Alternation Of Phases

Alternation of generations (also known as metagenesis or heterogenesis) is the predominant type of life cycle in plants and algae. It consists of a multicellular haploid sexual phase, the gametophyte, which has a single set of chromosomes alternating with a multicellular diploid asexual phase, the sporophyte which has two sets of chromosomes.

A mature sporophyte produces haploid spores by meiosis, 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 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 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 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 a few cells which grow entirely inside the sporophyte.

Animals develop differently. They 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 is describing such an organism as having a diphasic ontogeny). 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

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 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.

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. In seed plants, the female gametophyte develops totally within 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 " orophyte and gametophyte effectively function as a single organism". The alternative term 'alternation of phases' may then be more appropriate.

History

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.).

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. 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.

Wilhelm Hofmeister demonstrated 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 ( Čelakovský, 1874) and homologous (Pringsheim, 1876) alternation of generations. Čelakovský coined the words sporophyte and gametophyte.

Eduard Strasburger (1874) discovered 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 is no change in nuclear phase, but the alternation of generations is maintained.

Alternation of generations in plants

Fundamental elements

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:

* 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. 2''n'' chromosomes in total.
* The single-celled diploid zygote germinates, dividing by the normal process (mitosis), which maintains the number of chromosomes at 2''n''. 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 (2''n'') 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 (2''n'') individual 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 multi-cellular generation. Some insects have haploid males that develop from unfertilized eggs, but the females are all diploid.

Variations

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 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:
* ''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. 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.
*** ''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: " orophyte and gametophyte effectively function as a single organism". Some authors have preferred the term 'alternation of phases'.
* ''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.
** ''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.
*** ''One large and sessile, one small and motile'' (oogamy). The larger sessile megagametes are eggs (ova), and smaller motile microgamete

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