Alternation of Generations

Alternation of Generations (also known as metagenesis or heterogenesis ) [1] is a type of life cycle that occurs in plants and algae in the Archaeplastida and Heterokontophyta that separate haploid sexual and diploid asexual stages. In these groups, a multicellular haploid gametophyte with n chromosomes substitutes with a multicellular diploid sporophyte composed of n chromosomes, with 2 n pairs. a mature sporophyte meiosisProduces haploid spores by spores , a process that halves the number of chromosomes from 2n to n .

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

The relationship between sporophyte and gametophyte varies among different groups of plants. In algae in which there is an 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 largely depends on it. Although mosses and hornwort sporophytes can photosynthesize, they require additional photosynthesis from the gametophyte to maintain growth and spore development and depend on it for supply of water, mineral nutrients and nitrogen. [2] [3] In contrast, all modern vascular plantsThe gametophyte in the U.S. is less developed than the sporophyte, although their Devonian ancestor had a gametophyte and sporophyte of nearly equal complexity. [4] In ferns the gametophyte is a small flattened autotrophic prothallus on which the young sporophyte depends for some time for its nutrition. In flowering plants , the lack of the gametophyte is more extreme, consisting of only a few cells that are fully developed inside the sporophyte.

Animals develop differently. They directly produce haploid gametes. No haploid spores capable of dividing are produced, so there is generally no multicellular haploid stage. (Some insects have a sex-determination system whereby haploid males produce from haploid eggs, although females produced from fertilized eggs are diploid.)

Plants and algae alternate with haploid and diploid multicellular stages of the life cycle, referred to as diplohaplontic (equivalent terms haplodiplontic , diplobiontic and dibiontic , are also in use as a two-stage being of such an organism Description is Personality [5] ). The life cycle, such as that of animals, in which there is only one diploid multicellular stage, is called diploid . The life cycle in which there is only one haploid multicellular stage is called haplontic .

Definition

Alternation of generations is defined as the alternation of multicellular diploid and haploid forms in the life cycle of an organism, even though these forms are free-living. [6] In some species, such as the alga ulva lactuca , the diploid and haploid forms are in fact independent organisms that are essentially identical in appearance and are therefore called isomorphic. Free-swimming, haploid gametes form a diploid zygote that germinates into a multicellular diploid sporophyte. The sporophyte produces free-swimming haploid spores by meiosis which germinate into the haploid gametophyte. [7]

However, in some other groups, either the sporophyte or the gametophyte are greatly reduced and are unable to live free. For example, in all bryophytes the gametophyte generation is dominant and the sporophyte is dependent on it. In contrast, gametophytes are strongly reduced in all modern vascular land plants , although fossil evidence indicates that they were derived from isomorphic ancestors. [4] Seed plants inOf course, the female gametophyte develops entirely within the sporophyte, which protects and nurtures it and the embryonic sporophyte that it produces. Pollen grains, which are male gametophytes, are reduced to only a few cells (in many cases only three cells). Here the notion of two generations is less clear; As Bateman and DiMichel say, “the porophyte and the gametophyte effectively function as one organism”. [8] The alternative term ‘alternation of steps’ may then be more appropriate.

History

The debate about the alternation of generations in the early twentieth century can be confusing because of the different ways of classifying “generations” as coexistence (sexual versus asexual, gametophyte versus sporophyte, haploid versus diploid, etc.). ^[10]

Initially, Chamisso and Steenstrup described the succession of differently arranged generations (sexual and asexual) as “alternation of generations” in animals, during the evolutionary studies of tunicates, cnidarians and trematode animals. ^[10] This phenomenon is also known as heterosexuality. Currently, the term “alternation of generations” is almost exclusively associated with the life cycle of plants, particularly with the alternation of haploid gametophytes and diploid sporophytes.

Wilhelm Hofmeister demonstrated the morphological alternation of generations in plants, [11] between a spore-bearing generation (sporophyte) and a gamete-bearing generation (gametophyte). [12] [13] By that time a debate had emerged focusing on the origin of asexual genera of land plants (i.e., sporophyte) and is recognized as a conflict between the theories of traditional anagrams (Čelakovský, 1874) and homologous (Pringsheim, 1876) The alternation of generations. [10] Selakovsky coined the terms sporophyte and gametophyte.
Eduard Strasberger (1874) discovered the alternation between diploid and haploid nuclear phases, [10] also known as cytological alternation of nuclear phases. [14] Although most often coincidental, morphological alternation and nuclear phase alternation are sometimes independent of each other, for example, in many red algae, the same nuclear phase may correspond to two diverse morphological generations. [14] In some ferns that have lost sexual reproduction, there is no change in the nuclear stage, but the alternation of generations remains.

alternation of generations in plants

Fundamental element

The diagram above shows the basic elements of alternation of generations in plants. The many variations found in different groups of plants are described using these concepts later in the article. The processes starting from the right side of the diagram are as follows: ^[16]

• Two single-celled haploid gametes, each containing n unpaired chromosomes, fuse to form a single-cell diploid zygote, which now contains n pairs of chromosomes, that is, a total of 2 n chromosomes.
• Single-celled diploid zygotes germinate, dividing by the normal process (mitosis), which maintains the number of chromosomes at 2n . The result is a multicellular diploid organism, called a sporophyte (since it produces spores at maturity).
• When it reaches maturity, the sporophyte produces one or more sporangia (singular: sporangium) which are organelles that produce diploid spore mother cells (sporocytes). These divide by a special process (meiosis) that halves the number of chromosomes. This initially results in four single-celled haploid spores, each containing n unpaired chromosomes.
• Single-celled haploid spores germinate, dividing by the normal process (mitosis), which maintains the number of chromosomes at n . The result is a multicellular haploid organism, called a gametophyte ( since 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 type of gamete has some mechanism to merge with another gamete to reach it.

The ‘alternation of generations’ in the life cycle thus occurs 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 produces haploid ( n ) gametes directly by meiosis . In animals, spores (i.e. haploid cells capable of undergoing mitosis) are not produced, so there is no asexual multi-cell generation. Some insects have haploid males that develop from fertilized eggs, but females are all diploid.

Shift

The diagram shown above is a good representation of the life cycle of some multi-cellular algae (such as the genus Cladophora ), which have nearly identical looking sporophytes and gametophytes and do not contain different types of spores or gametes. ^[17]

However, there are many possible variations in the fundamentals of the life cycle, including the alternation of generations. Each variation can occur separately or in combination, resulting in a bewildering variety of life cycles. The words used by botanists to describe these life cycles can be equally astonishing. As Bateman and Dimichael put it “[…] the alternation of generations has become a terminological quagmire; often, one word represents several concepts or one concept is represented by several words.” ^[18]

Possible variations are:

• Relative importance of sporophyte and gametophyte.
  • Similar ( symmetry or symmetry ). Filamentous algae of the genus Cladophora
    , which are mainly found in fresh water, have diploid sporophytes and haploid gametophytes that are externally indistinguishable. ^[19] No living land plant has equally dominant sporophytes and gametophytes, although some theories of evolution by alternation of generations suggest that ancestral land plants did.
  • unequal ( heteromorphy or anisomorphy ).The gametophyte of Ammonium hornum , a moss.
    • The dominant gametophyte ( gametophytic ).
      In liverworts, mosses and hornworts, the predominant form is the haploid gametophyte. The diploid sporophyte is not capable of independent existence, receives most of its nutrition from the parent gametophyte, and does not contain chlorophyll when mature. ^[20]The sporophyte of Lomaria discolor , a fern.
    • The 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 very small and simple in structure. In seed plants, the gametophyte is further reduced (minimum to only three cells), deriving all its nutrition from the sporophyte. The extreme reduction in the size of the gametophyte and its retention within the sporophyte means that the term ‘alternation of generations’ is somewhat misleading when applied to seed plants: “The porophyte and gametophyte effectively function as one organism.” We do”. ^[8] Some authors have preferred the term ‘alternation of steps’. ^[9]
• Differentiation of gametes.
  • Both gametes are identical ( isogamy ). Like other species of Cladophora , C. Colicomas have flagellated gametes that are similar in appearance and ability to move. ^[19]
    
  • Two different sizes of gametes ( anisogamy ).
    • Both have the same mobility . Ulva
      species, the sea lettuce, have gametes that all have two flagella and are therefore motile. However they are of two sizes: larger ‘female’ gametes and smaller ‘male’ gametes. ^[21]
    • One big and slow , one small and moving ( Ugami ). Large sessile megagametes are eggs (ova), and small motile microgametes are spermatozoa (spermatozoa). The degree of motility of the sperm may be very limited (as in the case of flowering plants) but all eggs are capable of moving towards the egg. When (as is almost always the case) the sperm and egg are producing different types of gametangia, the sperm-producing ones are called antheridia (singular antheridium) and egg-producing archegonia (singular archegonium).Gametophyte of Pelia epiphylla, with sporophytes growing from remnants of archegonia.
      • Antheridia and archegonia are found on the same gametophyte, which are then called monoicous . (Many sources, including those related to bryophytes, use the term ‘monocious’ for this condition and ‘dioecious’ for the opposite. ^[22] [23] Here ‘monocious’ and ‘dioecious’ are used only for sporophytes The
        liverwort Pelia epiphylla has the gametophyte as the dominant genera . It is monoecious: small red colored sperm-producing antheridia are scattered along the middle vein while the egg-laying archegonia grows closer to the plant division tips. ^[24]
      • Antheridia and archegonia occur on separate gametophytes , which are then called dioecious .
        The moss Ammonium hornum has the gametophyte as the dominant genera . It is dioecious: male plants produce only antheridia in terminal rosettes, female plants produce only archegonia in the form of stalked capsules. ^[25]Seed plant gametophytes are also dicotyledonous. However, the parent sporophyte can be monoecious, producing both male and female gametophytes, or dioecious, producing gametophytes of only one sex. Seed plant gametophytes are greatly reduced in size; The archegonium consists of only a small number of cells, and the entire male gametophyte can be represented by only two cells. ^[26]
• Differentiation of spores.
  • All spores of the same size ( homospory or isospory).
    Horsetails ( species of Equisetum ) have spores that are all the same size. ^[27]
  • Spores of two different sizes ( heterospory larger: or anisospory) are megaspores and smaller microspores . When two types of spores are produced in different types of spores, they are called megasporangia and microsporangia . A megaspore often (but not always) develops at the expense of the other three cells as a result of meiosis, which is aborted.
    • Megasporangia and microsporangia occur on a single sporophyte, which is then called monoecious .
      Most flowering plants fall into this category. Thus a lily flower has six stamens (microsporangia) that produce microspores that develop into pollen grains (microgametophytes), and three fused carpels that produce complete megasporangia (eggs), each of which is a megaspore. which develops inside the megasporangium. Megagametophyte. In other plants, such as hazel, some flowers have only stamens, others only carpels, but both types of flowers occur in the same plant (ie sporophyte) and are therefore monoecious.Flowers of European holly, a diploid species: male above, female below (leaves cut to show flowers more clearly)
    • Megasporangia and microsporangia occur on separate sporophytes , which are then called dioecious .
      An individual tree of the European holly ( Ilex aquifolium ) produces either ‘male’ flowers containing only functional stamens (microsporangia) that produce microspores that develop into pollen grains (microgametophytes) or ‘female’ flowers that contain only There are functional carpels that produce integumentary megasporangia ( ovules ). ) containing a megaspore that develops into a multicellular megagametophyte.

There are some correlations between these variations, but they are just that, correlation, and not absolute. For example, in flowering plants, microspores eventually produce microgametes (sperm) and megaspores eventually produce megagametes (eggs). However, ferns and their allies have groups containing undivided spores but differentiated gametophytes. For example, the fern Ceratopteris thalictrioides has only one type of spore, which varies continuously in size. Small spores germinate into the gametophyte which produces only antheridia that produce sperm. ^[27]

A complex life cycle

The diagram shows the alternation of generations in a species that is heterozygous, sporophytic, oogametic, dioecious, heterosporic and dioecious. An example of a seed plant might be a willow tree ( most species of the genus Salix are dioecious ). ^[28] Starting from the center of the diagram, the processes involved are:

• A stationary 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 meiosis by meiosis.
    In a willow (like all seed plants) the zygote first develops into an embryonic microsporophyte within the ovule (a megasporangium is enclosed in one or more protective layers of tissue known as the integument). At maturity, these structures become seeds. The seed is later shed, germinates and develops into a mature tree. A ‘male’ willow tree (a microsporophyte) produces flowers with only stamens, the anthers of which are microsporangia.
  • Microspores germinate to produce microgametophytes; At maturity one or more antheridia are produced. Sperm develop within antheridia.
    In a willow, microspores are not released from the anther (microsporangium), but develop into pollen grains (microgametophytes) within it. The entire pollen grain is carried (for example by an insect or by wind) to an ovule (megagametophyte), where a sperm is produced which travels down the 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; Normally one of the four spores produced by meiosis gains the bulk at the expense of the remaining three, which disappear.
    ‘Female’ willow trees (megasporophytes) produce flowers with only carpels (modified leaves that bear megasporangia).
  • Megaspores germinate to produce megagametophytes; At maturity one or more archegonia are produced. The eggs develop within the archegonia.
    The carpels of a willow produce eggs, megasporangia enclosed in integuments. Within each ovule, a megaspore develops into a megagametophyte by mitosis. Within the megagametophyte an archegonium develops and produces an egg. The entire gametophyte ‘generation’, except for the pollen grain, is protected by the sporophyte (which is reduced to only three cells contained within the microspore wall).

Life cycle of different plant groups

The word “plants” here refers to the Archiplastida, i.e. 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, generations are homomorphic (isomorphic) and free-living. Some species of red algae have a complex triadic alternation of generations, consisting of a gametophyte stage and two distinct sporophyte stages. For more information, see Red Algae: Reproduction.

Land plants have heteromorphic (anisomorphic) alternations of all generations, in which the sporophyte and gametophyte are clearly differentiated. The gametophyte generation is most conspicuous among all bryophytes, i.e. liverworts, mosses and hornworts. As an example, consider a monoicus moss. Antheridia and archegonia develop on the mature plant (gametophyte). In the presence of water, biflagellate spermatozoa from antheridia swim to the archegonia and fertilization occurs, producing the diploid sporophyte. The sporophyte grows from the archegonium. Its body consists of a capsule atop a long stalk within which spore-producing cells undergo meiosis to form haploid spores. Most mosses rely on the wind to disperse these spores, although Splachnum sphaericumis insectivorous, recruiting insects to spread their spores. For more information see Liverwort: Life Cycle, Moss: Life Cycle, Hornwort: Life Cycle.

• Diagram of alternation of generations in liverworts.

In ferns and their allies, including clubmosses and horsetails, the plant observed in the specific region is the diploid sporophyte. The haploid spores develop on the underside of the fronds in sori and are spread by wind (or in some cases, floating on water). If the conditions are right, a spore will germinate and develop into an inconspicuous plant body called a prothallus. The haploid prothallus does not resemble the sporophyte, and as such there is a heteromorphic alternation of generations in ferns and their allies. The prothallus is short lived, but reproduces sexually, producing a diploid zygote which then exits the prothallus as the sporophyte. For more information, see Fern: Life cycle.

• A gametophyte (Prothallus) of Dixonia sp .
• A sporophyte of Dixonia antarctica .
• A Dixonia antarctica showing sori, or spore-producing structures, beneath the frond.

In phloem, seed plants, the sporophyte is the major multicellular stage; The gametophytes are greatly reduced in size and vary greatly in morphology. The entire gametophyte generation, with the sole exception of pollen grains (microgametophytes), is contained within the sporophyte. The life cycle of the willow, a dioecious flowering plant (angiosperm), is outlined in some detail in an earlier section (A Complex Life Cycle). The life cycle of gymnosperms is similar. However, in flowering plants there is also a phenomenon called ‘double fertilization’. Two sperm nuclei from the pollen grain (microgametophyte), instead of one sperm, enter the archegonium of the megagametophyte; One joins with the nucleus of the egg to form the zygote, the other with the other two nuclei of the gametophyte to form the ‘endosperm’, which nourish the developing embryo. See Double fertilization for more details.

Development of the major diploid stage

It has been proposed that the basis for the emergence of the dominant stage of the diploid phase (sporophyte) of the life cycle (such as in vascular plants) is that diploidy allows the expression of deleterious mutations to be hidden through genetic complementation. ^[29] [30]Thus, if diploid cells have mutations in one of the parental genomes that cause defects in one or more gene products, these deficiencies can be compensated for by the other parental genome (which is still in other Your fault may be genes). As the diploid phase was becoming dominant, the masking effect likely allowed genome size, and therefore information content, to increase without the constraint of improving the accuracy of DNA replication. The opportunity to enhance the information content at a lower cost was advantageous as it allowed new variations to be encoded. This view has been challenged, with evidence that selection is not more effective in the haploid than in the diploid stages of the lifecycle of mosses and angiosperms.

Tip of tulip stamens showing pollen (microgametophytes)

(megagametophytes): gymnosperm ovule on the left, angiosperm ovule (inside the ovary) on the right

Similar processes in other organisms

Rhizaria

Some organisms are currently classified in the clade Rhizaria and thus exhibit alternation of generations, not plants in the sense used here. Most foraminifera undergo a heteromorphic alternation of generations between the haploid egmont and diploid egmont forms . A single-celled haploid organism is usually much larger than a diploid organism.

Fungus

Fungal mycelia are usually haploid. When mycelia of different mating types mate, they produce two multi-central ball-shaped cells, which connect via a “mating bridge”. Nuclei move from one mycelium to another, 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 the sporangia are formed. Karogamy produces a diploid zygote, a short-lived sporophyte that soon undergoes meiosis to form haploid spores. When spores germinate, they develop into new mycelia.

Mud molds

The life cycle of slime molds is similar to that of fungi. The haploid spores germinate to form swarm cells or myxamoebae . These fuse to form a diploid zygote in a process referred to as plasmogamy and karyogamy . The zygote develops into a plasmodium, and the mature Plasmodium produces one to several fruiting bodies containing haploid spores, depending on the species.

Animals

The alternation between a multicellular diploid and a multicellular haploid generation never occurs in animals. ^[32] In some animals, there is a choice between parthenogenic and sexual reproductive stages (heterosexuality). Both phases are diploid. This is sometimes called “alternation of generations”, ^[33] but it is quite different. In some other animals, such as hymenopterans, males are haploid and females are diploid, but this is always the case, rather than an alternation between different generations.