Chemicals in Evolution

A

Arkantos

Registered Senior Member
So there are some amoebas that can release a chemical that signals them to clump together after they have used up food, and then they can move to a spot and form spores to be blown away.

How could these type of chemicals signalling single-celled organisms to clump together have played a role in the development of multi-celluar life?
 
Yes indeed, this is an excellent question!

You are, of course, referring to one of the most amazing laboratory organisms – the cellular slime mold <I>Dictyostelium discoideum</I> (I presume).

Cellular slime molds have a remarkable life cycle. They exist as single celled amoeba, but when deprived of nutrients the cells begin to associate into an aggregate consisting of up to 100,000 cells. The multicellular aggregate secretes material that forms a sheath around the entire structure resembling a slug. The cells cooperate as though they were part of a single multicellular organism. The ‘slug’ migrates as a single ‘organism’. Eventually the slug contracts and the anterior tip begins to rise to form a fruiting body. As the fruiting body forms, the cells differentiate into a base, stalk, and apical mass containing encapsulated spores. The spores can tolerate a wider range of environmental conditions than the slug and germinate into new single cells following dispersal, thus renewing the cycle.

So Dictyostelium is an organism that is a weird combination of unicellular, pseudo-multicellular, plant and animal life form.

But there are, of course, many differences between genuine multicellular organisms and Dictyostelium. Dicty is only a "part-time multicellular organism" that does not form many cell types, and the more complex multicellular organisms do not form by the aggregation of formerly independent cells. Nevertheless, many of the principles of development demonstrated by this "simple" organism also appear in embryos of more complex animals. Many of the mechanisms used by individual cells within an animal embryo to sense a chemical gradient and migrate within the growing embryo are the same mechanisms which the individual Dicty amoeba use to sense other amoeba and migrate together to form a slug. There are strong molecular parallels between amoeba cell migration and morphogenesis during animal development. Moreover, there is also strong conservation in cell surface proteins that mediate cell cohesiveness in both Dicty slugs and embryos throughout the animal kingdom. Dicty is also a model organism that is very useful for studying differentiation-inducing molecules which (surprise surprise) are also found in metazoan organisms.
 
Actually these multicellular aggregations can even be found in bacteria.
Myxococcus for instance is famous for the formation of fruiting bodies (somewhat similar to fungi) in which spores are formed.
 
I am very ignorant in biology, but believe that there must be environmental chemical signaling agents outside of the cell (perhaps made by other near by cells) that help with cell differentiation.

I have a mental image, perhaps incorrect of an embryo developing a little "bud" growing away from the main body, which will soon be come an arm or a leg. I also tend to believe this bud must grow mainly form the "tip."

If, for example, the tip is making a forearm and later will make a hand, my question is how in the hell does it "know" to make a right hand (instead of a left hand) at the end of the right arm bud which is well away from the body and the "right tip" at least looks very much the "left tip" before the hand starts to develop.

There never seems to any failures in the symmetry system but there are "counting failure" in that you may get four or six fingers instead of five. Any ideas? Is this question one biologists have worried about?
 
there's probably a structure or common order to most animal genes that causes all physical structures to grow in symmetry. it really wouldn't be that complicated, comparitively.
 
Here’s another interesting Dicty tidbit I found:

In addition to this asexual cycle, there is a possibility for sex in Dictyostelium. Two myxamoebae can fuse to create a giant cell, which digests all the other cells of the aggregate. When it has eaten all its neighbors, it encysts itself in a thick wall and undergoes meiotic and mitotic divisions; eventually, new myxamoebae are liberated.
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Wow! :eek: So <I>Dictyostelium</I> cells are capable of differentiation and meiotic divisions. Perhaps this really was the route by which true multicellular organisms arose from unicellular eukaryotes.

Fascinating stuff! :)
 
spuriousmonkey said:
Failures in the symmetry system are common. The best known is the straightforward 'mirror image duplication'....
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Thanks for the reference. It was hard for me to understand, but I got the impression that some times a fly etc may develop extra wings. I.e. a "right one" and a "left one" on each side. That is not exactly my question, which can be boiled down to "Why is there always a right hand at the end of my right arm? (instead of sometimes a left one)"

Your reference, if I understood it, is more like what I called a "counting error" in my original post and yes that does happen inhumans too as I illustrated in that post. (I think one of the Chineses emporiers even had three eyes.)
 
Billy T said:
Why is there always a right hand at the end of my right arm? (instead of sometimes a left one)
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The answer is that the cells in the developing limb bud “know” their position. There are numerous genes whose expression is spatially restricted to specific parts of the limb bud. The differential gene expression means that cells know where they are with respect to the anterior-posterior and proximal-distal axes of the limb bud based on what genes are expressed there. Cells in the anterior part of the limb bud know that they are to form digit I (ie. the thumb or equivalent) whereas cells in the posterior part know that they are meant to form digit V.

For example, here is a diagram of the spatially restricted expression of a Hox gene in a mouse limb bud. Cells in the posterior-distal part of the limb bud see high expression, cells in the distal part see low levels and cells in the anterior part see no expression.

limb-bud-hox-expr.jpg


So, hopefully you can see that this system produces mirror image limb buds on each side of the embryo because the anterior-posterior axis of the embryo is fixed. Interestingly, there have been many very elegant experiments with chick embryos where cells from one part of a developing limb bud have been transplanted to other parts, and this results in ectopic digits and, as you eluded to, left hands on right limbs.
 
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As an aside, it is varying expression levels of the same set of genes that dictates the morphology of limbs in different species. Mammalian limbs are all formed from the same set of bones. All that differs is the relative length and strength of these bones.

fig15_7.jpg


I attended a fascinating seminar once where the speaker described an experiment that involved making genetically engineered mice that had the normal set of mouse genes involved in limb formation but under control of gene regulatory elements from a bat. This altered the timing and levels of gene expression in the developing limb buds. The mice developed limbs with all the normal complement of bones, but the bones were all longer than normal, reminiscent of the elongated bones of bat wings!
 
Thanks Hercules (your strong man name here is well chosen)

I understand the right hand on right arm puzzle well enough now to stop wondering about it. Perhaps you can also help with the other bio-problem that has also eluded me for more than 40 years:

After I began to understand that there is a biological cost to every talent a cell masters, I began to wonder: Why are peas green? Or at least: Why is the interior of a pea green? No light reaches the interior and the pods are only slightly translucent. I wonder about this even more now that you solved the right-on-right mystery by showing how easy relative global location can be encoded.

I had guessed that the biological cost of telling an interior pea cell: "You are deep inside the pea, don't bother to be green." would be too high - more than the saving to be achieved by avoiding needlessly making chlorophyll deep inside the pea, but you have weaken this rationalization/ guess by educating me about genetic location encoding.

All this got started years ago when I was reading that the reason why the guinea pig is so good a model for humans in some cases is that it, like humans, has lost the ability to make vitamin C. That is, the early high fruit and vegetable diet of both provided enough vitamin C so the capacity to make vitamin C was no longer worth the biological cost and were eliminated by Mr. Darwin. Etc. Seemed possible to me (What do you think?) but if that is true, why are pea interiors still green?

If you can, lay this old puzzle, which you have made worse, to rest for me also. I would appreciate it. Thanks, again for at least killing one.
 
What color would you expect them to be?

There are plenty of fruits and vegetables where the eatable parts of the plant serve no part in photosynthesis. They DO ensure that their seeds will be eaten, and dispersed.

Maybe your looking at this the wrong way? Maybe I am.
 
I know it's off topic but
Billy T said:
why are pea interiors still green?
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I don't think it takes much thought to explain this one, seriously 40 years? :confused:

I would guess that it is because we have selected them that way over centuries of cultivation.
Take the well known work of Gregor Mendel, did he not work with green, yellow, wrinkled and smooth seeded varieties?
How about the commonly grown sweet pea, it's seeds are brown, not green, as it has been selected for flower colour not edible seed preference.
Any gardener knows that most seeds are not viable until they change colour, usually from green to brown or black and are usualy pale yellow/white in the interior. Most pea seeds for planting are hard and pale in colour through to the middle, not green.
I would suggest that what you think of as green seeds are just picked immature and selected for their appeal to us to be edible. I can't find a reference, but I would bet that the interiors of wild peas are not green, at least when they are mature.
Have you ever held a peapod up to the light, you can usually see some light coming through, so I would also suggest that it is beneficial for maturing seeds, in a translucent pod, to be green so that as they develop they can photosynthesise to aid in their rapid development.
But in short, they are tastier that way. :)
 
BSFilter said:
What color would you expect them to be?...
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Whitish, brownish or gray, like beans. To make any well defined color, red, green, blue etc. you need* a very wavelength selective (read that as complex with narrowly defined reflection or absorption band. Green in this case as red of sunlight is strongly absorbed**) molecule. That ain't cheap to make. Why do it where there is no light?

I understand why flowers spend this energy/effort - they have co-evolved with the bees etc to the mutual benefit of both, but they are not so silly as peas. I.e. The interior of a rose stem is not red. etc.
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*Blue sky is an exception, but I understand that well. (Scattering law's lambda to the fourth power, molecular density statistics, etc.)

**Note I mentioned that the pod is translucent in first post, but will say a little more now: The chlorophyll is interested in getting the dominate energy from the sun (red, yellow region.) When you look thru a green pod, essentially none of this part of the spectrum is coming thru. - Thus there is very little left even for the outside of the pea's chlorophyll to use. Effectively the pod is opaque to the light the green pea could use. - This should make the mystery more clear. Some here have been confused by fact some green light comes thru the pod.
 
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Billy T said:
Why is the interior of a pea green?
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To be honest, I don’t think I can answer this question to your satisfaction. :( I cannot come up with much of an explanation or add much to what has already been said above. A lot of different fruits and vegetables have coloured inner flesh of various hues, which is the result of pigment production. I don’t imagine that the metabolic cost of producing one type of coloured pigment is any greater than other types of coloured pigments. Peas and beans produce a green pigment inside and out which just happens to be the same pigment used for photosynthesis. Whether this is functionally significant for the peas or merely a coincidence, I do not know.
 
Hercules Rockefeller said:
To be honest, I don’t think I can answer this question to your satisfaction. :( I cannot come up with much of an explanation or add much to what has already been said above. A lot of different fruits and vegetables have coloured inner flesh of various hues, which is the result of pigment production. I don’t imagine that the metabolic cost of producing one type of coloured pigment is any greater than other types of coloured pigments. Peas and beans produce a green pigment inside and out which just happens to be the same pigment used for photosynthesis. Whether this is functionally significant for the peas or merely a coincidence, I do not know.
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Thanks anyway. Let me make a stab at answer, based on what you taught me about left hand not being on right arm mechanism, etc.

Peas (and many fruits) are basicly spheres,or at least rotationally symmetric (pair, etc). They do not have the anterior, posterior axis, only the distal / proximal one, related to where the stem is. What do you think about that? (I do note that the non-spherical beans are not producing colors insides and this lends some support to this idea.) I.e. to keep the skin colored at 1cm from the stem on the Prx/dist axis they must color every thing at 1cm from the stem. This seems a bit of a streach to me as I would think some level of something (oxygen perhaps) varies with depth from the outside and could be used to say to an individual cell: "You are inside. Do not bother to activate pigment production."

You said:
"I don’t imagine that the metabolic cost of producing one type of coloured pigment is any greater than other types of coloured pigments."

Possibly true, at least to first order, but why produce any deep inside? (Color ain't cheap. Look at the price of B/W vs. color TVs! :D )

I would hate to think that plants are less lazy than me, doing hard, complex things they have no use for, etc. but perhaps they are more lazy as they do not walk, run around, etc. ;)

BTW I feel a little better about peas being green and probably will forget the problem in a few years. Hope you are not cursed with it for 40* as I was, if you shoot down my "they are spheres and lack well defined second axis" answer.
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*I have forgotten 100s of more important problems, but when the frozen peas are so dam green they torment me at least 10% of the time. Roughly 5 times a year, times 40 years - hell that 200 times! DAM PEAS ANYWAY. If you also have this problem henceforth, I am sorry. Try canned peas - I find that helps a lot.
 
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