Alternation of generations - Wikipedia
A. Zygotic: An example of a generic fungal life cycle (zygotic): note the short single Algae Life cycle (isomorphic alternation of generations): mutualism. asexual reproduction and sexual reproduction with alternation of generations. For example most terrestrial plants form symbiotic relationships with fungi. an association between a fungus and its photosynthetic partner (usually an alga). For example most terrestrial plants form symbiotic relationships with fungi. association between a fungus and its photosynthetic partner (usually an alga). .. The lifecycle of basidiomycetes includes alternation of generations (Figure ) .
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 microsporangiumbut develop into pollen grains microgametophytes within it. The whole pollen grain is moved e. 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.
Megaspores germinate producing megagametophytes; at maturity one or more archegonia are produced. Eggs develop within the archegonia.
- Alternation of generations
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 Archaeplastidai. 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: Land plants all have heteromorphic anisomorphic alternation of generations, in which the sporophyte and gametophyte are distinctly different.
All bryophytesi. 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 entomophilousrecruiting insects to disperse its spores.
For further information, see Liverwort: Life cycleMoss: Life cycleHornwort: Diagram of alternation of generations in liverworts. Moss life cycle diagram Hornwort life cycle diagram In ferns and their allies, including clubmosses and horsetailsthe 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: 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 spermatophytesthe 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 microgametophytesis contained within the sporophyte. The life cycle of a dioecious flowering plant angiospermthe 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 microgametophyterather 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.
Lichens : Symbiotic Relation Between Algae and Fungi
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. As the diploid phase was becoming predominant, the masking effect likely allowed genome sizeand 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. Gymnosperm ovule on left, angiosperm ovule inside ovary on right Double fertilization 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 karyogamyand 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. Both phases are diploid. This has sometimes been called "alternation of generations",  but is quite different.
Graphis, Lecanora, Haematomma In this case, thallus has leaf-like lobes. They are fixed from the substrate by hairy rhizoids like structure called rhizines. They are attached only at central points. Parmellia, Collema, Peltigera Their thalli are cylindrical ribbon-like and branched. It is attached only at the base by basal mucilagenous disc. They are commonly called as shrubby lichens.
Two species of Lecanora have been used as food in the barren plains and mountains of Western Asia and Northern Africa. Certain classes of East Siberian inhabitants use lichens as vegetable diet. It is also used for the preparation of chocolates and pastries. It is a food for reindeer and cattle. Species of Cladonia, Citraria, Evernia, Parmelia are used as fodder. Usnic acid obtained from the Usnea and Cladonia species is used as an antibiotic against Gram-positive bacteria. Peltigera canina, the dog lichen is used as medicine for hydrophobia in ancient days.
Lobaria pulmonaria, lungwort are used for the diseases of lungs respiratory diseases and T. Parmelia is useful against epilepsy. Usnea species are good against urinary diseases. Xanthoria sp is used in jaundice etc Some lichens possess anticarcinogenic properties.
Lichens : Symbiotic Relation Between Algae and Fungi
The lungwort lichen is used in tanning, in perfumery. The cell walls of the fungi of certain lichens contain colouring matters. Species of Rocella and Lecanora yield a most important colouring matter known as Orchil or Cudbear. This is used in colouring woolen and silk fabrics. It is also used for manufacturing litmus papers which are used in laboratories for acid and base identification.
It is obtained from Roccella montagnei. In Russia and Sweden, lichens are used for the production of alcohol. Some lichens are harmful to industrial products because they cause considerable damage to glass surfaces and marble stones due to etching. Ecological Importance of Lichens: Lichens Contribute to Soil Formation: Lichens thrive in undisturbed sites where nothing else will grow.
They grow on rocks, barren soil and the bark of dead or live trees. They are not parasitic when they grow on trees, they just use the tree bark as a home.