Is sexual reproduction an example of convergent evolution?

[Nereid]

Second thread on the evolution of like, in the Archaen and Proterozoic eons.

There are many kinds of eyes, and many have arisen quite independently; the classic example of convergent evolution.

Did sexual reproduction arise more than once, independently? Is it too an example of convergent evolution? If so, what are the details?

Specifically, did sexual reproduction arise independently (if at all) in each of, or some proper subset of, the six eukaryote supergroups?

I'm not really interested in cases where sexual reproduction was lost, then re-emerged; I'm only interested in independent 'firsts'.

 

[Nereid]

Bump.

347 reads, zero replies!

Anyone?

 

[stevenb]

Bump.

347 reads, zero replies!

Anyone?

Good question, but 348 and counting don't know the answer.

It's just a matter of time. Someone must know.

 

[nismaratwork]

I have no idea, and I'm not sure that there is a verifiable answer to give, only conjecture. You're talking about something that arose a VERY long time ago, and I don't think molecular biology or fossil records have been able to pinpoint the answer. Personally, given how advantageous sexual reproduction can be, I would be surprised if it didn't arise independently, but I can't back that with anything solid.

 

[epenguin]

I have no idea, and I'm not sure that there is a verifiable answer to give, only conjecture. You're talking about something that arose a VERY long time ago, and I don't think molecular biology or fossil records have been able to pinpoint the answer. Personally, given how advantageous sexual reproduction can be, I would be surprised if it didn't arise independently, but I can't back that with anything solid.

Personally, given how advantageous sexual reproduction can be, I would be surprised if it did arise independently, but I can't back that with anything solid.

 

[Ken Natton]

As it so happens this is a subject I have visited previously on another forum. And I have something very solid to offer in the form of a book to read. It is called ‘The Origins of Life, From the Birth of Life to the Origins of Language’ by John Maynard Smith and Eors Szathmary. It contains a chapter titled ‘The Origin of Sex’ that would answer your question. Be warned it is not an easy read. It is intended as a popular science book, not a science textbook, but as popular science books go, it is quite challenging.

For all those that have struggled to understand how male and female versions of a species could have evolved, and those who have seized on its impossibility as evidence of another flaw in the Theory of Evolution, the answer, when you get it, is one of those head smacking ‘of course!’ moments. The point is, of course, the move from parthenogenesis to sexual reproduction didn’t happen at the level of higher level, complex organisms, it happened at the level of cell cloning changing into diploid cells that divided into haploid gametes that recombined into diploid zygotes. The question then of how male and female forms of any species evolved is no more complex than the question of how the species itself evolved.

As a point of interest, there is known to be a species of lizard that has reverted to parthenogenesis. But having done so, it cannot revert to sexual reproduction, because, for the very reason that everyone struggles to figure out how sexual reproduction evolved, it really can’t happen at the level of the higher level, complex organism.

In answer to your original question, I suppose it is possible that sexual reproduction evolved more than once independently. But personally, my guess would be that the overwhelming majority of modern, sexually reproducing species have a common ancestor, that was an extremely simple life form, and that it was that common ancestor that evolved sexual reproduction.

 

[Jon Richfield]

As it so happens this is a subject I have visited previously on another forum. And I have something very solid to offer in the form of a book to read. It is called ‘The Origins of Life, From the Birth of Life to the Origins of Language’ by John Maynard Smith and Eors Szathmary. It contains a chapter titled ‘The Origin of Sex’ that would answer your question. Be warned it is not an easy read. It is intended as a popular science book, not a science textbook, but as popular science books go, it is quite challenging.

Ken, I have made a move towards answering that question in another thread, one I think that you initiated, though not in direct response to you. For starters, JMS was one of my favourite and formative writers on evolution-related material, and together with ES,they produced some really terrific material in both the book you mention and its predecessor "The Major Transitions in Evolution".

For the rest, IMO (but as always in science, you takes your pick) sex began with recombination as opposed to division, and happened because of the nature of the molecular-biological processes of mitosis, in whatever its original forms took. You could say that early forms of mitosis pre-adapted organisms for recombination and eventually meiosis.

So much for sex.

Now, as for gender: this (Did I say IMO? Good!) amounts to splitting the recombinational roles asymmetrically. Big gametes being "female". Such asymmetry occurs in microbes at least as far down as some fungi and algae. It clearly occurred many times over in many barely-related ranches of living things. If you call this "convergent" or "parallel", suit yourself.

Note that this takes the history back to waaaaayyyyy before there were any chordates, let alone mammals, in sight.

Similarly, suit yourself about how much of my line you swallow. I wasn't there to see it happen. :-)

Cheers,

Jon

 

[Ken Natton]

Hi Jon,

Thanks for engaging. No, I only found these forums very recently. I made my first post anywhere on this website as recently as July 7th, and the two posts I made, one last night on this thread and then again earlier this morning on the other thread, are the first two that I’ve made on this biology forum. So no, I haven’t engaged with you before.

One of the recurring themes in discussions I have read between serious professional biologists on different forums has been not just a great professional respect, but also a deep personal liking for John Maynard Smith. I recall two such biologists who actually knew him agreeing with each other that he was a ‘dude’. He is another one that can be seen being interviewed, and talking fascinatingly about his work, on the Web of Stories website.

One of the most surprising points that Maynard Smith and Szathmary discuss in that chapter on the origin of sex was how it ever evolved in the first place. As nismaratwork and epenguin suggest above, my understanding had been that it was a done deal that sexual reproduction was better. But Maynard Smith and Szathmary point out that many of the supposed advantages of sex operate at the group level, rather than at the level of the individual – a gene pool can only be advantageous to a population, it can’t offer any particular advantage to an individual – and, of course, it is a subject of some controversy whether or not group selection is real. But the key thing is, whatever the truth about its advantages to a population leading to the widespread existence of sexual reproduction among species, for it ever to have evolved in the first place, it must have offered some selective advantage to the individual. And Maynard Smith and Szathmary demonstrate that it is very difficult to come up with a water-tight Darwinian explanation for why and how it did.

 

[nismaratwork]

Ken, I have made a move towards answering that question in another thread, one I think that you initiated, though not in direct response to you. For starters, JMS was one of my favourite and formative writers on evolution-related material, and together with ES,they produced some really terrific material in both the book you mention and its predecessor "The Major Transitions in Evolution".

For the rest, IMO (but as always in science, you takes your pick) sex began with recombination as opposed to division, and happened because of the nature of the molecular-biological processes of mitosis, in whatever its original forms took. You could say that early forms of mitosis pre-adapted organisms for recombination and eventually meiosis.

So much for sex.

Now, as for gender: this (Did I say IMO? Good!) amounts to splitting the recombinational roles asymmetrically. Big gametes being "female". Such asymmetry occurs in microbes at least as far down as some fungi and algae. It clearly occurred many times over in many barely-related ranches of living things. If you call this "convergent" or "parallel", suit yourself.

Note that this takes the history back to waaaaayyyyy before there were any chordates, let alone mammals, in sight.

Similarly, suit yourself about how much of my line you swallow. I wasn't there to see it happen. :-)

Cheers,

Jon

In other words, as I said there has been no way to pinpoint this with molecular biology (and the obvious lack of eyewitnesses *joke*), so... your opinion as you say. This still doesn't go very far towards answering whether or not this could have arisen independently, or not. Opinions with little hope of confirmation are not worth much in event his softest of the sciences.

 

[Jon Richfield]

One of the recurring themes in discussions I have read between serious professional biologists on different forums has been not just a great professional respect, but also a deep personal liking for John Maynard Smith.

So it is not only me, huh?
Something about his writing, not only informal but formal, suggests an unassuming, thoughtful courtesy that I find irresistible. It also is a component of the clarity of his work.

One of the most surprising points that Maynard Smith and Szathmary discuss in that chapter on the origin of sex was how it ever evolved in the first place. As nismaratwork and epenguin suggest above, my understanding had been that it was a done deal that sexual reproduction was better. But Maynard Smith and Szathmary point out that many of the supposed advantages of sex operate at the group level, rather than at the level of the individual – a gene pool can only be advantageous to a population, it can’t offer any particular advantage to an individual – and, of course, it is a subject of some controversy whether or not group selection is real. But the key thing is, whatever the truth about its advantages to a population leading to the widespread existence of sexual reproduction among species, for it ever to have evolved in the first place, it must have offered some selective advantage to the individual. And Maynard Smith and Szathmary demonstrate that it is very difficult to come up with a water-tight Darwinian explanation for why and how it did.

I cannot remember any details, but I have an impression that they were not the first to come up with this difficulty, though I also have an impression that they have been the most prominent exponents of the concept. It is something of an education to read their work and note how non-confrontational and their approach is. This is not to say that it is not formidable, but its power lies in the force of logic, not of polemics.

The essence of the issue of course is the more general concept of the meaningfulness of group selection as a Darwinian process, as compared to individual selection or, more strictly speaking, gene selection. Now this is an extremely controversy-fraught field, and if you want my view of it (not everyone does, but fortunately it is optional for those who prefer to ignore it or go into conniptions) the reason is failure to recognise and sort out certain pivotal concepts.

The best-publicised, and for some decades, the most popular (and simultaneously, most bitterly-reviled) view of this complex of issues was Richard Dawkins' gene-selection theory. Now, but Dawkins is arguably the best science writer since Peter Medawar, and he is a formidable thinker as well. His argument has a lot of merit, and he has honed and adjusted it in rational reaction to criticism. On the other side, group selection was not very competently presented, and not very cogently thought out. It did not fare well. For a considerable period it was reduced to pockets of resistance among the incompetent and inarticulate.

I exaggerate of course, but certainly the pendulum swung so far that it became quite difficult to find anything like balanced discussions of the theme. That was the good news.

The bad news was, as I saw it, was a confusion of concepts, but it is not my field and in any case I still do not have it properly thought out. It is a difficult conceptual field.
As I see it the key conceptual stumbling-block is in our difficulties in dealing with the concept of entity. That has been a pervasive, largely unrecognised and implicit trap for the unwary in every field of science that I can think of. In spite of the attitudes of exponents of fuzzy logic, that useful and ingenious discipline does not in itself solve the problem, though it certainly has scope for wider application and, I suspect, deeper and wider development. In Darwinism, ecology and related fields things are nearly as bad as they could get. The very concept of natural selection assumes a cogent concept of entity, and no such concept has been established. In fact, I have a nasty suspicion that part of the problem is that no one concept is sufficient for the requirements of this field, or, for that matter, many other fields. And what is more, I suspect that different mental toolkits would be necessary to deal with the concept of entity in say, Darwinism and quantum mechanics.

Part of Dawkins' argument that the gene was the only, or nearly the only, or at least far and away the most important, entity in Darwinism, he modified by an expanded view of the definition of a gene. He did so well and characteristically articulately, but I still think that he fell short.

Why? Well, to deal with that I think we need to stop and ask ourselves some things about the concept of entity and of what would comprise an entity in biology.

Let's break there for now. Think over it while I ponder on the next sermon. Remember, I have not matured even my own ideas on the subject, and may never do so. I have started a book on it but have not had time even to develop it properly, let alone half finish it.

Cheers for now,

Jon

 

[Jon Richfield]

In other words, as I said there has been no way to pinpoint this with molecular biology (and the obvious lack of eyewitnesses *joke*), so... your opinion as you say. This still doesn't go very far towards answering whether or not this could have arisen independently, or not. Opinions with little hope of confirmation are not worth much in event his softest of the sciences.

NAW, you have an interesting line in choice of words and the concepts you allocate to them. Pinpointing? In this field what would that mean? The nearest half billion years or so? Don't you think the relative sequence of events would be most of what we could expect, and most of what we would need? A matter of opinion, you reckon? And opinions are interchangeable, I suppose? And what do you suppose the role and form of confirmation of opinion might be in scientific matters, beyond a defensive salve for the feelings of the thin-skinned?

And "softest of the sciences"? Which would that be? Would you feel comfortable defining "softness" in science, Professor Rutherford? Do enlighten us please.

Having done so, kindly favour us with your encore, and explain what the softness of the science entails for exponents. Does it mean that you can say whatever you please, or make meaningless statements, each protagonist safely cocooned in his own bed of softness? If you can do so, you will revolutionise science.

Turning to pleasanter, if more trivial, matters, such as biology and Darwinism, you no doubt will accept it as generally recognised biological fact, and not specifically my isolated opinion (as if that had anything to do with scientific cogency) that, if not sex, then at least gender has arisen in at least three kingdoms (Fungi, Plantae, and Animalia) and you might wish to add more. Sex, in the sense of genetic exchange and recombination between individuals, probably is much older, so much so, that we probably would need to develop molecular-biological arguments to pursue the topic of its origin. No matter; you yourself seem to be insisting on gender rather than sexual reproduction as the topic at issue, so that should suit both of us.
Well, then if one thing is clearer than another, it is that none of those three kingdoms seems to have derived its various genders from the others.

Right?

Really right??

Then we have a possible multiple origin for sex, and possibly not, but a dead cert for multiple origin for gender in various forms.
If you can bounce that one, drop science and go into politics as a spin doctor; clients would KILL for your services!

Cheers,

Jon

 

[Ken Natton]

Hi again Jon,

It might seem an obvious point to make, but it has always been at the centre of my contention that the function and purpose of a forum is very different than the function and purpose of a book. Judicious use of both is a great strategy to enhance understanding. My point is, neither you nor I are pretending that we are writing a science textbook here. This is a forum. I don’t see the need to keep reinforcing the point that what we are saying is only our own opinion or our own perspective. For me, perspectives are what a forum is all about. Of course, it does help if your contribution has some basis in genuine knowledge.

A couple of thoughts in response to your last post.

It is something of an education to read their work and note how non-confrontational and their approach is. This is not to say that it is not formidable, but its power lies in the force of logic, not of polemics.

I came to the Maynard Smith and Szathmary book straight off a reading of Dawkins’ The Blind Watchmaker. Believe me, I share your respect for Dawkins but it is clear that he is the arch polemicist. The Blind Watchmaker’s very raison d’etre is to highlight the fallacies of the anti-evolutionist’s arguments. So then, as you might imagine, the contrast in approach with Maynard Smith and Szathmary was very striking. The latter writers just take it as read that their readership consists of rational thinkers. If you don’t agree with the viewpoint behind anything they say, you are reading the wrong book.


Dawkins’ gene centred view always had something of the ring of Douglas Adams about it to me. You know, planet Earth is just an experiment set up by the mice experimenting on the humans. A viewpoint that seems to perceive the function of higher life forms as to serve the interests if their genes does seem to be an odd way of looking at things. But yes, if there is one (philosophical) thing that quantum physics has taught us, it is that sometimes the scientific explanation does not accord with intuitive, rational thought processes!

 

[Jon Richfield]

...This is a forum. I don’t see the need to keep reinforcing the point that what we are saying is only our own opinion or our own perspective. For me, perspectives are what a forum is all about. Of course, it does help if your contribution has some basis in genuine knowledge.

Yes Ken. I agree and I probably would not have wasted our time on such obviosities if I had not been side-tracked by other parties with more demanding views.

I came to the Maynard Smith and Szathmary book straight off a reading of Dawkins’ The Blind Watchmaker. Believe me, I share your respect for Dawkins but it is clear that he is the arch polemicist. The Blind Watchmaker’s very raison d’etre is to highlight the fallacies of the anti-evolutionist’s arguments.

Mmmm... I suppose you have a point, but it never struck me as strongly. Possibly this fact simply reflects the ubiquity of non-scientific anti-Darwinistic sniping, that, sadly, has succeeded to the extent of more or less orienting much biological thinking into either an obsessively confrontational or excessively defensive attitude.

For my part I read Dawkins largely for the substance and the sheer quality of his writing. (It is not of course uniform; "The Greatest Show on Earth" is far better than most popular books on evolution, but for a Dawkins fan it was a disappointment. One does not read the works of a master for the sake of mediocrity.) Anyway, perhaps I was generally too much absorbed in the substance to notice polemicism. No bad thing sometimes.

Dawkins’ gene centred view always had something of the ring of Douglas Adams about it to me. You know, planet Earth is just an experiment set up by the mice experimenting on the humans. A viewpoint that seems to perceive the function of higher life forms as to serve the interests if their genes does seem to be an odd way of looking at things. But yes, if there is one (philosophical) thing that quantum physics has taught us, it is that sometimes the scientific explanation does not accord with intuitive, rational thought processes!

Hum. Here I cease to resonate. Did you perchance read his first edition or a later one, of "The Selfish Gene"? If only the first, I could understand what you say, though the book never struck me that way at all. If the second or third, then I simply am puzzled. As a point of view, what you say strikes me as personifying the genes, almost anthropomorphically, in a way that I never recognised in the books. Am I misunderstanding something, or missing your point?

By way of (strained) analogy, would you attribute the behaviour of a computer game or word processor to the character of the central processor of the computer? Or perhaps more usefully, to an adder in the arithmetic unit or a pixel on the screen? Such items do not act in any way explicitly isomorphic to the objectives or strategies of the whole. Genes do not plan for the behaviour of brains or muscles.

I don't feel that I am being very clear here; do you feel that we might be at cross purposes?

Here I feel that entity thing rising again. To speak of either an individual, or a population, or a genetic line as being something a gene planned, or partly "intended" to "control" doesn't seem to me to make sense. A cell in a cell in a mammal (a mitochondrion if you like) or the enclosing cell in an organ (together with some billions or trillions of essentially identical cells), or the organ in a body, or the body in a community, or the community in a population, or the population in a species occupying perhaps several regions for a couple of millions of years -- each of those is an entity. Right? Which of those entities does the gene control, and how? There is no doubt that just one allele could in principle could be enough to make all the difference between the entity coming to be or surviving, versus never emerging, or dying out. Is that in any way like experimenting after the fashion of Adams' mice?

Could you please comment or clarify? I don't feel that I am getting any closer.

Cheers,

Jon

 

[Nereid]

Thanks everyone for your posts.

Perhaps I can clarify my question a bit.

While the phylogenetic tree of the eukaryotes is, today, rather uncertain, I think some simple questions can be asked, wrt sexual reproduction.

For example, whether there are six super-groups, or more, questions of the following kind can be asked:

Are there any examples of sexual reproduction within the Amoebozoa, the Rhizaria, the Excavata, or the Chromalveolata?

There are certainly examples of sexual reproduction within the Opisthokonta and the Archaeplastida!

Yes, the period in Earth's history my questions refer to is waaaay before the Cambrian; it may go back to the first appearance of eukaryotes, or shortly afterwards.

 

[Jon Richfield]

Thanks everyone for your posts.

Perhaps I can clarify my question a bit.

While the phylogenetic tree of the eukaryotes is, today, rather uncertain, I think some simple questions can be asked, wrt sexual reproduction.

For example, whether there are six super-groups, or more, questions of the following kind can be asked:

Are there any examples of sexual reproduction within the Amoebozoa, the Rhizaria, the Excavata, or the Chromalveolata?

There are certainly examples of sexual reproduction within the Opisthokonta and the Archaeplastida!

Yes, the period in Earth's history my questions refer to is waaaay before the Cambrian; it may go back to the first appearance of eukaryotes, or shortly afterwards.

Nereid, I cannot answer you in respect of the groups you mention; sorry! I am under the impression that most of most of them have nothing like any sort of gender, even though they do have what I have been calling "sex" in the sense of exchange and recombination of genetic material.

However, as I read your question and its intention, I am not sure why you are in doubt. We are pretty sure that genders in plants have no common ancestor with genders in animals, for just one example. It seems pretty clear to me that gender in that sense has arisen repeatedly, with major variations in the details within the same theme.
That theme seems to me to be:
1: (Presumably usually) symmetrical genetic combination, possibly in a special life-cycle stage, followed by splitting/recombination in the F1 cells.
2: Development of (presumably usually) symmetrical production of (presumably usually) motile sexual cells from a (presumably usually) long-lived parent. This amounts to the production of what we recognise as gametes.
3: Development of structures that permit some (typically non-motile) gametes to remain with the (female equivalent of possibly only part of one parent's) structure, permitting such gametes to grow larger or to be protected or otherwise advantaged. Call such gametes ova for the purposes of this note. To match the captive, relatively massive ova, development of far smaller, more motile (or passively borne) cheaper, more plentiful, gametes (hereinafter referred to as sperms). This amounts to what I have been calling gender, as opposed to "mere" sex.

Developments along those lines seem to have happened repeatedly and in several kingdoms (whatever current flavour of kingdoms seems to be favoured for the present!)

Note that no such stage has unique and universal advantages; living examples of all such stages occur in many taxa. However, one can identify certain contingent advantages of the more advanced forms. For example, sperms specialised for long-range travel can reach more distant ova that have the advantages of parental supplies and possibly protection.

Am I making sense?

Is this the sort of thing you have in mind?

Do you think it would help if you discussed the lines of thought you have been following?

Go well,

Jon

 

[nismaratwork]

NAW, you have an interesting line in choice of words and the concepts you allocate to them. Pinpointing? In this field what would that mean? The nearest half billion years or so? Don't you think the relative sequence of events would be most of what we could expect, and most of what we would need? A matter of opinion, you reckon? And opinions are interchangeable, I suppose? And what do you suppose the role and form of confirmation of opinion might be in scientific matters, beyond a defensive salve for the feelings of the thin-skinned?

And "softest of the sciences"? Which would that be? Would you feel comfortable defining "softness" in science, Professor Rutherford? Do enlighten us please.

Having done so, kindly favour us with your encore, and explain what the softness of the science entails for exponents. Does it mean that you can say whatever you please, or make meaningless statements, each protagonist safely cocooned in his own bed of softness? If you can do so, you will revolutionise science.

Turning to pleasanter, if more trivial, matters, such as biology and Darwinism, you no doubt will accept it as generally recognised biological fact, and not specifically my isolated opinion (as if that had anything to do with scientific cogency) that, if not sex, then at least gender has arisen in at least three kingdoms (Fungi, Plantae, and Animalia) and you might wish to add more. Sex, in the sense of genetic exchange and recombination between individuals, probably is much older, so much so, that we probably would need to develop molecular-biological arguments to pursue the topic of its origin. No matter; you yourself seem to be insisting on gender rather than sexual reproduction as the topic at issue, so that should suit both of us.
Well, then if one thing is clearer than another, it is that none of those three kingdoms seems to have derived its various genders from the others.

Right?

Really right??

Then we have a possible multiple origin for sex, and possibly not, but a dead cert for multiple origin for gender in various forms.
If you can bounce that one, drop science and go into politics as a spin doctor; clients would KILL for your services!

Cheers,

Jon

In other words, as I said there has been no way to pinpoint this with molecular biology (and the obvious lack of eyewitnesses *joke*), so... your opinion as you say. This still doesn't go very far towards answering whether or not this could have arisen independently, or not. Opinions with little hope of confirmation are not worth much in event his softest of the sciences.

Or again you could just read my entry in post 4 which says the same thing. Where in this thread am I insisting on gender?

Given that this is about sex and not gender, I stand by my assertion that, as you say, there are no current methods to tell when this exchange of genetic material as a means of reproduction began or whether it occurred, vanished, re-emerged, converged or arose independently.

 

[Nereid]

I still think there's considerable confusion over what I'm asking, so here goes one more attempt at clarification.

Meiosis is the form of cell reproduction which is at the heart of sexual reproduction; no meiosis, no sexual reproduction*.

Possibly with some exceptions (I'm still searching), all eukaryotes' reproduction involves meiosis.

There are three forms, zygotic meiosis, gametic meiosis (all animals have this form), and sporic meiosis (most plants have this form).

Some eukaryotes' life history is complicated, and doesn't fit into these neat three divisions.

However, to repeat, AFAIK, all eukaryotes include at least one stage in their life history in which meiosis plays a key role ... except for a few which once reproduced sexually but no longer do so.

If, indeed, all eukaryotes reproduce at least in part via meiosis (or did once, in their evolutionary past), then this seems to be a second characteristic of eukaryotes which happened just once (the other is the endosymbiosis which resulted in the mitochondria).

* the converse is interesting to consider: is there meiosis which is not part of sexual reproduction (but is still part of reproduction)?

 

[Jon Richfield]

I still think there's considerable confusion over what I'm asking, so here goes one more attempt at clarification.

* the converse is interesting to consider: is there meiosis which is not part of sexual reproduction (but is still part of reproduction)?

Let's deal with this one first. It seems to me something of a contradiction in terms to speak of meiosis as having anything but a sexual function. I suppose that much of the recombination function could happen outside meiosis, but it is hard to think of convincing examples. The only ones that spring to my mind are certain viruses such as flu.
The other function is reduction of ploidy. Not just any old polyploidy, but specifically the halving of ploidy. One could base a science fiction biology on some such function, but I know of none on this planet. Not as part of a standard life cycle anyway. Possibly some tetraploid somewhere has regained a normal haploid or diploid status in that way, but nothing of that type sense of interest here.

 

[nismaratwork]

Let's deal with this one first. It seems to me something of a contradiction in terms to speak of meiosis as having anything but a sexual function. I suppose that much of the recombination function could happen outside meiosis, but it is hard to think of convincing examples. The only ones that spring to my mind are certain viruses such as flu.
The other function is reduction of ploidy. Not just any old polyploidy, but specifically the halving of ploidy. One could base a science fiction biology on some such function, but I know of none on this planet. Not as part of a standard life cycle anyway. Possibly some tetraploid somewhere has regained a normal haploid or diploid status in that way, but nothing of that type sense of interest here.

God help me, I agree.

 

[Jon Richfield]

Okay, now let's deal with this aspect:

Possibly with some exceptions (I'm still searching), all eukaryotes' reproduction involves meiosis.
There are three forms, zygotic meiosis, gametic meiosis (all animals have this form), and sporic meiosis (most plants have this form).
Some eukaryotes' life history is complicated, and doesn't fit into these neat three divisions.

Well, I can hardly complain about that part of the statement for logic and comprehensiveness. Let's see where it gets us!

If, indeed, all eukaryotes reproduce at least in part via meiosis (or did once, in their evolutionary past), then this seems to be a second characteristic of eukaryotes which happened just once (the other is the endosymbiosis which resulted in the mitochondria).

Well, first let's deal with the mitochondria. Possibly I once again misunderstand your intention, but why do you say they are unique? What about plant plastids? What about hydrogenosomes?
They might not be as ubiquitous as mitochondria, but that does not affect the question of their acquisition (or at least the acquisition of their ancestors) occurring through a similar mechanism.

All right, now meiosis. At our current, still-pathetic level of knowledge of the fundamentals, we cannot yet answer your question categorically. However, some lines of thought might be at least intellectually helpful.

What came before meiosis?

An easy answer is: mitosis. We should not be too glib about this, because mitosis is a tremendously sophisticated and advanced process. There may well have been earlier forms of division which would hardly have resembled the the cell division that we now take for granted. Very likely the modern synkaryon of some fungi is a secondary development (which it certainly is in the case of say, vertebrate muscle cells) but it may all the same bear some hints at the nature of early cell wall structures.

Other analogies, noncellular in these cases, would be certain viruses, such as influenza, in which the inclusion of segments of genetic material in a given capsid is largely haphazard. It certainly is possible for that approach to work very well, both reproductively and genetically, as any flu sufferer can attest. It effectively amounts to mitosis and meiosis combined in one process.

Startlingly decisive, if rather brutal, approach to the Gordian knot, I always thought!

Now, a later stage of development very probably was a more efficient, rigorous, and versatile development of a cell wall, and possibly at about the same stage, of a fairly distinct and consistently structured cell, together with its, typically diploid, nucleus. Whether the nuclear structure of Giardia is a secondary development or not, it does have suggestive aspects in this connection.

Now, a very important point is that at such a stage it is already conceivable that we could have large populations of viable structures that could have gone off and taken the next steps towards eukaryote status independently and not fully compatibly.

For just one example, that could explain the difference between say, mitosis in the kingdoms Animalia and Plantae. Speculative? You bet! Anyone got anything better? So what is the merit of such a speculation, you ask? It gives us something to base hypotheses on. No hypotheses, nothing to test. Nothing to test, and we are not talking science any more.

Well, if it is possible that mitosis was not a unique development, then there are important implications. For a start, it seems to me (and we must by now be very close to being able to test this) almost inescapable that meiotic machinery is based on the mechanisms on which mitosis is based. If that is the case, your question is answered, and with no money to put where my mouth is, that still is the way that I would bet.

I doubt that either advanced mitosis or meiosis was a unique development.

I suspect that there were various other parallel developments that fell by the wayside, either contingently or by the agency of direct competition.

Let's leave it at that for now. If your patience will take the foregoing, then mull it over, and give me a hint at your reactions.

Remember, I am at least as inquisitive about it as you, and I should love to hear your thoughts.

Cheers,
Jon

 

[Jon Richfield]

God help me, I agree.

Console yourself; it happens to all of us, Nismar!

 

[nismaratwork]

Console yourself; it happens to all of us, Nismar!

Yeah, I know, and your next post was very lucid as well. I'm pleased that Jackson put the brakes on the previous spat... this is far more productive. I also have to agree that meiosis had to be an offshoot of mitosis, with the possibility of the two arising completely independently being beyond credulity. I wonder if there are any studies showing that both maximize efficiency compared to any competitor which, as you say, didn't persist. It's a good hypothesis, but difficult to prove when the notion of efficiency in such a context is... elusive.

 

[Nereid]

I still think there's considerable confusion over what I'm asking, so here goes one more attempt at clarification.

* the converse is interesting to consider: is there meiosis which is not part of sexual reproduction (but is still part of reproduction)?

Let's deal with this one first. It seems to me something of a contradiction in terms to speak of meiosis as having anything but a sexual function. I suppose that much of the recombination function could happen outside meiosis, but it is hard to think of convincing examples. The only ones that spring to my mind are certain viruses such as flu.

And in the general context of my question, viruses are out of scope.

The other function is reduction of ploidy. Not just any old polyploidy, but specifically the halving of ploidy. One could base a science fiction biology on some such function, but I know of none on this planet. Not as part of a standard life cycle anyway. Possibly some tetraploid somewhere has regained a normal haploid or diploid status in that way, but nothing of that type sense of interest here.

Scifi aside ... for meiosis itself, there are lifeforms which handle reduction (and subsequent return to normal) in ploidy in, um, interesting ways (most plants, for example). However, they all fall under the general heading of sexual reproduction.

It's good to rule this out though.

 

[Nereid]

[...]

If, indeed, all eukaryotes reproduce at least in part via meiosis (or did once, in their evolutionary past), then this seems to be a second characteristic of eukaryotes which happened just once (the other is the endosymbiosis which resulted in the mitochondria).

Well, first let's deal with the mitochondria. Possibly I once again misunderstand your intention, but why do you say they are unique? What about plant plastids? What about hydrogenosomes?
They might not be as ubiquitous as mitochondria, but that does not affect the question of their acquisition (or at least the acquisition of their ancestors) occurring through a similar mechanism.

I'd be very interested to continue this discussion in another thread I started, https://www.physicsforums.com/showthread.php?t=414883"

In short, the endosymbiosis which resulted in mitochondria is unique in the sense that, as far as we know, the mitochondrial DNA of all present-day eukaryotes has a single ancestor. This is not true of plastids.

All right, now meiosis. At our current, still-pathetic level of knowledge of the fundamentals, we cannot yet answer your question categorically. However, some lines of thought might be at least intellectually helpful.

What came before meiosis?

An easy answer is: mitosis. We should not be too glib about this, because mitosis is a tremendously sophisticated and advanced process. There may well have been earlier forms of division which would hardly have resembled the the cell division that we now take for granted. Very likely the modern synkaryon of some fungi is a secondary development (which it certainly is in the case of say, vertebrate muscle cells) but it may all the same bear some hints at the nature of early cell wall structures.

Other analogies, noncellular in these cases, would be certain viruses, such as influenza, in which the inclusion of segments of genetic material in a given capsid is largely haphazard. It certainly is possible for that approach to work very well, both reproductively and genetically, as any flu sufferer can attest. It effectively amounts to mitosis and meiosis combined in one process.

Startlingly decisive, if rather brutal, approach to the Gordian knot, I always thought!

Now, a later stage of development very probably was a more efficient, rigorous, and versatile development of a cell wall, and possibly at about the same stage, of a fairly distinct and consistently structured cell, together with its, typically diploid, nucleus. Whether the nuclear structure of Giardia is a secondary development or not, it does have suggestive aspects in this connection.

Now, a very important point is that at such a stage it is already conceivable that we could have large populations of viable structures that could have gone off and taken the next steps towards eukaryote status independently and not fully compatibly.

For just one example, that could explain the difference between say, mitosis in the kingdoms Animalia and Plantae. Speculative? You bet! Anyone got anything better? So what is the merit of such a speculation, you ask? It gives us something to base hypotheses on. No hypotheses, nothing to test. Nothing to test, and we are not talking science any more.

Following this line, and keeping within my general scope, the key questions would be (I think) something like: which of the six (YMMV) supergroups of Eukaryota have forms of mitosis which are not, apparently, shared by other supergroups? And, to the extent could we knock the question into a quasi-hypothesis form, to what extent can we say that one form of mitosis is descended from another (vs, say, an independent innovation)?

And to stress again, the level I'm interested in is not Animalia and Plantae, but Opisthokonta and Archaeplastida (vs the other four supergroups). So, for example, is mitosis in the Fungi different from that in the Animalia? Is mitosis in Glaucophyta different from that in the Plantae? If yes to either, then how did the different forms of mitosis evolve?

Well, if it is possible that mitosis was not a unique development, then there are important implications. For a start, it seems to me (and we must by now be very close to being able to test this) almost inescapable that meiotic machinery is based on the mechanisms on which mitosis is based. If that is the case, your question is answered, and with no money to put where my mouth is, that still is the way that I would bet.

I doubt that either advanced mitosis or meiosis was a unique development.

What I'm interested in is whether meiosis evolved only once, independently. Or is it like the eye, something which evolved independently many, many times?

I hadn't thought to ask about mitosis, partly because I'd assumed (wrongly?) that we have no way whatsoever of understanding its origin (today anyway), but with meiosis we may have some possible approaches (e.g. the genetic machinery which is involved).

I suspect that there were various other parallel developments that fell by the wayside, either contingently or by the agency of direct competition.

No doubt ... but there's no way to know, is there?

Let's leave it at that for now. If your patience will take the foregoing, then mull it over, and give me a hint at your reactions.

Remember, I am at least as inquisitive about it as you, and I should love to hear your thoughts.

Cheers,
Jon

Thanks very much!

This all started, for me, when reflecting on the uniqueness of life of Earth, Sagan (or Ward) vs Mayr (or Gould), if you will.

 

[Jon Richfield]

Yeah, I know, and your next post was very lucid as well. I'm pleased that Jackson put the brakes on the previous spat... this is far more productive. I also have to agree that meiosis had to be an offshoot of mitosis, with the possibility of the two arising completely independently being beyond credulity. I wonder if there are any studies showing that both maximize efficiency compared to any competitor which, as you say, didn't persist. It's a good hypothesis, but difficult to prove when the notion of efficiency in such a context is... elusive.

Thank you for taking it in such a generous spirit. I agree that this is much more rewarding.

Studies? I am not aware of any, but that means nothing, as this is quite out of my field. Not that I take "fields" seriously, but the current ferocious rate of publication means that it takes concentrated, focussed study just to stay in touch, never mind keep up with the cutting edge!

No, what I wrote was the purest speculation based on general knowledge. I have for some time been interested in a detached way in the concept of how the earliest cells and cell mechanisms formed. It is an intriguing field, but one which palaeontology can give us only the vaguest clues to.

Efficiency... That is a hard one. Efficiency in those days would practically come down to finding anything that would work at all. I am reminded of the difference between the walk and the sprint of a cheetah. The walk maximises energy efficiency, whereas the run maximises speed, and to hell with efficiency. If it catches its meal it can get far more energy than it expended; if not, it loses far more than any efficiency could have economised.

I am in a general state of feeling lost. That twilight stage at the start of life on Earth is very tantalising.

 

[Jon Richfield]

In short, the endosymbiosis which resulted in mitochondria is unique in the sense that, as far as we know, the mitochondrial DNA of all present-day eukaryotes has a single ancestor. This is not true of plastids.

I see. It certainly is not true of a lot of organelles that might be endosymbiotic relics. It is quite conceivably is true of mitochondria, but I am cautious. For one thing, how are we to distinguish between a unique source being a single event perpetuated throughout mitochondria-bearing life, and totally independent ancestors independently picking up the same species of eo-mitochondrial bacteria in different events?

This is not simple-minded niggling. There could be significant implications for the validity of recognition of monophyletic groups.

By way of analogy, suppose that a new strain of E.coli evolved that developed an endosymbiotic relationship with humans, certain birds, and guinea pigs, conferring say, independence from dietary sources of such vitamins as ascorbate and vitamin B 12. Imagine say, in 100 million years a future palaeontologist and comparative physiologist arguing that those groups (or their descendants of course) must surely be monophyletic.
I am not of course, arguing that this is the case, but that we need a great deal of caution. You see, I do have a deep suspicion that in our earliest stages of development of life on this planet, species were not nearly as exclusively separated as we take for granted nowadays. In fact, I am not really speaking about the very earliest days but about the ages leading into early forms or approaches to eukaryotic biology.

Following this line, and keeping within my general scope, the key questions would be (I think) something like: which of the six (YMMV) supergroups of Eukaryota have forms of mitosis which are not, apparently, shared by other supergroups? And, to the extent could we knock the question into a quasi-hypothesis form, to what extent can we say that one form of mitosis is descended from another (vs, say, an independent innovation)?

[...]
If yes to either, then how did the different forms of mitosis evolve?

Here you go beyond what I know or have an inkling of. In fact I am confident that not all we need to know to comment on some of those questions is yet known at all. They are very important questions, not only for palaeontological reasons, but for understanding the very workings of living cells of all kinds.
What I suspect, but here I am groping in such abject ignorance, that I hardly dare dignify it with the term "speculation", is that when some sort of reasonably effective (I am not saying a word about "efficiency") cell division developed in some of our proto-cellular ancestors, that they rapidly developed de facto parapatry. No doubt (well, all right, plenty of doubt, but live with it!) Much of the crucial machinery had by then been developed, in particular microtubules and various other cellular machinery at all near the molecular level.
On those rather generous assumptions it is easy to imagine how independent improvements to cell division could have developed, all of them culminating in one form of mitosis or another, and not necessarily building on each other's developments.
Conversely, if such early cells were able to fuse, or to enter into endosymbiotic relationships, it is quite conceivable that some may have appropriated say, the ciliary or flagellar motors of others, or what ever they used for making centrioles and the like.
Does that sound excessively vaguely inclusive? That is because it is excessively vaguely inclusive out of ignorance. I leave it to you to imagine the depths of comparative molecular biological research still to be addressed before we can achieve anything like confidence in the details.
And I emphasise again that the ignorance I so glibly refer to hear, is not just the current universal ignorance of humanity, but the far more thorough ignorance of my own.
However, what I do like about this proto-speculation, is that to me at least it suggests a picture of how partly independent development of mitotic machinery, differing in details similar to some items that we already have seen about us, might conceivably have evolved. One involving out of another need not have happened. It could well have been parallel development from common ancestors.
The differences might have been due, not simply to differences in efficiency, but differences in circumstances. Mitosis within nuclear membranes for example might well be an adaptation suited to the environment with in a synkaryon. It might have happened many times, using the same machinery. To us it would look persuasively monophyletic.

What I'm interested in is whether meiosis evolved only once, independently. Or is it like the eye, something which evolved independently many, many times?

Obviously I cannot tell you that, as well you realize, but I find it hard to imagine an early world in which some of the assumptions that I make would not apply in some form or degree. In particular, no matter how wrong my assumptions or deficient my imagination, I find it hard to imagine the mechanism or any of its equivalents being single-event adaptations.
Note that this is totally independent of the question of how many of the component mechanisms and resources may have developed once only. For example, you mention the eye. As I understand it, eyes as structures may have developed umpteen times beyond those that we have identified, but it looks persuasively as though the rhodopsin light sensing mechanism or some precursor, may indeed have evolved just once. Similarly, mitosis, even some forms of mitosis that we regard as effectively identical, might indeed be highly polyphyletic, but all using microtubules of monophyletic origin.

I hadn't thought to ask about mitosis, partly because I'd assumed (wrongly?) that we have no way whatsoever of understanding its origin (today anyway), but with meiosis we may have some possible approaches (e.g. the genetic machinery which is involved).

Well yes, but I don't see why say, the genetic machinery should be dramatically more diagnostic, or at least more suggestive, than comparative work on the machinery of cytokinesis, spindle formation and the like.
Having speculated as far as I have, I do not expect to see resolution of many such questions in my lifetime, but maybe in the lifetimes of my sons. I certainly have been very surprised by some successes that I never had expected, and perhaps more surprised by the routes by which they were discovered.

No doubt ... but there's no way to know, is there?

Not yet!

This all started, for me, when reflecting on the uniqueness of life of Earth, Sagan (or Ward) vs Mayr (or Gould), if you will.

Right. This takes me back to some rather vigorous debates I have engaged in, concerning panspermia.

Something to think about!
Cheers,
Jon

 

[physicsdude30]

Did sexual reproduction arise more than once, independently? Is it too an example of convergent evolution? If so, what are the details?

I wonder if there's a way to find the genes which are responsible for how cells combine for sexual reproduction, then look at a variety of organisms, then use the molecular clock to see if they have the same of those type of genes and how much they've mutated to get an estimate of time period for a common ancestor for sexual reproduction? Although some may say they're not sure if they're aware of a molecular or fossil record, I wonder anyway.

 

[nismaratwork]

I wonder if there's a way to find the genes which are responsible for how cells combine for sexual reproduction, then look at a variety of organisms, then use the molecular clock to see if they have the same of those type of genes and how much they've mutated to get an estimate of time period for a common ancestor for sexual reproduction? Although some may say they're not sure if they're aware of a molecular or fossil record, I wonder anyway.

When Jon and I talk about the future of molecular biology, in part, what you're talking about is just that.

 

[Ygggdrasil]

There was a study published this week in Nature that compares the origin of sex chromosomes in birds (where males are ZZ and females are ZW) and mammals (where males are XY and females are XX). I've only skimmed through this discussion and the paper (it's not my area of specialty, so I don't know how much I can contribute), but perhaps some of you might find the paper interesting. From the paper:

Our comparison of the finished sequences of the chicken Z and human X chromosomes reveals that each evolved independently from different portions of the ancestral genome, from separate pairs of ordinary autosomes. In each lineage, different portions of the ancestral genome were substantially remodelled to become specialized sex chromosomes. The Z and X chromosomes have converged on a set of structural features that distinguish them from autosomes: a high density of interspersed repeats, and long intergenic distances resulting in low gene density. Furthermore, the Z and X chromosomes have both gained multicopy gene families that are expressed in testis, biasing the gene content of both chromosomes towards male reproductive functions.

You can read the full study at the link below (subscription required):
Bellot et al. (2010). Convergent evolution of chicken Z and human X chromosomes by expansion and gene acquisition. Nature 466:612–616. http://dx.doi.org/10.1038/nature09172 .

 

[Jon Richfield]

There was a study published this week in Nature that compares the origin of sex chromosomes in birds (where males are ZZ and females are ZW) and mammals (where males are XY and females are XX). I've only skimmed through this discussion and the paper (it's not my area of specialty, so I don't know how much I can contribute), but perhaps some of you might find the paper interesting.

Thanks Yg. I could only access the abstract, but it is not my field either, so that was enough for practical purposes. On the one hand the independent origin of the sex chromosomes was no surprise, given the great variety of sex determination mechanisms in nature. This of course does not mean that it was of no interest; confident prediction in the absence of actual observations in science have a long history of being, if not actually necessarily completely wrong, at least missing matters of interest and importance. And of course sometimes they really are radically wrong.

However, there are other aspects of interest to this paper. Not only was there the question of the independence of the origins of sex chromosomes, but the nature of the differences. And such questions are highly nontrivial, both in importance and in interest.

Other aspects of interest include the nature of the convergence and the fact that there are convergent "sex chromosome attributes" in both schemes of adaptation. To my mind this raises interesting questions concerning the control and information theory (in a handwaving sense of course). For one thing, if there can be adaptation for sex chromosomes as one functional subset of the karyotype, then might there not be similar subsets with less obvious associated classes of function?

We patently have a huge amount to learn.

Thanks again,

Jon.

 

[Jon Richfield]

When Jon and I talk about the future of molecular biology, in part, what you're talking about is just that.

Right!

 

[Jon Richfield]

And in the general context of my question, viruses are out of scope.

I cannot remember whether I have said this explicitly elsewhere, so forgive if I am getting repetitious, but certain viruses, the flu virus in particular, are in my opinion of enormous interest in the comparative study of mechanisms and evolutionary origins.
Just a reminder: the capsid of the flu virus is not of a particularly well defined shape or size. It ranges from a roughly spherical blob to a long tube. More to the point the genome is split, roughly gene by gene, into separate segments roughly analogous to chromosomes. These segments get packed apparently haphazardly into the capsids, so that as a rule one must be infected by a large number of capsids if one is to develop flu.
If the same cell is infected by different strains of flu, one gets incidental recombination, so that a totally new strain (or even several new strains) may emerge.
Note that this apparently inelegant mechanism not only is extremely, even dramatically, effective in nature, but on the one hand is far simpler than meiosis, therefore entailing a correspondingly smaller selective burden, and on the other it combines the functions of reduction and division.
The fact that we have such a radically different mechanism giving such nearly analogous functions illustrates how carefully we should consider the possibility that there might be unobvious alternatives to mechanisms that we otherwise might have considered as fundamental.
Items such as this have made me very pleased to have stumbled across a book of virology years ago. I had always regarded the field as uninspiring, but I found the book full of breathtaking examples, often with implications far beyond virology.
Cheers,
Jon