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Author Topic:   Cells into Organs: could it evolve?
Asgara
Member (Idle past 2303 days)
Posts: 1783
From: Wisconsin, USA
Joined: 05-10-2003


Message 16 of 39 (186013)
02-16-2005 9:31 PM
Reply to: Message 15 by Quetzal
02-16-2005 9:23 PM


Re: Depends, do you get my question?
Fascinating Q !!!
Do you have anymore?

Asgara
"Embrace the pain, spank your inner moppet, whatever....but get over it"
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This message is a reply to:
 Message 15 by Quetzal, posted 02-16-2005 9:23 PM Quetzal has replied

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Quetzal
Member (Idle past 5872 days)
Posts: 3228
Joined: 01-09-2002


Message 17 of 39 (186120)
02-17-2005 8:16 AM
Reply to: Message 16 by Asgara
02-16-2005 9:31 PM


Re: Depends, do you get my question?
Thanks. You're insatiable! Hmm, other examples. Well, I guess that depends on what you're looking for. There are about a zillion "irreducible" symbiont examples, from lichens to shallow-water corals to various trichomonads, etc. Some of these symbioses are not just cross-phyla, but cross-kingdom (different "kinds" ). If you're looking for more of the Volvox-type example of primitive cellular differentiation, I'm sure there are (I am a firm believer that nothing in nature is ever invented only once), but I'd have to dig a bit to find them. Let me know what you're interested in, and I'll try and come with more examples.

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Wounded King
Member
Posts: 4149
From: Cincinnati, Ohio, USA
Joined: 04-09-2003


Message 18 of 39 (186195)
02-17-2005 12:31 PM
Reply to: Message 15 by Quetzal
02-16-2005 9:23 PM


Re: Depends, do you get my question?
This is a bit different than the Volvox set up. In Physalia, these are evolved from different, formerly free-living organisms. In Volvox we're seeing cooperation from a single type of organism that is differentiating "roles" within the colony.
Are you suggesting that Physalia is actually some sort of cross species symbiosis? As I understand it it is an obligate colonial certainly but I don't remember ever coming across the idea that the seperate zooids originated with distinct species.
I also don't recall off hand a species of Volvox with a free living individual stage of the life cycle. I know that Chlamydomonas is often given as the 'unicell' ancestor of the colonial Volvocaceae and that some are classed as colonial and some as multicellular. What particular species of volvox were you thinking of? The seasonality in your example makes me think of the seasonal sexual reproduction of V. carteri which produces a spore like zygote which can survive a ponds drying up.
Heat shock elicits production of sexual inducer in Volvox.
Kirk DL, Kirk MM.
Science. 1986 Jan 3;231(4733):51-4.
In the green alga Volvox carteri, heat shock had an unusual and adaptive effect mediated by induced production of a well-defined effector molecule. Females of this species normally reproduce asexually in the absence of a potent sexual inducer produced by mature sexual males, but they generated egg-bearing sexual daughters after a brief exposure to elevated temperatures. This response involved an "autoinduction" of sexuality, in which heat-shocked somatic cells made and released the sexual inducer, which then redirected development of the reproductive cells. Males, including a sterile mutant incapable of producing inducer in the usual manner, also produced the inducer in response to heat shock. The phenomenon probably is of significance in the wild, where Volvox reproduces asexually in temporary ponds in spring but becomes sexual and produces dormant, overwintering zygotes before the ponds dry up in the summer heat.
Can you be a bit more specific about the source of your examples? References always appreciated.
TTFN,
WK

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Ooook!
Member (Idle past 5815 days)
Posts: 340
From: London, UK
Joined: 09-29-2003


Message 19 of 39 (186265)
02-17-2005 3:20 PM
Reply to: Message 18 by Wounded King
02-17-2005 12:31 PM


Evolution of differentiation
I hope Q doesn't mind be butting in here, but this is something I find fasinating.
I also don't recall off hand a species of Volvox with a free living individual stage of the life cycle. I know that Chlamydomonas is often given as the 'unicell' ancestor of the colonial Volvocaceae and that some are classed as colonial and some as multicellular. What particular species of volvox were you thinking of? The seasonality in your example makes me think of the seasonal sexual reproduction of V. carteri which produces a spore like zygote which can survive a ponds drying up
I'm not too sure (not being too versed in these matters), but I think he may indeed be talking about V. carteri. I can't give any definitive references (I'm at home now), but there's a few pages about it in the first chapter of Developmental Biology by Scott F. Glbert if you've got one lying around. The sexual stage of their lifecycle is not the only interesting aspect of them apparently; the asexual stage has a definite division of labour.
From what I can gather from the pictures and the text, they resemble a ball of somatic cells which have flagella surrounding a collection of larger cells that never have functional flagella and are purely reproductive. Once these inner (germ?)-cells mature they divide rapidly (including some asymmetrical divisions) to form an embryo which eventually gets released from the 'parental' somatic cells (which subsequently commit suicide). You and Q are bound to know a lot more about this kind of thing than I do, so does this ring any bells?
The book mentions a couple of interesting things
1. A simple mutation can cause the somatic cells to not commit suicide, and go on to become gonidia cells in their own right. This IMO shows that the step between a colony made up of identical multipurpose cells and a differentiated one is relatively small.
2. The embryos that are inside the colony are originally inside out (ie with all the larger cells dotted around on the outside). They turn themselves the right way around by folding movements which resemble (superficially at least) those involved in gastrulation, (and the production of the different types of cells in more complex organisms).
Any thoughts?

This message is a reply to:
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Wounded King
Member
Posts: 4149
From: Cincinnati, Ohio, USA
Joined: 04-09-2003


Message 20 of 39 (186290)
02-17-2005 4:10 PM
Reply to: Message 19 by Ooook!
02-17-2005 3:20 PM


Re: Evolution of differentiation
What you recall corresponds to what I remember learning about Volvox. I've had a brief scan on pubmed and google and I haven't found what Q was talking about yet, I'm wondering if it is a Volvocine but not actually a species of Volvox, but the problem is that there is just so much stuff out there.
If you have access to journals you might be interested in a new paper.
A twelve-step program for evolving multicellularity and a division of labor.
Kirk DL.
Bioessays. 2005 Feb 15;27(3):299-310
The volvocine algae provide an unrivalled opportunity to explore details of an evolutionary pathway leading from a unicellular ancestor to multicellular organisms with a division of labor between different cell types. Members of this monophyletic group of green flagellates range in complexity from unicellular Chlamydomonas through a series of extant organisms of intermediate size and complexity to Volvox, a genus of spherical organisms that have thousands of cells and a germ-soma division of labor. It is estimated that these organisms all shared a common ancestor about 50 +/- 20 MYA. Here we outline twelve important ways in which the developmental repertoire of an ancestral unicell similar to modern C. reinhardtii was modified to produce first a small colonial organism like Gonium that was capable of swimming directionally, then a sequence of larger organisms (such as Pandorina, Eudorina and Pleodorina) in which there was an increasing tendency to differentiate two cell types, and eventually Volvox carteri with its complete germ-soma division of labor.
There is no need to actually own Gilbert as Pubmed has the entire text online in a searchable format.
TTFN,
WK
This message has been edited by Wounded King, 02-17-2005 16:15 AM

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Quetzal
Member (Idle past 5872 days)
Posts: 3228
Joined: 01-09-2002


Message 21 of 39 (186298)
02-17-2005 4:42 PM
Reply to: Message 18 by Wounded King
02-17-2005 12:31 PM


Combined Response: WK & Oook
First off, thanks to Oook for clarifying an aspect of Volvox that is indeed fascinating.
WK, I’m curious as to why you chose that article? It has nothing to do with either the phylogeny of Volvox or its lifecycle. The article discusses how heat stress provides an environmental trigger for the organism to change from asexual to sexual reproduction. In point of fact, it’s actually oxidative stress brought on by heat that appears to be the proximate cause of this transformation (see, for instance, Nedelcu AM, Marcu O, Michod RE, 2004 "Sex as a response to oxidative stress: a twofold increase in cellular reactive oxygen species activates sex genes", Proc R Soc Lond B Biol Sci. 271:1591-6).
As to the phylogeny, please see Kirk, DL 2005 A twelve-step program for evolving multicelluarity and a division of labor, Bioessays 27:299-310.
quote:
The volvocine algae provide an unrivalled opportunity to explore details of an evolutionary pathway leading from a unicellular ancestor to multicellular organisms with a division of labor between different cell types. Members of this monophyletic group of green flagellates range in complexity from unicellular Chlamydomonas through a series of extant organisms of intermediate size and complexity to Volvox, a genus of spherical organisms that have thousands of cells and a germ-soma division of labor. It is estimated that these organisms all shared a common ancestor about 50 +/- 20 MYA. Here we outline twelve important ways in which the developmental repertoire of an ancestral unicell similar to modern C. reinhardtii was modified to produce first a small colonial organism like Gonium that was capable of swimming directionally, then a sequence of larger organisms (such as Pandorina, Eudorina and Pleodorina) in which there was an increasing tendency to differentiate two cell types, and eventually Volvox carteri with its complete germ-soma division of labor.
This article discusses the relationship among the Volvacinae, and does a pretty good job of tracing the evolution of Volvox from a single-cell Chlamidomonas-like ancestor. For additional information, please see Nozaki H, Misawa K, Kajita T, Kato M, Nohara S, Watanabe MM, 2000, "Origin and evolution of the colonial volvocales (Chlorophyceae)as inferred from multiple, chloroplast gene sequences", Mol Phylogenet Evol. 17:256-68.
quote:
A combined data set of DNA sequences (6021 bp) from five protein-coding genes of the chloroplast genome (rbcL, atpB, psaA, psaB, and psbC genes) were analyzed for 42 strains representing 30 species of the colonial Volvocales (Volvox and its relatives) and 5 related species of green algae to deduce robust phylogenetic relationships within the colonial green flagellates. The 4-celled family Tetrabaenaceae was robustly resolved as the most basal group within the colonial Volvocales. The sequence data also suggested that all five volvocacean genera with 32 or more cells in a vegetative colony (all four of the anisogamous/oogamous genera, Eudorina, Platydorina, Pleodorina, and Volvox, plus the isogamous genus Yamagishiella) constituted a large monophyletic group, in which 2 Pleodorina species were positioned distally to 3 species of Volvox. Therefore, most of the evolution of the colonial Volvocales appears to constitute a gradual progression in colonial complexity and in types of sexual reproduction, as in the traditional volvocine lineage hypothesis, although reverse evolution must be considered for the origin of certain species of Pleodorina. Data presented here also provide robust support for a monophyletic family Goniaceae consisting of two genera: Gonium and Astrephomene.
As to the Volvox lifecycle, the gonidia (produced during the asexual version) are motile, and divide to produce daughter cells outside the colony. Indeed, Volvox aureus produces free- wimming "male" versions somewhat equivalent to sperm cells during sexual reproduction. See, for example, Desnitski AG, 2000, "Development and reproduction of two species of the genus Volvox in a shallow temporary pool", Protistology 1:195—198
quote:
The reproduction and the formation of resistant dormant spores in Volvox aureus Ehr. and V. tertius Meyer were studied in a temporary pool during July—August 1996 and 1997. It has been shown that the populations of two species of Volvox (L.) Ehr. from this pool were characterized by different reproductive traits. Comparatively small quantity of male individuals and dormant zygotes (zygospores) episodically formed in the population of V. tertius. By contrast, in the population of V. aureus the formation of dormant parthenospores occurred and the male individuals were never observed. The ratio V. aureus : V. tertius was different in different years and underwent changes during one and the same summer.
The abstract, unfortunately doesn't do the article justice. Beyond these, if you want more information, I'll try and dig it up.
I'll work up the references on your question concerning Physalia later, but for reference: Yes, I'm claiming the zooids represent formerly free-living organisms, as they don't appear to be based on cellular differentiation from within a single organism, as was the case with Volvox. In the meantime, do you have any quibbles on the Mixotricha example, or can we let than one stand without you trying to shoot holes in it?

This message is a reply to:
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Brad McFall
Member (Idle past 5033 days)
Posts: 3428
From: Ithaca,NY, USA
Joined: 12-20-2001


Message 22 of 39 (186355)
02-17-2005 7:06 PM
Reply to: Message 21 by Quetzal
02-17-2005 4:42 PM


Re: Combined Response: WK & Oook
and there is the point. to have been free living or not?
On seeing the narative in the Botany text ( and I guess now I really need to find this book so as to cite chapter and verse) the linkage from a single cell to a colony of cells was dependent on the images of these zooids as not drawn dependent on any thing but the artists free style. The narration only described common biological process in the mean time.
On reading this I was led to understand that the zooids ARE NOT independent artist free creations and thus free evolutionary argument ornaments but dependent also projectively in the illustrations ON THE ANGLE between the nucleus and the cilia on a line DRAWN between the pigment spot and Pyrenoid BEHIND WHICH is thePHOTOSYNTHETIC SPACE and THE CYTOPLASM in the foreground. Thus I cam to appreciate a new "view" of the critters, that, no matter whether living free or with any other number of zooids the division is INSIDE the place the memebrane attaches between the cilia and the rest of the life provided there are two functions actually of motion from the chemcial heat source and motion to the sun's heat source. That still keeps the same images but does not support the simple narrative that must invest on the free vs not free issue of the zooids othewise.
Thus when it came to issues of Behe and sexuality and difference say between Chlamydomonas and Volvox argument AGAINST irreducible complexity (the need to narrate the zooids obviously ADDS complexity in Behe's sense) could only be maintained if the WORD "colony" had a projective geometric grammer as it is about what, how the cilia affect in the cycle of sex, almost remincient of human reproduction in that way. Of course creationism could be correct but I assume most here are not trying that out at first. I know by personal observation that the community of biologist does not have a more sophisticated use of math to suggest I am simply naive in this.
Maybe all this information would be good for is to prepare to write as hard for biology as Newton did physics, less creationism, for I certainly would rather see more research than stratgies against creationist thought but I still dont understand why physicists cant use Newton directly as a community and thus make contact with the triple intelligence of OQowK?
This message has been edited by Brad McFall, 02-17-2005 19:26 AM

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Wounded King
Member
Posts: 4149
From: Cincinnati, Ohio, USA
Joined: 04-09-2003


Message 23 of 39 (186366)
02-17-2005 7:26 PM
Reply to: Message 21 by Quetzal
02-17-2005 4:42 PM


Life cycle
It has nothing to do with either the phylogeny of Volvox or its lifecycle. The article discusses how heat stress provides an environmental trigger for the organism to change from asexual to sexual reproduction.
It's a seasonal switch from asexual to sexual reproduction. It may not strictly be 'The' life cycle, in as much as colonies can simply cycle asexually during the spring period, but it is fairly significant in terms of the continued existence of the population over the year.
It is particularly relevant as it is reminiscent of the seasonality you were suggesting is shown by free living Volvox individuals clustering together to form colonies. All I was suggesting is that you might have got mixed up between seperate individuals clustering to form colonies and overwintering zygotes being reactivated and forming colonies.
As to the Volvox lifecycle, the gonidia (produced during the asexual version) are motile, and divide to produce daughter cells outside the colony.
I thought it was more usual for the daughters to form inside the colony.
I'm not sure why you think I should have chosen a paper on phylogeny, that seems entirely peripheral to the question of the Volvox lifecycle, unless you were actually thinking of another member of the Volvocales.
Your abstract from Desnitski(2000) actually seems to suggest that V. tertius, rather than V. aureus, produces
the 'male' individuals. A free swimming (haploid?)'sperm' also hardly constitutes the sort of free living individuals you suggested in your original post.
Yes, I'm claiming the zooids represent formerly free-living organisms, as they don't appear to be based on cellular differentiation from within a single organism, as was the case with Volvox.
Why don't they appear to be based on cellular differentiation? What do the gonozooids do? Does each seperate zooid type have its own reproductive subpopulation allowing a 'budding' of the colony by growing seperate zooids in situ. Have the gonozooids accumulated all of the genetic information neccessary to form the individual zooid systems and if so how does this differ from the sort of specialisation seen in volvox? As far as i can find out the gonophores release gametes into the water where they meet up and form planula larvae, do the various different zooids just then meet up in some sort of post-breeding assortment? As I understand it the larva produces the individuals for the colony by asexual reproduction.
In the meantime, do you have any quibbles on the Mixotricha example, or can we let than one stand without you trying to shoot holes in it?
Relax, I'm just asking for more details, I am totally unaware of the situations you are describing, is it so unreasonable to ask for some clarification and sources? Does being on the evolutionary side somehow absolve you of having to support things with evidence?
I don't know anything about Mixotricha, but I suspect that Lynn Margulis loves it.
TTFN,
WK

This message is a reply to:
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Replies to this message:
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Brad McFall
Member (Idle past 5033 days)
Posts: 3428
From: Ithaca,NY, USA
Joined: 12-20-2001


Message 24 of 39 (186391)
02-17-2005 8:32 PM
Reply to: Message 23 by Wounded King
02-17-2005 7:26 PM


Re: Life cycle
I would like to see the info on why it is not cell differentation too. It would be interesting to understand & how the following logic does not apply

My analysis would find the cilia of any kind within Davenports dotted lines and the zooids as figure rather than ground.

This message is a reply to:
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LDSdude
Inactive Member


Message 25 of 39 (186395)
02-17-2005 8:48 PM


okay, small little shift here then....
quote:
______________________________________________________________________Species evolve by accumulation of change. There is no question of "figuring out" anything. Living creatures just do their thing. Some survive, and some don't. This is due mostly the vagaries of fortune; a predator happens by, food runs out, a tree falls in the forest and sets up a suitable environment. But there is also a contribution of how well adapted living things are to take advantage of opportunity or to surmount obstacles
______________________________________________________________________
You are right that there is no "figuring out", and that is my point. Cells such as these slugs do not grow up with the ability to observe and then immitate their parents. Turning into this slug is then an obvious instinct which all of the examples I have been given show.
I hate to deviate on this, but I think you should understand what I'm saying.
My question was how did this process of turning into a slug become instinct to the cells? This slug(and also the algae fish) are quite complex little animals which require all of their parts to operate(more irreducible complexity... man, it's everywhere, isn't it?). They could not have evolved through slow changes over time. And even if one colony somehow defied Darwin's wildest dreams and accomplished it, it's an action, not a gene.
For example, if you do something, unless your kids can observe it and mimmick it, they might never know how to do it. Just becuase you did it doesn't mean it's going to become part of your DNA and your kids well naturally do it in their lifetimes. Cells like these don't have the ability to observe on that high level. They don't have the ability to LEARN.
So somehow these cells attained within their DNA the ability to naturally turn into an irreducibly complex organism. (According to evolution). And I'd like an explanation(if you aren't too busy).
Thanks for all the examples, people. But although they answered my first questions, in a way, they just raise more. Since this topic is still young I'm going to go ahead and add this question of instinct of turning into "organs" to the whole topic, seeing as they both relate very well.
I asked how cells stopped working for themselves and started working for the whole---you gave me examples---I'm asking now how the trait is passed on.
I think that somes it up pretty well.
fixed quote tags, changed parens to brackets - The Queen
This message has been edited by AdminAsgara, 02-17-2005 19:54 AM

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Loudmouth
Inactive Member


Message 26 of 39 (186401)
02-17-2005 9:05 PM
Reply to: Message 25 by LDSdude
02-17-2005 8:48 PM


Re: okay, small little shift here then....
quote:
You are right that there is no "figuring out", and that is my point. Cells such as these slugs do not grow up with the ability to observe and then immitate their parents. Turning into this slug is then an obvious instinct which all of the examples I have been given show.
And those instincts are the result of their genes. As you said, they can't learn by watching so that leaves one option, genetics.
quote:
So somehow these cells attained within their DNA the ability to naturally turn into an irreducibly complex organism. (According to evolution). And I'd like an explanation(if you aren't too busy).
It's quite simple, actually. Mutations allowed for offspring to work together. This was beneficial, as were other mutations that made this relationship more effecient. As the cells became better and better at working together the genes needed for individual survival withered away. The cells could no longer survive on their own, but survived fine working as a colony. Therefore, mutations that reduced a cells ability to live on it's own did not hurt the cells chances of spreading it's DNA (ie kin selection) and so these mutations were neutral. Mutations that made working together more effecient did help the cells spread their DNA and so the mutations were selected for.

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Sylas
Member (Idle past 5260 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 27 of 39 (186405)
02-17-2005 9:17 PM
Reply to: Message 25 by LDSdude
02-17-2005 8:48 PM


Re: okay, small little shift here then....
LDSdude writes:
My question was how did this process of turning into a slug become instinct to the cells?
I don't think there is any "instinct" here at all. The word instinct, as I use it, refers to an ingrained behaviour mediated by the nervous system.
The creatures we are looking at here don't have any nervous system. The level of organization above the level of the cell is minimal. The cells are reactive to stimuli. If this is "instinct", then any reaction is instinct, and the word becomes so inclusive that it no longer serves as a way of distinguishing different behaviours.
But whether you call it instinct or not, the cellular chemistry of the cells responds to external stimuli in a manner characteristic of that species of cell. Evolution can change such instincts, just as they can change any other heritable aspect.
The complexity of these creatures is not actually "irreducible" in the sense defined by Behe, because you can in fact remove or modify bits and pieces and have a workable system. It is only irreducible if you take a gross view of removing whole slabs of functionality, which is a complete non-sequitur, because evolution does not work by adding functionality in large slabs. Evolution tends to tweak or tinker with a system.
The claim that this behaviour cannot evolve over time is false on the face of it. We see living creatures evolving over time just fine. We see slight graduations of behaviour in many related organisms, spanning the gammut of possibilities from single cells to obligate multi-cellularity, with no particular gap or hurdle that could pose the slightest problem for evolutionary change. The many degrees of behaviour in currently living cells shows right now, for those willing to actually look at biology in the real world, that there is ample opportunity for such behaviours to evolve.
There are even experiments in which the colonial habit is shown to vary with evolutionary change. I don't have a reference right handy, but I'll see if I can find one.
The claim that colonial behaviour and differentiation can't evolve is mere assertion, completely lacking in any sensible argument or evidence, and conflicting with every indication we have about how living things, and these critters in particular, work in the real world.
You're still talking about "learning". This is totally irrelevant. They don't learn. They merely change, from generation to generation, and the changes that work out persist.
You've been given the explanation. The way it happens is by evolution. The tendency to congregate under stress is inherent in the cells, and it can vary as the cellular chemistry and its mediating genetic basis varies. If the tendency to congregate tends to result in more of the organisms propagating to subsequent generations, than that tendency gets amplified, simply by selection.
I asked how cells stopped working for themselves and started working for the whole---you gave me examples---I'm asking now how the trait is passed on.
Traits are passed on because they are founded in a cells chemistry and genome, which is passed on. Traits of an organisms that are not passed on to the next generation cannot be used to identify a species; only an individual.
The colonial habit of slime moulds is passed on. That's an observation. The replication of cells is complicated, but quite well understood by now. So what is your problem? I don't get it.
Cheers -- Sylas
PS. LDSdude; check out Message 1. It can give some clues about quoting and formatting you may find handy.
This message has been edited by Sylas, 02-17-2005 21:20 AM

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Brad McFall
Member (Idle past 5033 days)
Posts: 3428
From: Ithaca,NY, USA
Joined: 12-20-2001


Message 28 of 39 (186414)
02-17-2005 9:40 PM
Reply to: Message 27 by Sylas
02-17-2005 9:17 PM


Re: okay, small little shift here then....
There is a slight problem at this level as it is not clear that one can ipso fact apply thoughts of animals to plants. A harvard neuroendocrinologist tranlated Dostal's work on plant phylogeny and ontogeny because he wanted to try to relate animal and plant ideas together chemically. There are others here who have a better handle on the current lit than me but I dont think Dostal's comments below dont apply("On Integraion in Plants Integraion at the Embryonal level)
"The embryo is the primary source of the future plant, for, aside from one or more envelopes, it contains the rudiments of the root and stem. The directing forces that caused the arragement of cells into these fundamental organs of the plant, the root and stem are not clear if only because it is impossible to carry out experiments inside the seed as it develops on the mother plant without endangering its whole development. Studies of the development of an embryo are therefore more or less restricted to descriptions fo the normal process of division of the fertilized egg cell, as shown in microscopic preperations. Such work has brought to light a great variety in the configuration (topography) of the particular process, but the deep causes of these variations remain concealed."
bookonsale
This message has been edited by Brad McFall, 02-17-2005 21:45 AM

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Wounded King
Member
Posts: 4149
From: Cincinnati, Ohio, USA
Joined: 04-09-2003


Message 29 of 39 (186445)
02-18-2005 2:25 AM
Reply to: Message 25 by LDSdude
02-17-2005 8:48 PM


Re: okay, small little shift here then....
In terms of the Dicty 'slug' or Grex, I notice that they also have an aggregated sexual pathway and I wondered if the asexual fruiting system was actually a derived characteristic, i.e. it arose subsequently to the sexual reproductive pathway. A quick literature search showed that this is not an original idea (what a surprise).
Developmental cheating and the evolutionary biology of Dictyostelium and Myxococcus.
Dao DN, Kessin RH, Ennis HL.
Microbiology. 2000 Jul;146 ( Pt 7):1505-12.
Not as irreducibly complex as you might imagine.
TTFN,
WK

This message is a reply to:
 Message 25 by LDSdude, posted 02-17-2005 8:48 PM LDSdude has not replied

  
Quetzal
Member (Idle past 5872 days)
Posts: 3228
Joined: 01-09-2002


Message 30 of 39 (186477)
02-18-2005 9:29 AM
Reply to: Message 23 by Wounded King
02-17-2005 7:26 PM


Re: Life cycle
Why don't they appear to be based on cellular differentiation? What do the gonozooids do? Does each seperate zooid type have its own reproductive subpopulation allowing a 'budding' of the colony by growing seperate zooids in situ. Have the gonozooids accumulated all of the genetic information neccessary to form the individual zooid systems and if so how does this differ from the sort of specialisation seen in volvox? As far as i can find out the gonophores release gametes into the water where they meet up and form planula larvae, do the various different zooids just then meet up in some sort of post-breeding assortment? As I understand it the larva produces the individuals for the colony by asexual reproduction.
Your understanding is correct. The zooids reproduce asexually once the initial form occurs - each after its "kind" (by budding, not division). I'm basing my stance on discussion by E.O. Wilson primarily, who classifies the zooids as "individuals" (see Wilson 2000, Sociobiology: 25th Anniversary Edition, ppg 383-396, but especially his discussion of siphonophores 383-84). I've dug around a bit, and can find no reference that suggests the zooids are derived cells. In point of fact, most of the articles I checked continue to call the various polyps that make up Physalia individuals, making the organism not just a colony, but a symbiont (see, for example, the UMich Animal Diversity entry on Physalia physalis and the UCMP Berkely Hydrozoan page are pretty typical.
quote:
In colonial hydroids, the individual polyps, or zooids, are differentiated for different functions: gastrozooids feed, dactylozoids capture prey, and gonozooids give rise to medusoids with gametes. Some colonial hydrozoans are so integrated that they behave like a single animal and are often mistaken for jellyfish. The "sailors-by-the-wind," or chondrophorines, are such colonial hydroids. Even more integrated are the siphonophores, which not only bear feeding and reproductive zooids but often nectophores, or pulsating swimming bells, and/or pneumatophores, or gas-filled floats. (from the latter)
I've been unable to find any peer-reviewed articles (or any other articles, for that matter) that support the idea that Physalia represents an aggregate of cells derived from a single organism. Perhaps you could indicate why you feel that the symbiotic nature of the siphonophores in general and Physalia in particular may be in error?
Interestingly, the argument over whether zooids are "individuals" or "organs" appears to have been bubbling along for over a century (I came across a reprint of a 1904 encyclopedia article with this discussion in it: Order VI.) However, I think you'll be hard-pressed to find any article that doesn't describe these organisms as symbiotic colonies of different polyps/medusae.

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 Message 23 by Wounded King, posted 02-17-2005 7:26 PM Wounded King has replied

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