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Author | Topic: Cells into Organs: could it evolve? | |||||||||||||||||||||||||||
Quetzal Member (Idle past 5902 days) Posts: 3228 Joined: |
Jar's probably going to use my favorite example of this. I'll let him get first crack and then fill in the gaps.
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Quetzal Member (Idle past 5902 days) Posts: 3228 Joined: |
Hokey dokey. I'll wait to see if LDS responds to your question. Otherwise not much point (hint: it's an algae).
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Quetzal Member (Idle past 5902 days) Posts: 3228 Joined: |
I think I understand your questions, especially in light of your response to Sylas. I've got a few examples, mostly for your information. None of them resemble a "heart lying on a sidewalk", and not all of them are "irreducibly complex". However, each of them is suggestive in one respect or another.
The first example I'd like to give you is Volvox. This is an autotrophic protist - an algae. It's very common in the East and Northeastern US (I don't know whether they have them out in Utah, however). Each cell has two flagellum, and the usual protist accoutrements (chloroplast, eye spot, etc). It is motile, and its eyespot is a simple light sensitive pigment that allows the protist to use its flagella to move toward light. During the spring, these independent organisms (quite capable of independent existence) form colonies in ponds of between 500-1000+ cells, some of which are large enough to be seen with the naked eye. The cells link together with cytoplasmic threads, and then secrete a mucus-like jelly that serves as a colony boundary. So far nothing special, right? What happens next is truly amazing (to me). The colony develops a distinct "front" and "rear" end as a colony. The eyespots at one "end" of the colony grow substantially larger than those at the "rear" or "interior" - although all cells retain the basic parts. The flagella of these "eye" cells stop beating. The cells at the "rear" of the colony increase the frequency of their flagella. Somehow, and no one knows for sure exactly how this is accomplished, the entire colony now functions as a single organism. The "front", with its enlarged eyespots draws the colony toward light, and the flagella at the "rear" push the colony forward. IOW, we have a very primitive version of the cellular differentiation we see in "higher" metazoans. Note, in addition, that during its colonial phase, a very small fraction of the cells become gonidia - reproductive cells very similar to metazoan germline cells. The rest remain same-old motile Volvox. My second example is the Portuguese man-of-war (Physalia physalis). Although commonly called a "jellyfish", it isn't a cnidarian. It's a colony composed of four different types of organism: a pneumatophore (single individual which serves as a floatation device and supports the rest of the colony); numerous dactylozooids which detect and capture food and convey their prey (the tentacles) to the digestive gastrozooids (basically living mouths); finally reproduction is carried out by gonozooids. The colony is irreducible - none of the organisms can live without the others. Each member of the colony performs a unique function that aids the whole organism. 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. My final example is another odd-ball. Mixotricha paradoxa is quite possibly the poster-child of endosymbiosis. It is a composite organism that outwardly resembles a protist. It lives inside the gut of a termite (Mastotermes spp.), and helps the termite digest cellulose. However, M. paradoxa is actually four organisms. It has a nucleated cell, but in place of mitochondria, it contains spherical bacteria. Instead of cilia and flagella, it contains two different types of spirochete for motility. Although I don't know whether each of the subordinate organisms can live without the others, it's certain the Mixotricha can't live without its symbionts - each of which perform a different function for the whole. To sum up: Volvox shows primitive differentiation at the cellular level in a single organism. Physalia and Mixotricha show irreducible symbiosis between different organisms functioning as a single organism. These examples are suggestive, rather than absolute, answers to your question.
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Quetzal Member (Idle past 5902 days) Posts: 3228 Joined: |
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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Quetzal Member (Idle past 5902 days) Posts: 3228 Joined: |
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: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: 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 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?
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Quetzal Member (Idle past 5902 days) Posts: 3228 Joined: |
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: 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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Quetzal Member (Idle past 5902 days) Posts: 3228 Joined: |
The Bluebottle or Portuguese Man-of-War describes it thusly:
quote: Waikiki Aquarium information page describes the organism like this:
quote: On life-cycle:
quote: I will say that I misspoke slightly: there are only one or two spots that appear to be "zooid producers", and produce all the different types by budding based on some trigger. This goes against what I said about the zooids each budding their own type. For a bit more on siphonophore organization and lifecycle, see Siphonophores. The article as this to say about your "individuality" question: quote: As to how we can develop a phylogeny from a colony/symbiont: how do we develop a phylogeny from any other symbiotic organism? For example, the phylogeny of lichens (which even you'll have to admit are symbionts, yes?), is quite doable. See, for instance: Ekman and Jrgensen PM, 2002, "Towards a molecular phylogeny for the lichenfamily Pannariaceae (Lecanorales, Ascomycota)", Can. J. Bot. 80:625—634. quote: Developing a phylogeny is apparently a bit more complicated that a straight molecular phylogeny of more complex metazoans. This doesn't surprise me unduly. In the lichen case, they investigated one of the two organisms making up the symbiont, apparently, and derived their phylogeny from that. In the case of Physalia, or any of the other hydroids, it would probably be easier - the zooids are genetically identical except (apparently) developmentally. However, this does not indicate that they were differentiated from a single organism - rather the concensus appears to be as I stated. PS: I'm afraid I don't have access to the 25th aniversary edition of 'Sociobiology' but I might go to the library to get out the 1980 edition. I have to say that 1980 isn't really a very current reference, although that isn't neccessarily a point against it. If there truly were distinct evolutionary origins of the Zooids I'm surprised there are no molecular papers discussing their differing genomes. The page numbering may be different in another edition. It is, however, Chapter 19: The Colonial Organisms and Invertebrates. Is there some point to your argument, or are you simply interested in a parallel to your discussion of the Chlorella evidence? I mean, dueling references is all fun and everything, but it would seem to obscure the basic point I'm trying to get across: there are examples in nature that contradict/answer LDS's question. You know, it might be an interesting exercise for you to find your own examples for a change rather than simply nit-picking everyone elses. I'm happy to continue this discussion with you, however. I stand by what I have posted, and nothing you've presented thus far substantively contradicts what I've written.
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Quetzal Member (Idle past 5902 days) Posts: 3228 Joined: |
You must have missed this bit from one of the quotes I provided. Here, let me repost it for you:
quote: I'm not making this shit up, as you seem to think. There is a gradation in hydroids from single medusae/polyp to sessile colonies composed of different polyps to free-floating colonies like Physalia. All the references I have posted have referred to the component zooids as individuals. And most of them have referred to the various different zooids as different kinds of organism. This is where I get my idea. I never claimed I couldn't be wrong. Although admittedly I'd be surprised - this is the way I've been considering this organism from the first time I was introduced to it. IF, in fact, as you state the different zooids are simply different developmental forms of a single organism that are somehow differentiated in form in the colonial version, then don't we have a much more complicated affair to explain? How did this differentiation occur? Why do these forms resemble other, free-floating/swimming versions of the hydrozoa? Why do some hydrozoa have different zooids (for example, the absence of the float)? If we're not dealing with a symbiont, then we have some very weird developmental/organizational issues to address.
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Quetzal Member (Idle past 5902 days) Posts: 3228 Joined: |
Let me back up a minute. I had something of a rude shock last night. Your comment about evolutionary lineages triggered it. For over 30 years I have been thinking of, and referring to, this organism as a symbiont. The shock came when I realized that I had never considered the implications of this categorization. OF COURSE it would have to have different evolutionary lineages if it was a symbiont. All the other symbionts I know do. However, for a bit, there I stuck: I couldn't think of any other organism which showed such extreme morphological and functional differentiation that WASN'T a symbiont, not to mention the free-living polyps, etc. So I rolled over and said "okay, no problem. Obviously there's another explanation." Then, of course, my (treasonous) brain obligingly pulled up an example. Now don't laugh: the example bears no relation to the siphonphores. However, it does have a functional analog: it is a non-symbiotic colonial organism with functional and morphological differentiation: the Hymenoptera. The order even has solitary individuals. With drones, queens and workers, the organization was similar enough to make things click: obviously these are not distinct organisms (although individual agents), and they show a degree of differentiation that in the siphonophores is taken to the extreme. I've been filtering all the data through the "symbiont" lens (hence all my confusion about "individual" vice "distinct organism"). Don't bother looking up the Wilson reference: he never said symbiont. I "translated" that chapter just like all the other references.
So you were right, and I've been wrong for 30+ years. I suppose I should thank you for correcting my long-standing misapprehension. However, I'm absolutely not feeling remotely gracious about this. So, credibility shattered, I'll simply retire leaving you in sole posession of the field.
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