Talk:Alternation of generations

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Do someone know the biology history about "alternation of generations".

Contents

[edit] Cleanup needed

This article has many typos and run-on sentences. Someone should proofread the article. Also, the article needs to have the current information about hypotheses on the origin of alternation of generations. The situation has evolved several times independently (each time multicellularity evolved). Protists in particular exhibit many different forms of alternation of generations (hey, even the green algae display several major variations). There's a LOT that an expert could add to this article. -- EncycloPetey 11:00, 12 November 2005 (UTC)

Actually, the problem is much deeper. In the intro it states that alternation-of-generations involves "two free-living phases" (a life-cycle or one generation includes both phases), then the article goes on to discuss flowering plants in which there are not two "free-living" phases. In other words, the article is confusing several related concepts - Marshman 21:41, 15 November 2005 (UTC)
Yes, it mixes together several concepts. At the core of them lies the fact that alternation of generations if the result of several cases of independent elaboration of some stage of the sexual reprocutive process. However, it is correct to say that all land plants have an alternation of generations. The alternation does not require the stages be free living. The term alternation of generations is usually restricted to those life cycles where there is both a multicellular haploid stage and a multicellular diploid stage. Thus, the term does not apply to animals or to those protists which never develop a multicellular stage. -- EncycloPetey 02:04, 16 November 2005 (UTC)
No, I do not agree. Since all sexual organisms "alternate" in that way, it is even worse to call that "alternation of generation". A life cycle is one generation, so the term is really wrong if you apply it to organisms that are not even independent in each phase (the only possible way one could see them as separate "generations"). That would mean each of us is the alternate generation of a previous generation of eggs and sperm. The concept you are pushing is called sexual reproduction, meiosis, or some such. Without that inclusion of "free living" the concept becomes trite, and is the reason this article is so confusing. - Marshman 03:03, 16 November 2005 (UTC)
But not all sexual organisms do. Humans do not have an alternation of generations because there is no multicellular haploid stage. I know of only a very few animal species with a multicellular haploid stage in the lifecycle, and in those cases, the haploid stage is sterile. Also, no botanist I know would call a single generation in a plant a life cycle. A life cycle by definition is a return to the starting point, and in plants that always means passing through two generations. That is one term that has a very, very different usage between botany and zoology. Except, of course, among parasitologists where the parasite life cycle typically involves several stages.
The problem is whether one defines the other. I agree a life cycle is just that, from egg to egg, or seed to seed. But I would not view an egg or seed as another generatioon separate from the adult it will grow into. I cannot figure out what your botanisrt friends are saying (only what they are not saying), so it is hard to respond to that. - Marshman
Neither do botanists consider a seed to be a separate generation. The seed is a container for an immature sporophyte. It is not my "botanist friends" that are saying this, it is all botanists. If you don't understand, try D. H. Campbell's (1918) Mosses and Ferns p.562-575. Some of the language and ideas are a bit outdated now, but it is an entire chapter on the subject, and one of the key sources still referenced today. -- EncycloPetey 08:10, 18 November 2005 (UTC)
If you force "free living" onto the definition of alternation of generations, then the only plants with alternation of generations would be the pteridophytes, which is not at all what a botanist means by the term. See any standard botany textbook to see support for my definition...
All I see is confusion over the term "generation", which is defined as one of two phases of a life-cycle. Indeed, the text I picked up says this: "The word generation is used here in a different sense from the usual one, which refers to the time interval between the birth of the parents and that of the progeny." How silly: redefine generation to confuse yet another generation of young botanists. I hold science in a little higher regard than that. I have no problem including your sense of the term "alternation of generation" as long as it is put in a proper context. To just go on redefining terms to suit a thought may be ok for a tetbook (the authors control the "unverse" of the text), but not a dynamic source of information like Wikipedia. Here, definitions need to be internally consistent or there better be some text explaining why not. It should not be our mission to simply regurgitate confusing, inconsistent, and sometimes outdated pronouncements from textbooks or "botanists we know" because they say so. Question and clarify, or this source of information is not worth consulting. - Marshman 18:58, 16 November 2005 (UTC)
The term "generation" is not merely a time interval, but a cohort of organisms produced by a parental group. Even in casual English this dual definition exists, hence "generation-X", which is a group of people, and not a time interval. It is thus not my sense of the word, but the scientific communitity's sense. If you have reason to disagree, why not provide a source instead of a personal opinion? By the way, you do know who the authors of the various texts I quoted are, don't you? Peter H. Raven is director of the Missouri Botanical Garden, and one of the world's most respected botanists. Gifford & Foster are two of the world's most respected paleobotanists, and their text is one of two standards used in college courses in English speaking countries. Again, if you truly believe that their ideas are inconsistent and outdated, please provide some reason why you believe this. Is it a personal belief, or is it foudned in some reason? -- EncycloPetey 08:10, 18 November 2005 (UTC)
"Multicellular haploid organisms that appear in alternation with diploid forms are found in plants; in some brown, red, and green algae; and in two closely related genera of chytrids and one or more other groups of protists not discussed in this book. Such organisms exhibit the phenomenon known as alternation of generations." p.236 Raven, Evert, & Eichhorn (2005) Biology of Plants, (7th ed.).
Statement's only problem is the oft expressed (but not by me) idea that algae are not plants. But nothing in that text says "free-living" should not be part of the definition; and in fact if you look up "organism" you will see they are saying exactly that ("haploid organisms that appear in alternation with diploid forms" = the n plant gives rise to the 2n plant gives rise to the n plant, ad infinitim) - Marshman
Nothing in that particular statement forbids "free-living" from being part of the definition, but if you look at the section of their book where the quote appears, you'll see that it cannot be logically included. The diagrams which accompany the discussion make it clear that a sporophyte "diploid individual" alternated with a gametophyte "haploid individual", and that this alternation applies equally to bryophytes (where the sporophyte individual remains attached to the gametophyte), pteridophytes (where the two generations live independently), and to seed plants (where the gametophyte individual remains attached to and is nutritionally dependent upon the sporophyte). -- EncycloPetey 08:10, 18 November 2005 (UTC)
"Alternation of generations, or phases in the life cycle is a consistent feature of all groups of vascular plants and hence represents the basic pattern of reproduction in these dominant plants of the modern world." p.15 Gifford & Foster (1988) Morphology and Evolution of Vascular Plants, (3rd ed.).
Only true to the extent that the pattern can be traced up through all the higher plant groups, but there is significant (I think) shift in what results in the conifers and angiosperms where the gametophyte phase ceases to be independent in any sense (i.e., cannot be regarded as a "generation"); and note again the use of "phases", recognition by these authors that "generation" is not necessarily the correct term - Marshman
So, logically, you would argue that a fetus is neither an individual nor a generation because it is dependent upon and attached to its mother? In both cases (fetus and angiosperm GPT), the dependent generation is genetically distinct from its parent. Generation is the correct term, since it develops as a multicellular organism from a single cell. (see below) -- EncycloPetey 08:10, 18 November 2005 (UTC)
"Sexual reproduction in most plants involves an alternation of dramatically different generations, whereas in animals reproduction involves the direct production of sperm and eggs as a result of meiosis in the germ cell line. The life cycle of a flowering plant is shown in Fig. 1.14. Plants have two alternating generations: the dominant sporophytic generation and a much reduced, parasitic gametophytic generation." p.27 Fosket (1994) Plant Growth and Development: A Molecular Approach.
I consider the above not a good statement, including the caveate about "most" plants and the lack of clear understanding of a term like generation. It emphasizes a point-of-view that even uses the idea that the gametophyte is a "parasite" to justify the somewhat fog-bound thinking that there are still really two generations in the higher plants. You don't think it is pushing things more than a bit to regard the multicellular gametophyte in a flower as a "parasite"? - Marshman
In the strictest sense of what a "parasite" is, yes. A parasite is an organism that derives its nutrition from another organism. That is exactly what is going on in this situation. Johannes Proskauer did significant experimental research on the relative independence of generations in hornworts, and found that they were not quite as dependent as is often cited. The sporophyte, though usually regarded as a strict parasite, was able to survive on its own. In any event, if total independence of existence is required to define an individual organism, then most forest trees do not qualify. Approximately 80-90% of all forest tree species rely on symbiotic fungi to survive. -- EncycloPetey 08:10, 18 November 2005 (UTC)
As you can see from these quotes (from the first three books I had at hand), (1) all plants have an alternation of generations, (2) the two generations are part of a single life-cycle, (3) animals do not have an alternation of generations, and (4) the alternating generations do not have to be free-living. You may not agree with this definition, but it is the definition that everyone else is using.
-- EncycloPetey 05:08, 16 November 2005 (UTC)
I think read more into those than is stated. Only the last really supports your contention about what is the definition, and I agree it certainly is one point of view. I think we need to do better here. - Marshman 18:58, 16 November 2005 (UTC)
What is the alternative point of view? Does anyone but you support it? -- EncycloPetey 08:10, 18 November 2005 (UTC)

[edit] What to do?

Rather than expend a lot of time and energy pushing forth ideas that are really not that far apart, let's admit that it is the use of confuising terminology that constitutes the real muddle here. I understand the evolutionary significance for the higher plants of the concept of "alternation of generations" and even the point that Fosket (1994) is making; but he dug himself a hole and is simply digging deeper to avoid admitting his confusion of terminolgy.

I'm not clear what you mean by this. His caveats are intended to cover those cases where plant species are known to reproduce only asexually, without meiosis and syngamy. There are many plant species that propogate asexually and no other way, so it is quite natural for someone who is technically minded to use the caveats when talking about plant reproduction. Among sexually reproducing plants, all species undergo an alternation of gametophyte and sporophyte generations. In some groups (e.g. seed plants) one generation is reduced but that makes it no less of a generation. You do not have an issue with Fosket, you have an issue with the evolution of development in plants.

Let's write this article so it clarifies the confusing aspects and not assume the reader is a "botanist" (wink, wink). The flowering plants can (and should) be brought in, but in the context of the evolutionary importance, not a blanket "Sexual reproduction in most plants involves an alternation of dramatically different generations..." which is silly for the use of "most" and "dramatically" and frankly, pablum for non-thinkers. I like the Gifford & Foster (1988) approach: "...phases in the life cycle is a consistent feature of all groups of vascular plants" – stated that way makes sense and is a good lead-in to what you want to include in this article - Marshman 19:19, 16 November 2005 (UTC)

I certainly agree that the concept is a confusing one. I have taught introductory botany for years (at Duke and UC Berkeley among other places). You expect that the students at such institutions are of top caliber in biology, but even they might take three or four weeks of digesting the idea before its meaning would sink in.
I can agree to some sort of "middle ground" as long as two critically important points are maintained. First, it must be clear that an alternation of generations is a life cycle that is split between two multicellular organisms that are genetically different, each being the offspring of the other. When the morphologist William Hoffmeister discovered this feature was common to (and homologous across) all land plants, the idea revolutioned botany. It is a core concept holding together plant life-cycles and plant evolution. Second, we should not attempt to redfine alternation of generations into something that it does not mean. The phrase is used quite consistently in the literature, unless you are familiar with a publication that I am not. The purpose of an encylcopedia is to make the information accessible, but is not the place to redfine words or formulate a personal position. -- EncycloPetey 04:33, 18 November 2005 (UTC)
I agree with what you are saying here. I'm not trying to "redefine" anything. If there is no conflict (all biologists accept that generation means a certain thing; perhaps as you describe below) then lets go with it. But I don't think that is the case. And again, I fully accept that the concept is important in understanding evolution in the plant kingdom, so I'm not trying to argue against including that; it should be the ultimate outcome that a complete article leads to here. - Marshman 18:19, 18 November 2005 (UTC)

[edit] Life-cycle vs generation

I've been reading and re-reading your suggested text changes, and think I've come across one specific issue that could be clarified. You are assuming that the terms "generation" and "completion of a life cycle" are necessarily one and the same. They are not. A life-cycle is a complete turn through the stages of organismal reproduction from one point back to that same point. I think we agree on that as a definition, at least. A generation, however, is the production of a multicellular organism from a single cell. This happens twice in organisms with a haplo-dipontic or a diplo-haplontic life cycle, since both the zygote and the spore undergo multiple mitotic divisions, to produce the diploid sporophyte and haploid gametophyte, respectively. The two terms are indeed synonymous among animals (and all organisms with a haplontic or diplontic life-cycle), but only because the life cycle includes a single multicellular stage. -- EncycloPetey 04:43, 18 November 2005 (UTC)

I just do not think (at this point) that your definition of generations is necessarily the generally accepted one (although I do see your point). I've run across the ideas, multiple times, that in "alternations of generations" generations is defined differently (quote I gave above) and that it should be alternation of phases within a generation to be correct. Obviously, the "problem" does revolve around that definition, which seems to be not generally fixed among biologists. But even your sources (you quoted above) used "phases" (in addition to generation in one case, presumably to satisfy both "camps"). While I agree, the term usually used is "alternation of generations", to call the gametophyte of the higher plants another generation becomes a real stretch (another phase within the generation is a lot cleaner). I think it is an idea some botanists like to promote (Fosket, 1994 for example) because it connjurs up an interesting picture of the gametophyte being "captured" by the sporophyte over evolutionary time; but that is just a metaphore. One can also view it as simply the reduction of importance of the gametophyte phase to its basic functions, paralleling animal evolution. Obviously with such confusions between botanists and everyone else and within botany itself, it is safest to avoid the term "generation"; but we cannot do that, because it is in the article title (for good reason). Therefore I suggest the "explanatory" approach I have been e3diting towards. That includes pointing out the conflicting definitions of "generation" used by biologists. It might help to first get the life cycle article tuned up. - Marshman 18:12, 18 November 2005 (UTC)

[edit] More Diagrams?

I think that having more diagrams to illustrate the various types of alternation of generations (ie: moss, gymnosperms, angiosperms) would make this article more readily understood. Alex Ng 04:23, 4 May 2006 (UTC)

Be BOLD and add images that are already located in the various sub-articles on groups of plants. In particular, LadyofHats has done some very nice diagrams, and might be asked to do a few more. --EncycloPetey 09:05, 4 May 2006 (UTC)


[edit] More examples would be nice

The real problem is that there are not enough examples. Experts in the subject have a bunch of examples in their heads that keep all these ideas straight; regular folk scratch their heads and don't see how it all fits together. For example, in plain language how do the apple trees in my yard go through this process? What are the different stages?? How do the flowers fit into this system? How does moss pass through this process? It just isn't clear to non experts. The article may be accurate but it is completely worthless to a high school freshman trying to read her biology text.

[edit] All or some vascular plants

Alternation of generations is a reproductive cycle of certain vascular plants, fungi, and protists.

Should it not be "all vascular plants" ? I do not know exceptions but the introduction suggests that it is rare. From what I have read there is always alternation of generations, the timespan/length of the haploid and diploid stages however vary. [1] Shyamal 03:59, 4 December 2006 (UTC)

Oops, looks like this has been discussed in the threads above. But the final introduction is still not satisfactory as per the earlier discussion. Shyamal 12:10, 4 December 2006 (UTC)
This has still not been cleared up. The article refers to "certain plants", and then below in the section "vascular plants", only "ferns and their allies" are mentioned. This implies that other vascular plants do not exhibit alternation of generations, contrary to what I thought was the case, although I'm not a botanist and I'm not sure. I suggest that a clear statement should be made one way or the other, either "most other vascular plants exhibit alternation of generations also", or "most other vascular plants do not exhibit alternation of generations". Bayle Shanks 03:34, 14 July 2007 (UTC)
Strangely, I too noticed that this has not been fixed just now and was further surprised by the GA status. I would think this article needs rework on citations, layout and wording. Unfortunately my botany is a little weak and rusty for this. Shyamal 03:40, 14 July 2007 (UTC)

[edit] GA Re-Review and In-line citations

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[edit] Cnidaria

Reference in the text to this phenomena in the Cnidaria (medusa to polyp forms) needs to be made Goldfinger820 05:38, 13 July 2007 (UTC)

Please elaborate on it, then. Speciate 06:03, 13 July 2007 (UTC)

Cnidarians do not undergo alternation of generations. They do have more than one life-cycle phase, but this is not the same as alternation of generations, which involves stages with different ploidy of tissues. In Cnidarians, all the stages are diploid, and the medusa stage is genetically identical to the stage from which it buds asexually. In alternation of generations, one stage is produced by meiosis and the other is produced by syngamy. --EncycloPetey 02:30, 14 July 2007 (UTC)

fair comment - however the term (although a misnomer) is still used in Cnidarian textbooks to describe the change between medusa and polyp phases. this should be made clear here, with a note that it is not true alternation of generations as alluded to above. Goldfinger820 03:57, 14 July 2007 (UTC)

If you will provide references to these textbooks, I can probably acquire them and add the information. The invertebrate texts I have available do not use this terminology. --EncycloPetey 20:16, 15 July 2007 (UTC)
although not a invertebrate only text, "Zoology Miller S.A and Harley, J.B. (1999) 4th ed IBSN 0-697-34555-6" has a reference to Cnidarian AoG on page 269. it is used here as a subtitle to describe the change in body form (but as you correctly point out -not in ploidy). will try to find some other refs if I get some time cheers Goldfinger820 04:44, 14 August 2007 (UTC)