Cells into Organs: could it evolve?

Jar's probably going to use my favorite example of this. I'll let him get first crack and then fill in the gaps.

Hokey dokey. I'll wait to see if LDS responds to your question. Otherwise not much point (hint: it's an algae).

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.

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.

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.

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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.

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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.

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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.

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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

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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.

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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?

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.

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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)

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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.

The Bluebottle or Portuguese Man-of-War describes it thusly:

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The Bluebottle or Portuguese Man-of-War is not a single animal but a colony of four kinds of highly modified individuals (polyps). The polyps are dependent on one another for survival.

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The float (pneumatophore) is a single individual and supports the rest of the colony. The tentacles (dactylozooids) are polyps concerned with the detection and capture of food and convey their prey to the digestive polyps (gastrozooids). Reproduction is carried out by the gonozooids, another type of polyp.

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Waikiki Aquarium information page describes the organism like this:

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Within the man-of-war colony, one individual is modified into a gas sac (pneumatophore) which supports the colony and keeps it floating at the ocean surface. A ridge along the top of the sac acts like a sail, and the movement of the colony depends upon the wind and ocean currents. It is the sail that gives the man-of-war its name, early explorers thought its shape resembled the helmets worn by Portuguese soldiers. Other members of the colony perform the tasks of food capture and feeding. The long, trailing tentacles (dactylozooids) are armed with stinging cells that contain nematocysts. Nematocysts inject a barbed thread and paralyzing toxin to capture and subdue prey like small fish. The tentacle transfers prey to the mouths of vase-shaped individuals (gastrozooids) that perform digestion. Nutrients from the meal are shared through a common gut system that connects all members of the colony. Communication between individuals is maintained through a network of nerve fibers. (MARINE LIFE PROFILE: INDO-PACIFIC PORTUGUESE MAN-OF-WAR)

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On life-cycle:

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Physalia- spend their entire life as a colony attached to a gas filled (mostly nitrogen) float-Pneumatophore; the
pneumatophore is not the same as a medusa, which is one individual
* have all three types of individuals: gastrozoids, gonozoids, and dactlyozoids
* the individuals are attached to tentacles trailing from pneumatophore; tentacles may be 9 meters long
* reproduces sexually and forms a modified planula larva- this develops into a medusa the comes to the surface and eventually develops into a colony through asexual reproduction (from zoology course outline: CNIDARIA: THE JELLYFISH, ANEMONES AND CORALS)

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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:

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Siphonophores challenge us to think about what we mean when we call something an individual, a concept that we usually think of as being quite straightforward. Is a single zooid or an entire colony the siphonophore individual? The answer is that you have to specify what features you are interested in before you can expect a meaningful answer. Do you mean ecologically? The entire colony functions as a single organism whether it is predator or prey. So the colony is an ecological individual. The same can be said for behavior. How about evolutionarily? There are two different components to this question. If we ask how evolution acts on siphonophores now, they are individuals. All the parts of the colony are genetically identical and the colony lives or dies as a whole (except for the eudoxids described later). So siphonophores are evolutionary individuals with respect to how natural selection shapes them today. The other way to look at evolutionary individuals is by descent. We can do this by taking a look at two animals and asking which structures descend from the same feature of a common ancestor. Just as this leads us to recognize that bat wings are modified arms, it shows that siphonophore zooids are polyps and medusae, structures that can be free living animals in other species. So this argument leads to the conclusion that the zooids of siphonophores are individuals. This is not contradictory to our previous conclusions, we are just looking at a different feature of individuality.

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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 lichen
family Pannariaceae (Lecanorales, Ascomycota)", Can. J. Bot. 80:625—634.

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The phylogeny of the family Pannariaceae (Lecanorales, lichenized Ascomycota) was investigated using ITS1—5.8S—ITS2 nuclear ribosomal DNA sequences representing 21 species. Phylogenetic estimations were performed using parsimony and a Bayesian Markov chain Monte Carlo (MCMC) tree sampling procedure. Several phylogenetic null hypotheses were tested, also using MCMC. The results indicate that Pannariaceae, as currently treated, is polyphyletic and that Degelia sect. Amphiloma, Fuscopannaria subg. Micropannaria, and Moelleropsis s.str. do not belong in the family. The inclusion of Parmeliella in the Pannariaceae could not be rejected, although it falls outside the family in the optimal trees. Psoroma, Santessoniella, Protopannaria, Fuscopannaria subg. Fuscopannaria, Moelleropsis s.str., and Pannaria unequivocally belong to the family. The Pannaria sphinctrina group belongs in Pannaria despite its green-algal photobiont. Protopannaria pezizoides is not, as sometimes treated, a Pannaria, although a relationship with Psoroma could not be ruled out. In the optimal trees, Moelleropsis s.str. is nested inside Fuscopannaria subg. Fuscopannaria and Santessoniella inside Psoroma, but null hypotheses of their independence from these genera could not be rejected. Pannaria cannot be divided into two monophyletic subgenera, Pannaria and Chryopannaria. The photobiont has changed twice or three times and the ascus apex and hymenial amyloidity twice within the Pannariaceae.

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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: 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.

You must have missed this bit from one of the quotes I provided. Here, let me repost it for you:

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(on the question of individuality) There are two different components to this question. If we ask how evolution acts on siphonophores now, they are individuals. All the parts of the colony are genetically identical and the colony lives or dies as a whole (except for the eudoxids described later). So siphonophores are evolutionary individuals with respect to how natural selection shapes them today. The other way to look at evolutionary individuals is by descent. We can do this by taking a look at two animals and asking which structures descend from the same feature of a common ancestor. Just as this leads us to recognize that bat wings are modified arms, it shows that siphonophore zooids are polyps and medusae, structures that can be free living animals in other species. So this argument leads to the conclusion that the zooids of siphonophores are individuals. This is not contradictory to our previous conclusions, we are just looking at a different feature of individuality.

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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.

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.