Argentina's harsh desert regions, known as the plains viscacha rat (Tympanoctomys barrerae) has been reported as an exception to this 'rule'.[15] However, careful analysis using chromosome paints shows that there are only two copies of each chromosome in T. barrerae, not the four expected if it were truly a tetraploid.[16] This rodent is not a rat, but kin to guinea pigs and chinchillas. Its "new" diploid [2n] number is 102 and so its cells are roughly twice normal size. Its closest living relation is Octomys mimax, the Andean Viscacha-Rat of the same family, whose 2n = 56. It was therefore surmised that an Octomys-like ancestor produced tetraploid (i.e., 2n = 4x = 112) offspring that were, by virtue of their doubled chromosomes, reproductively isolated from their parents.

Polyploidy was induced in fish by Har Swarup (1956) using a cold-shock treatment of the eggs close to the time of fertilization, which produced triploid embryos that successfully matured.[17][18] Cold or heat shock has also been shown to result in unreduced amphibian gametes, though this occurs more commonly in eggs than in sperm.[19]John Gurdon (1958) transplanted intact nuclei from somatic cells to produce diploid eggs in the frog, Xenopus (an extension of the work of Briggs and King in 1952) that were able to develop to the tadpole stage.[20] The British Scientist, J. B. S. Haldane hailed the work for its potential medical applications and, in describing the results, became one of the first to use the word “clone” in reference to animals. Later work by Shinya Yamanaka showed how mature cells can be reprogrammed to become pluripotent, extending the possibilities to non-stem cells. Gurdon and Yamanaka were jointly awarded the Nobel Prize in 2012 for this work.[20]

Humans[edit]

Further information: Triploid syndrome

True polyploidy rarely occurs in humans, although polyploid cells occur in highly differentiated tissue, such as liver parenchyma and heart muscle, and in bone marrow.[21]Aneuploidy is more common.

Polyploidy occurs in humans in the form of triploidy, with 69 chromosomes (sometimes called 69,XXX), and tetraploidy with 92 chromosomes (sometimes called 92,XXXX). Triploidy, usually due to polyspermy, occurs in about 2–3% of all human pregnancies and ~15% of miscarriages.[citation needed] The vast majority of triploid conceptions end as a miscarriage; those that do survive to term typically die shortly after birth. In some cases, survival past birth may extend longer if there is mixoploidy with both a diploid and a triploid cell population present.

Triploidy may be the result of either digyny (the extra haploid set is from the mother) or diandry (the extra haploid set is from the father). Diandry is mostly caused by reduplication of the paternal haploid set from a single sperm, but may also be the consequence of dispermic (two sperm) fertilization of the egg.[22] Digyny is most commonly caused by either failure of one meiotic division during oogenesis leading to a diploid oocyte or failure to extrude one polar body from the oocyte. Diandry appears to predominate among early miscarriages, while digyny predominates among triploid zygotes that survive into the fetal period.[citation needed] However, among early miscarriages, digyny is also more common in those cases <8.5 weeks gestational age or those in which an embryo is present. There are also two distinct phenotypes in triploid placentas and fetuses that are dependent on the origin of the extra haploid set. In digyny, there is typically an asymmetric poorly grown fetus, with marked adrenalhypoplasia and a very small placenta.[citation needed] In diandry, a partial hydatidiform mole develops.[22] These parent-of-origin effects reflect the effects of genomic imprinting.[citation needed]

Complete tetraploidy is more rarely diagnosed than triploidy, but is observed in 1–2% of early miscarriages. However, some tetraploid cells are commonly found in chromosome analysis at prenatal diagnosis and these are generally considered 'harmless'. It is not clear whether these tetraploid cells simply tend to arise during in vitro cell culture or whether they are also present in placental cells in vivo. There are, at any rate, very few clinical reports of fetuses/infants diagnosed with tetraploidy mosaicism.

Mixoploidy is quite commonly observed in human preimplantation embryos and includes haploid/diploid as well as diploid/tetraploid mixed cell populations. It is unknown whether these embryos fail to implant and are therefore rarely detected in ongoing pregnancies or if there is simply a selective process favoring the diploid cells.

Plants[edit]

Polyploidy is pervasive in plants and some estimates suggest that 30–80% of living plant species are polyploid, and many lineages show evidence of ancient polyploidy (paleopolyploidy) in their genomes.[23][24][25] Huge explosions in angiosperm species diversity appear to have coincided with the timing of ancient genome duplications shared by many species.[26] It has been established that 15% of angiosperm and 31% of fern speciation events are accompanied by ploidy increase.[27]

Polyploid plants can arise spontaneously in nature by several mechanisms, including meiotic or mitotic failures, and fusion of unreduced (2n) gametes.[28] Both autopolyploids (e.g. potato [29]) and allopolyploids (e.g. canola, wheat, cotton) can be found among both wild and domesticated plant species.

Most polyploids display novel variation or morphologies relative to their parental species, that may contribute to the processes of speciation and eco-niche exploitation.[24][28] The mechanisms leading to novel variation in newly formed allopolyploids may include gene dosage effects (resulting from more numerous copies of genome content), the reunion of divergent gene regulatory hierarchies, chromosomal rearrangements, and epigenetic remodeling, all of which affect gene content and/or expression levels.[30][31][32][33] Many of these rapid changes may contribute to reproductive isolation and speciation. However seed generated from interploidy crosses, such as between polyploids and their parent species, usually suffer from aberrant endosperm development which impairs their viability,[34][35] thus contributing to polyploid speciation.

Lomatia tasmanica is an extremely rare Tasmanian shrub that is triploid and sterile; reproduction is entirely vegetative, with all plants having the same genetic constitution.

There are few naturally occurring polyploid conifers. One example is the Coast Redwood Sequoia sempervirens, which is a hexaploid (6x) with 66 chromosomes (2n = 6x = 66), although the origin is unclear.[36]

Aquatic plants, especially the Monocotyledons, include a large number of polyploids.[37]

Crops[edit]

The induction of polyploidy is a common technique to overcome the sterility of a hybrid species during plant breeding. For example, Triticale is the hybrid of wheat (Triticum turgidum) and rye (Secale cereale). It combines sought-after characteristics of the parents, but the initial hybrids are sterile. After polyploidization, the hybrid becomes fertile and can thus be further propagated to become triticale.

In some situations, polyploid crops are preferred because they are sterile. For example, many seedless fruit varieties are seedless as a result of polyploidy. Such crops are propagated using asexual techniques, such as grafting.

Polyploidy in crop plants is most commonly induced by treating seeds with the chemical colchicine.

Examples[edit]

  • Triploid crops: some apple varieties (e.g. Belle de Boskoop, Jonagold, Mutsu, Ribston Pippin), banana, citrus, ginger, watermelon[38]
  • Tetraploid crops: very few apple varieties, durum or macaroniwheat, cotton, potato, canola/rapeseed, leek, tobacco, peanut, kinnow, Pelargonium
  • Hexaploid crops: chrysanthemum, bread wheat, triticale, oat, kiwifruit[9]
  • Octaploid crops: strawberry, dahlia, pansies, sugar cane, oca (Oxalis tuberosa)[39]
  • Dodecaploid crops: some sugar cane hybrids [40]

Some crops are found in a variety of ploidies: tulips and lilies are commonly found as both diploid and triploid; daylilies (Hemerocallis cultivars) are available as either diploid or tetraploid; apples and kinnow mandarins can be diploid, triploid, or tetraploid.

Fungi[edit]

Besides plants and animals, the evolutionary history of various fungal species is dotted by past and recent whole-genome duplication events (see Albertin and Marullo 2012[41] for review). Several examples of polyploids are known:

In addition, polyploidy is frequently associated with hybridization and reticulate evolution that appear to be highly prevalent in several fungal taxa. Indeed, homoploid speciation (i.e., hybrid speciation without a change in chromosome number) has been evidenced for some fungal species (e.g., the basidiomycotaMicrobotryum violaceum[49]).

As for plants and animals, fungal hybrids and polyploids display structural and functional modifications compared to their progenitors and diploid counterparts. In particular, the structural and functional outcomes of polyploid Saccharomyces genomes strikingly reflect the evolutionary fate of plant polyploid ones. Large chromosomal rearrangements[50] leading to chimeric chromosomes[51] have been described, as well as more punctual genetic modifications such as gene loss.[52] The homoealleles of the allotetraploid yeast S. pastorianus show unequal contribution to the transcriptome.[53]Phenotypic diversification is also observed following polyploidization and/or hybridization in fungi,[54] producing the fuel for natural selection and subsequent adaptation and speciation.

Chromalveolata[edit]

Other eukaryotic taxa have experienced one or more polyploidization events during their evolutionary history (see Albertin and Marullo, 2012[41] for review). The oomycetes, which are non-true fungi members, contain several examples of paleopolyploid and polyploid species, such as within the Phytophthora genus.[55] Some species of brown algae (Fucales, Laminariales [56] and diatoms[57]) contain apparent polyploid genomes. In the Alveolata group, the remarkable species Paramecium tetraurelia underwent three successive rounds of whole-genome duplication [58] and established itself as a major model for paleopolyploid studies.

Terminology[edit]

Autopolyploidy[edit]

Autopolyploids are polyploids with multiple chromosome sets derived from a single taxon. Two examples of natural autopolyploids are the piggyback plant, Tolmiea menzisii[59] and the white sturgeon, Acipenser transmontanum.[60] Most instances of autopolyploidy result from the fusion of unreduced (2n) gametes, which results in either triploid (n + 2n = 3n) or tetraploid (2n + 2n = 4n) offspring.[61] Triploid offspring are typically sterile (as in the phenomenon of 'triploid block'), but in some cases they may produce high proportions of unreduced gametes and thus aid the formation of tetraploids. This pathway to tetraploidy is referred to as the “triploid bridge”.[61] Triploids may also persist through asexual reproduction. In fact, stable autotriploidy in plants is often associated with apomictic mating systems.[62] In agricultural systems, autotriploidy can result in seedlessness, as in watermelons and bananas.[63] Triploidy is also utilized in salmon and trout farming to induce sterility.[64][65]

Rarely, autopolyploids arise from spontaneous, somatic genome doubling, which has been observed in apple (Malus domesticus) bud sports.[66] This is also the most common pathway of artificially induced polyploidy, where methods such as protoplast fusion or treatment with colchicine, oryzalin or mitotic inhibitors are used to disrupt normal mitotic division, which results in the production of polyploid cells. This process can be useful in plant breeding, especially when attempting to introgress germplasm across ploidal levels.[67]

Autopolyploids possess at least three homologous chromosome sets, which can lead to high rates of multivalent pairing during meiosis (particularly in recently formed autopolyploids, a.k.a. neopolyploids) and an associated decrease in fertility due to the production of aneuploid gametes.[68] Natural or artificial selection for fertility can quickly stabilize meiosis in autopolyploids by restoring bivalent pairing during meiosis, but the high degree of homology among duplicated chromosomes causes autopolyploids to display polysomic inheritance.[69] This trait is often used as a diagnostic criterion to distinguish autopolyploids from allopolyploids, which commonly display disomic inheritance after they progress past the neopolyploid stage.[70] While most polyploid species are unambiguously characterized as either autopolyploid or allopolyploid, these categories represent the ends of a spectrum between of divergence between parental subgenomes. Polyploids that fall between these two extremes, which are often referred to as segmental allopolyploids, may display intermediate levels of polysomic inheritance that vary by locus.[71][72]

About half of all polyploids are thought to be the result of autopolyploidy,[73][74] although many factors make this proportion hard to estimate.[75]

Allopolyploidy[edit]

Allopolyploids or amphipolyploids or heteropolyploids are polyploids with chromosomes derived from two or more diverged taxa. As in autopolyploidy, this primarily occurs through the fusion of unreduced (2n) gametes, which can take place before or after hybridization. In the former case, unreduced gametes from each diploid taxa – or reduced gametes from two autotetraploid taxa – combine to form allopolyploid offspring. In the latter case, one or more diploid F1 hybrids produce unreduced gametes that fuse to form allopolyploid progeny.[76] Hybridization followed by genome duplication may be a more common path to allopolyploidy because F1 hybrids between taxa often have relatively high rates of unreduced gamete formation – divergence between the genomes of the two taxa result in abnormal pairing between homoeologous chromosomes or nondisjunction during meiosis.[76] In this case, allopolyploidy can actually restore normal, bivalent meiotic pairing by providing each homoeologous chromosome with its own homologue. If divergence between homoeologous chromosomes is even across the two subgenomes, this can theoretically result in rapid restoration of bivalent pairing and disomic inheritance following allopolyploidization. However multivalent pairing is common in many recently formed allopolyploids, so it is likely that the majority of meiotic stabilization occurs gradually through selection.[68][70]

Because pairing between homoeologous chromosomes is rare in established allopolyploids, they may benefit from fixed heterozygosity of homoeologous alleles.[77] In certain cases, such heterozygosity can have beneficial heterotic effects, either in terms of fitness in natural contexts or desirable traits in agricultural contexts. This could partially explain the prevalence of allopolyploidy among crop species. Both bread wheat and Triticale are examples of an allopolyploids with six chromosome sets. Cotton is an allotetraploid with multiple origins. In Brassicaceous crops, the Triangle of U describes the relationships between the three common diploid Brassicas (B. oleracea, B. rapa, and B. nigra) and three allotetraploids (B. napus, B. juncea, and B. carinata) derived from hybridization among the diploid species. A similar relationship exists between three diploid species of Tragopogon (T. dubius, T. pratensis, and T. porrifolius) and two allotetraploid species (T. mirus and T. miscellus).[78] Complex patterns of allopolyploid evolution have also been observed in animals, as in the frog genus Xenopus.[79]

Paleopolyploidy[edit]

Main article: Paleopolyploidy

Ancient genome duplications probably occurred in the evolutionary history of all life. Duplication events that occurred long ago in the history of various evolutionary lineages can be difficult to detect because of subsequent diploidization (such that a polyploid starts to behave cytogenetically as a diploid over time) as mutations and gene translations gradually make one copy of each chromosome unlike the other copy. Over time, it is also common for duplicated copies of genes to accumulate mutations and become inactive pseudogenes.[80]

In many cases, these events can be inferred only through comparing sequenced genomes. Examples of unexpected but recently confirmed ancient genome duplications include baker's yeast (Saccharomyces cerevisiae), mustard weed/thale cress (Arabidopsis thaliana), rice (Oryza sativa), and an early evolutionaryancestor of the vertebrates (which includes the human lineage) and another near the origin of the teleostfishes. Angiosperms (flowering plants) have paleopolyploidy in their ancestry. All eukaryotes probably have experienced a polyploidy event at some point in their evolutionary history.

Karyotype[edit]

Main article: Karyotype

A karyotype is the characteristic chromosome complement of a eukaryotespecies.[81][82] The preparation and study of karyotypes is part of cytology and, more specifically, cytogenetics.

Although the replication and transcription of DNA is highly standardized in eukaryotes, the same cannot be said for their karotypes, which are highly variable between species in chromosome number and in detailed organization despite being constructed out of the same macromolecules. In some cases, there is even significant variation within species. This variation provides the basis for a range of studies in what might be called evolutionary cytology.

Paralogous[edit]

The term is used to describe the relationship between duplicated genes or portions of chromosomes that derived from a common ancestral DNA. Paralogous segments of DNA may arise spontaneously by errors during DNA replication, copy and paste transposons, or whole genome duplications.

Homologous[edit]

The term is used to describe the relationship of similar chromosomes that pair at mitosis and meiosis. In a diploid, one homolog is derived from the male parent (sperm) and one is derived from the female parent (egg). During meiosis and gametogenesis, homologous chromosomes pair and exchange genetic material by recombination, leading to the production of sperm or eggs with chromosome haplotypes containing novel genetic variation.

Homoeologous[edit]

The term homoeologous, also spelled homeologous, is used to describe the relationship of similar chromosomes or parts of chromosomes brought together following inter-species hybridization and allopolyploidization, and whose relationship was completely homologous in an ancestral species. In allopolyploids, the homologous chromosomes within each parental sub-genome should pair faithfully during meiosis, leading to disomic inheritance; however in some allopolyploids, the homoeologous chromosomes of the parental genomes may be nearly as similar to one another as the homologous chromosomes, leading to tetrasomic inheritance (four chromosomes pairing at meiosis), intergenomic recombination, and reduced fertility.

Example of homoeologous chromosomes[edit]

Durum wheat is the result of the inter-species hybridization of two diploid grass species Triticum urartu and Aegilops speltoides. Both diploid ancestors had two sets of 7 chromosomes, which were similar in terms of size and genes contained on them. Durum wheat contains two sets of chromosomes derived from Triticum urartu and two sets of chromosomes derived from Aegilops speltoides. Each chromosome pair derived from the Triticum urartu parent is homoeologous to the opposite chromosome pair derived from the Aegilops speltoides parent, though each chromosome pair unto itself is homologous.

Bacteria[edit]

Each Deinococcus radioduransbacterium contains 4-8 copies of its chromosome.[83] Exposure of D. radiodurans to X-ray irradiation or desiccation can shatter its genomes into hundred of short random fragments. Nevertheless, D. radiodurans is highly resistant to such exposures. The mechanism by which the genome is accurately restored involves RecA-mediated homologous recombination and a process referred to as extended synthesis-dependent strand annealing (SDSA).[84]

Azotobacter vinelandii can contain up to 80 chromosome copies per cell.[85] However this is only observed in fast growing cultures, whereas cultures grown in synthetic minimal media are not polyploid.[86]

Archaea[edit]

The archaeonHalobacterium salinarium is polyploid[87] and, like D. radiodurans, is highly resistant to X-ray irradiation and desiccation, conditions that induce DNA double-strand breaks.[88] Although chromosomes are shattered into many fragments, complete chromosomes can be regenerated by making use of overlapping fragments. The mechanism employs single-stranded DNA binding protein and is likely homologous recombinational repair.[89]

See also[edit]

References[edit]

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This image shows haploid (single), diploid (double), triploid (triple), and tetraploid (quadruple) sets of chromosomes. Triploid and tetraploid chromosomes are examples of polyploidy.
Schematic phylogeny of the fungi. Red circles indicate polyploidy, blue squares indicate hybridization. From Albertin and Marullo, 2012[41]
Schematic phylogeny of the Chromalveolata. Red circles indicate polyploidy, blue squares indicate hybridization. From Albertin and Marullo, 2012[41]

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