Alternation of generations - Wikipedia
Alternative Titles: diplohaplontic cycle, diplohaplontic life cycle, heterogenesis, metagenesis In algae, fungi, and plants, alternation of generations is common. green algae are flagellated. – Male gamete is flagellated in many plants Alternation of Generations. • sporophyte Fungi. • Hyphae. • Mycelium. • Dikaryotic. • Fruiting body. • Life cycle. • Lichens A Mutualism: Mycorrhizae. Campbell Fig. life cycle), Algae, Moss, Fern, Flowering Plant (including asexual) (sporic life cycles). 1. First some Algae Life cycle (isomorphic alternation of generations).
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. 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.
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. 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. 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 "[ Relative importance of the sporophyte and the gametophyte. Equal homomorphy or isomorphy. Filamentous algae of the genus Cladophorawhich are predominantly found in fresh water, have diploid sporophytes and haploid gametophytes which are externally indistinguishable. Unequal heteromorphy or anisomorphy.
Gametophyte of Mnium hornuma moss. 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.
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 cellsgaining 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: Both gametes the same isogamy. Like other species of CladophoraC. Both of similar motility.
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Species of Ulvathe sea lettuce, have gametes which all have two flagella and so are motile. However they are of two sizes: The larger sessile megagametes are eggs ovaand smaller motile microgametes are sperm spermatozoa, spermatozoids.Alternation of Generations
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.
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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. Many sources, including those concerned with bryophytes, use the term 'monoecious' for this situation and 'dioecious' for the opposite. The liverwort Pellia epiphylla has the gametophyte as the dominant generation. The moss Mnium hornum has the gametophyte as the dominant generation.
Sexual life cycles
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.
All spores the same size homospory or isospory. Horsetails species of Equisetum have spores which are all of the same size. 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 microgametophytesand 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. Flowers of European Holly, a dioecious species: 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.
A haploid spore 1n undergoes mitosis to produce a multicellular individual 1n with thread-like structures called hyphae.
Nuclear fusion then takes place, in which the haploid nuclei fuse to form diploid nuclei, and the cell containing the diploid nuclei is called the zygospore. The diploid nuclei in the zygospore undergo meiosis to produce haploid nuclei, which are released as unicellular spores 1nand the cycle repeats.
Because they were formed through meiosis, each spore has a unique combination of genetic material.
Alternation of generations
The spores germinate and divide by mitosis to make new, multicellular haploid fungi. Alternation of generations The third type of life cycle, alternation of generations, is a blend of the haploid-dominant and diploid-dominant extremes. This life cycle is found in some algae and all plants. Species with alternation of generations have both haploid and diploid multicellular stages. The haploid multicellular plants or algae are called gametophytes, because they make gametes using specialized cells.
Meiosis is not directly involved in making the gametes in this case, because the organism is already a haploid. Fertilization between the haploid gametes forms a diploid zygote.
The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte. Specialized cells of the sporophyte will undergo meiosis and produce haploid spores. The spores will then develop into the multicellular gametophytes. Example of alternation of generations: Haploid 1n spores germinate and undergo mitosis to produce a multicellular gametophyte 1n. Specialized cells of the gametophyte undergo mitosis to produce sperm and egg cells 1nwhich combine in fertilization to make a zygote 2n.
The zygote undergoes mitosis to form a multicellular, diploid sporophyte, the frond-bearing structure that we usually think of as a fern. On the sporophyte, specialized structures called sporangia form, and inside of them, haploid cells spores, 1n are formed by meiosis.
The spores are released and can germinate, starting the cycle over again. Although all sexually reproducing plants go through some version of alternation of generations, the relative sizes of the sporophyte and the gametophyte and the relationship between them vary among species.
In plants such as moss, the gametophyte is a free-living, relatively large plant, while the sporophyte is small and dependent on the gametophyte. In other plants, such as ferns, both the gametophyte and sporophyte are free-living; however, the sporophyte is much larger, and is what we normally think of as a fern.
In seed plants, such as magnolia trees and daisies, the sporophyte is much larger than the gametophyte: The gametophyte is made up of just a few cells and, in the case of the female gametophyte, is completely contained inside of the sporophyte within a flower. Why is sexual reproduction widespread?
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In some ways, asexual reproduction, which makes offspring that are genetic clones of the parent, seems like a simpler and more efficient system than sexual reproduction.
Two theories have been proposed to explain how alternation of generations evolved. Both theories hypothesize that the haploid generation is ancestral and that the diploid generation developed as a consequence of mitosis replacing immediate meiosis in the unicellular zygote.
One theory proposes that originally the developmental potential of the diploid zygote was identical to that of the haploid spores, resulting in isomorphic sporophytes and gametophytes. Sporophytes became structurally different from gametophytes as a result of spores and zygotes being exposed to different environmental pressures.
In land plants, for example, spores are released as unicells into the environment, while zygotes begin their development within the confines of the female gametangium. As a consequence, gametophytes, which develop from spores, and sporophytes, which develop from zygotes, are structurally very different.
The second theory proposes that the sporophyte generation evolved gradually by stepwise delays in zygotic meiosis, accompanied by the elaboration of vegetative diploid cells. The first sporophytes were little more than single sporangia, probably embedded in the much larger gametophytes. As evolution progressed, sporophytes became larger and larger, and gametophytes became more and more reduced. Even today, there is no consensus as to which theory best explains the diversity seen in modern organisms.
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