Experimental evolution. Testing Darwin regarding the origin of multicellularity.

[rjw]

Pretty cool stuff

I think so too. I'm finding a lot of information out there and online - stuff with makes it into the research journals.

It is hard to read, all that technical stuff. But worth trying to make sense of. And so often the introduction in its own right can be very educational. That goes for most scientific papers because they tell the reader a bit about the history of the idea as well as what it is they are trying to do, and more in layman's terms than the remainder of the article.

 

[rjw]

A reason for running the experiment in cheat purging (CP) mode was to better understand the results of the cheat embracing experiment.



The only difference between the two kinds of experiment was that a WS cell began the new life cycle as opposed to an SM cell in the CE mode. This corresponded to cheats destroying the mat but it being an old mat cell (the WS type) that got the new mat underway.



Predictions were that this system should have greater evolutionary potential using the previous definitions of fitness for both cells and mats. And indeed this was the case. After 10 life cycles, the evolved cells at least showed greater fitness than did the corresponding cells from the CE experiment. 

However, there was an interesting difference, and Figure 4b shows it. Cell fitness has increased along with mat fitness. Compare this with Figure 2b where while mat fitness increased, cell fitness decreased. This time mat fitness seemed to depend on cell fitness whereas before, both fitnesses were decoupled.



This observation confirmed for them that in the CE regime, the performance in mat fitness was driven by the alternating phases in the life cycles as opposed to being driven by the cells and their individual fitness.



They explored the adaptations in the CP regime that led to increased lineage performance and found that in this regime there was no improvement in the ability of WS to switch to SM cell types. In fact fewer SM types were produced when compared to the CE regime and furthermore, it took longer for the SM types to arise within a mat. They considered that cheater suppression may have begun to evolve within the CP regime, and certainly there was nothing like a genetic switch tuned to the phases the cells were in.



So the evidence suggested that improved line fitness was simply due to traits within the individual cells themselves. Improvements in lines was happening because selection was operating on the individual cells themselves and not operating on the mats.



The differences were clear. In the cheat embracing experiment, mat fitness was related to mats not cells, and in the cheat purging experiment, it was related to cell fitness. The ability to switch cell types was opposite for both regimes as well. There was a negative correlation in the cheat purging experiment as opposed to the embracing experiment, suggesting that the switch depended very much on mutational changes in individual cells when purging was done. In the purging experiment, selection seemed to be operating only at the level of the cells, where as in the cheat embracing experiment, it seemed to be operating at both levels with cell fitness decreasing while mat fitness increased.




Therefore, what did the researchers conclude from all this?






To be continued ...

 

[rjw]

In their summary/perspective section to conclude the article, the researchers note that multicellular organisms are descendants of free living single celled organisms. The fossil record supports this contention. Because single cells have the capacity for differential reproduction, then they were the units of selection. But when multicellular organisms arose, they became the focus of Darwinian selection in their own right. What made this possible was a means of reproduction. When multicellular organisms could reproduce then differential success at reproduction happened and thereby these organisms now became open to selection.



So how did the emergence of reproduction occur with the origin of multicellularity?



This experiment showed a possible pathway:-





... cheating cells—those types seemingly most detrimental to the persistence of newly formed cooperative entities—can function as a germ line within a life cycle that facilitates the reproduction of collectives.





The experiment also produced a novel kind of organism for selection to operate on, a two state organism where each state represents a different aspect of the organism. The first state was the mat and its growth and subsequent generation of a few germ cells. The second state was the death of the mat with the germ cells surviving, and switching to cells capable of generating a new generation of mat. These two phases were unified by the evolution of an underlying developmental program which incorporated a switch to flick between states and to cause a new organism (the mat) to form following the death of the old organism.



In contrast to this kind of entity was the single state system of previous experiments whereby mats grew, were overwhelmed by the subsequent development of cheats, causing the organism to die. The cells that remained went on to form new mats.

In this earlier system, the success of the mats really depended on the fitness of the individual cells from which the mats grew.

The authors noted that during the early stages of an evolutionary transition, 
levels of selection need to be considered because conflicts can arise, as indicated by this experiment. The design of the experiment established a two level ecology. On one level was that of the individual cells, reproducing every hour. Selection acting on these, clearly favoured short term success, given this hourly reproductive cycle. On the other level however, that of the cooperating cells, the mat, the cycle was 9 days. Here selection also acted but on a much larger time scale.



So if selection was acting for short term success, then it was unlikely to facilitate the persistence of the lineages which had cycles lasting days.



They argue that not only was a switch required to evolve, but also an underlying developmental program was needed to underpin the development of the multicellular organism, the mat. The switch would ensure the right kind of cell, depending on the phase of the life cycle. The program would bring about the onset of the mat. Therefore, as with a sexually reproducing multicellular organism like us, these experimental organisms had primitive germ cells which ensured life beyond the death of the mat. The switch and developmental program ensured the emergence of somite like cells from the germ cells, bringing on the formation of new mats.



The cells forming the body or the mat, played an ecological role. They allowed the mat to exist at the broth/surface interface where there was access to abundant oxygen for growth. But the body cells also played a reproductive role by producing some seed or germ cells (the cheat cells) which ensured the beginning of the next generation.



And so they conclude:-





Given sufficient variation among lineages, then selection over the longer timescale stands to conquer the short-term interests of individual cells. This appears to have happened in our CE regime with decoupling of fitness between levels supporting the view that selection has begun the process of transitioning to the higher (collective) level—with the lower level beginning to function for the good of the collective.

The authors describe the kind of situation their experiment was attempting to simulate in a Supplimentary Discussion paper.



On the second page, about two paragraphs down, they offer a thought experiment with the “glass boundaries” of their particular experiment removed. The paper, including the thought experiment is worth a read. However, given that this experiment is all about the origin of multicellularity, I thought it odd that they relied on reeds (multicellular organisms) to provide support columns for the mats. Surely a shallow flat pool with protruding rocks, or the vertical edge of a lake shore would have been a bit more realistic. 



But I quibble.



Anyway, in my next post, I will conclude this set of essays.






Final post to come ...

 

[rjw]

In conclusion. 



The origin of multicellularity has long been a mystery - and naturally it remains one.



In experiments it is quite easy to get many kinds of cells that normally operate individually, to join up into mats of cells. However, genetic mutations will always ensure that some cells turn cheats. That is, they live in the mat, drawing benefits by living in the mat, but contribute nothing in return to the mat. And so these cheat cells have resources to burn and end up out competing and out reproducing the mat cells. Hence the mat dies.



Obviously, the way to conduct experiments on the origins of multicellularity, is to work out plausible routes by which cheat removal from the system may have evolved.



However the experiment described here tested the idea that the cheats, which caused mat death, may act as primitive germ cells and thereby allow the reformation of a new generation of mat, following the death of the old mat. After all, cheat cells do have, more or less, the same genome as the mat cells.

And this is what was found. 

In fact, in the most successful of the experimental lineages, it was found that the evolution of a genetic switch and an underlying primitive developmental program brought on a novel kind of multicellular organism in which the switch was sensitive to the phase the organism was in. When the mat was developing, the switch would be thrown and the development of cheat cells more or less ensured. This would cause the mat to die, with the cheat cells being the survivors. The switch would act again, to ensure that some of them switched back to mat growing cells, and thus a new mat would begin to develop.

And so it went.

This had the effect of allowing mats to form and die in cycles that went well beyond the six (maximum) cycles found in experiments in which cheats were always purged from the system.



One of the properties of the new organism was that mat success did not depend on individual cell success. Individual cell fitness dropped off while mat fitness improved. That is, cell fitness was decoupled from mat fitness, in contrast to the cell purging system where mat fitness depended very much on cell fitness.

With the new organism then, selection was operating independently on two distinct levels, that of the individual cells and that of the organism as a whole.



The organism itself could now participate in Darwinian evolution in its own right.





This brings to an end this set of essays. I suspect that I missed a lot and got some things wrong. Or that I misunderstood the importance of some concepts and did not give them the attention I should have. Nevertheless, I think I understood the gist of the experiment enough to have learnt something from it, and to have been able to pass those ideas on.





The end.