Alternation of generations


Alternation of generations is the predominant type of life cycle in plants and algae. In plants, both phases are multicellular: the haploid sexual phase – the gametophyte – alternates with a diploid asexual phase – the sporophyte.
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 fuse to produce a diploid zygote, which divides repeatedly by mitosis, developing into a multicellular diploid sporophyte. This cycle, from gametophyte to sporophyte, 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 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. In many algae, the free-swimming, haploid gametes form a diploid zygote which germinates into a multicellular diploid sporophyte. The sporophyte produces free-swimming haploid spores by meiosis that germinate into haploid gametophytes.
However, in land plants, either the sporophyte or the gametophyte is very much reduced and is incapable of free living. For example, in all bryophytes the gametophyte generation is dominant and the sporophyte is dependent on it. By contrast, in all seed plants the gametophytes are strongly reduced, although the fossil evidence indicates that they were derived from isomorphic ancestors. In seed plants, the female gametophyte develops totally within 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. Here the notion of two generations is less obvious; as Bateman & Dimichele say "sporophyte and gametophyte effectively function as a single organism". The alternative term 'alternation of phases' may then be more appropriate.

History

In animals

Initially, Adelbert von Chamisso and Japetus Steenstrup described the succession of differently organized generations in animals as "alternation of generations". Later, the phenomenon in animals became known as heterogamy, while the term "alternation of generations" was restricted to the life cycles of plants, meaning specifically the alternation of haploid gametophytes and diploid sporophytes.

In plants

In 1851, Wilhelm Hofmeister demonstrated the morphological alternation of generations in plants, between a spore-bearing generation and a gamete-bearing generation. By that time, a debate emerged focusing on the origin of the asexual generation of land plants and is conventionally characterized as a conflict between theories of antithetic and homologous alternation of generations.
In 1874, Eduard Strasburger 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. There are many variations in different groups of plants. 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 2n chromosomes.
  • The single-celled diploid zygote germinates, dividing by the normal process, which maintains the number of chromosomes at 2n. The result is a multi-cellular diploid organism called the sporophyte.
  • When it reaches maturity, the sporophyte produces one or more sporangia which are the organs that produce diploid spore mother cells. These divide by a special process 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, which maintains the number of chromosomes at n. The result is a multi-cellular haploid organism, called the gametophyte.
  • When it reaches maturity, the gametophyte produces one or more gametangia 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 generation of multicellular sporophytes and a haploid generation of multicellular gametophytes.
The situation is quite different from that in animals, where the fundamental process is that a multicellular diploid individual produces haploid gametes by meiosis. In animals, spores are not produced, so there is no asexual multicellular 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 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.
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.Image:Mnium hornum 2005.04.02 14.55.41.jpg|thumb|Gametophyte of Mnium hornum, a moss
  • ** Dominant gametophyte.
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.Image:Blechnum discolor - crown fern.jpg|thumb|Sporophyte of Lomaria discolor, a fern
  • ** Dominant sporophyte.
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, 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: "porophyte 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.
Like other species of Cladophora, C. callicoma has flagellated gametes which are identical in appearance and ability to move.
  • * Gametes of two distinct sizes.
  • ** 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. The larger sessile megagametes are eggs, and smaller motile microgametes are sperm. The degree of motility of the sperm may be very limited but all are able to move towards the sessile eggs. When the sperm and eggs are produced in different kinds of gametangia, the sperm-producing ones are called antheridia and the egg-producing ones archegonia.Image:Pellia epiphylla IMG 1610.jpg|thumb|Gametophyte of Pellia epiphylla with sporophytes growing from the remains of archegonia
  • *** Antheridia and archegonia occur on the same gametophyte, which is then called monoicous.
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.
  • *** 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. 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.
  • Differentiation of the spores.
  • * All spores the same size.
Horsetails have spores which are all of the same size.
  • * Spores of two distinct sizes : 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 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 which produce microspores which develop into pollen grains, and three fused carpels which produce integumented megasporangia 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 has both kinds of flower and so is monoecious.Image:Hollyflowers.jpg|right|thumb|Flowers of European holly, a dioecious species: male above, female below
  • ** Megasporangia and microsporangia occur on different sporophytes, which are then called dioecious.
An individual tree of the European holly produces either 'male' flowers which have only functional stamens producing microspores which develop into pollen grains or 'female' flowers which have only functional carpels producing integumented megasporangia 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 and megaspores ultimately produce megagametes. 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.