Hi All,

Recently, an interesting bit of research dealing with experimental evolution was published in Nature. 


As sometimes happens, it caught my eye, and so I began reading with a hope to understanding it tolerably well, enough so that I could write up a series of posts explaining the experiment.


I think I do understand it and before I begin to describe the gory guts, let me introduce it and mention what I found so interesting about the research.


The article is:-
Life cycles, fitness decoupling and the evolution of multicellularity



Its abstract and conclusion read as follows:-



The research has to do with using a bacterium named Pseudomonas fluorescens as a means for exploring the origins of multicellularity. This organism is well established as a test bed for these kinds of experiments because what it can do is go from single celled organisms to clusters of cooperating organisms (called mats) fairly readily. But to go from single cells to mats of cells, the individual cells have to give up their individualistic identity and cooperate. By doing this, they reap benefits not open to the cells if they remain as individuals. As individuals, the cells remain immersed in a broth that can be toxic, thanks to the accumulation of waste products which deprive the cells of oxygen. As cooperators forming mats though, things change because the mats float, inhabiting the fluid/air interface and thus allowing the cells in the mat to gain access to oxygen.


Here is where it really starts to get interesting however.


The cells get access to oxygen by forming mats. In doing so, they have to expend food and energy producing a glue that binds them together. So, if cheats can learn to live with the cooperators, then the cheats also access the oxygen but dont have to give anything to the group to allow it to continue.


That is, cheats in the mat gain the benefits of being in the mat, without having to expend food and energy that allow the other cells to participate in mat formation.

As a result, the cheats outcompete the cooperators and soon overwhelm them and the mat dies. It disintegrates.
So mats only last for a while before dying.


As a result, experiments on the origins of multicellularity have focused on systems which purge the cheats from the habitat. It seemed an intuitive thing to do.

However, the results were of limited success.


Now here is where it begins to get even more interesting. The researchers wondered if there were ways in which the cheats may actually help the mats, not by preventing their deaths, but rather by acting as a kind of sex cell and on the death of the mat, the cheats being able to act as the nucleus for the next generation of mat, simply by virtue of the fact that a cheat cell also has the genome of the cooperating cell which it carries on into the future as the beginning of the next generation.


The experiment was able to show that there is a condition in which this is what happens. The cooperating mat cells behave as our somites - the non sex cells (skin, liver, hair, brain, eye cells) which ultimately die. The cheats in the mat behave as our sex cells, propagating the genome of the mat into the next generation, forming the nucleus of the new mat.


Like the cheats, our sex cells really dont contribute to the daily continuation of the body. They kind of bludge off it, using the food and energy produced by the rest of the body. But where they come into their own is on the death of the organism. When the organism dies, its the sex cells that get the new generation underway. Thus it was with the cheats.


Their research was not so much about the origins of sex, but rather it was about the origins of multicellularity.

The other fascinating thing was this. What is good for the individual cells in the mat, is not necessarily good for the mat itself. That is, if selection acts on the mat in a positive way, then it may at the same time, act on the individual cells in a negative manner, and vice versa. And certainly selection acts on cells over the life time of a cell, with is very much less than selection acting on a mat over the life time of a mat. Mats have a life time measured in days. Cells have a life time measured in hours.
Yet in this experiment, the mat and the cells became decoupled, such that selection at one level did not overly affect selection at the other level.
Hopefully I can describe the experiment to you, and give you an idea about what they found out, and what the genetics were that allowed this system of cooperating mat cells + cheat cells was all about, and how the cheats played a crucial role in bringing about the next generation of mat.


This set of essays may be slow in coming. I dont know. But it is that hectic time of the year and besides, Im busy digging up the back yard, remodeling it. Nevertheless, reading and learning some interesting idea is always a lot more fun than hot sweaty work in dust under a blazing sun or in mud and slush when a storm comes.  







To be continued ....

Recently, an interesting bit of research dealing with experimental evolution was published in Nature. 


As sometimes happens, it caught my eye, and so I began reading with a hope to understanding it tolerably well, enough so that I could write up a series of posts explaining the experiment.


I think I do understand it and before I begin to describe the gory guts, let me introduce it and mention what I found so interesting about the research.


The article is:-
Life cycles, fitness decoupling and the evolution of multicellularity



Its abstract and conclusion read as follows:-





link above said:Abstract

Cooperation is central to the emergence of multicellular life; however, the means by which the earliest collectives (groups of cells) maintained integrity in the face of destructive cheating types is unclear. One idea posits cheats as a primitive germ line in a life cycle that facilitates collective reproduction. Here we describe an experiment in which simple cooperating lineages of bacteria were propagated under a selective regime that rewarded collective-level persistence. Collectives reproduced via life cycles that either embraced, or purged, cheating types. When embraced, the life cycle alternated between phenotypic states. Selection fostered inception of a developmental switch that underpinned the emergence of collectives whose fitness, during the course of evolution, became decoupled from the fitness of constituent cells. Such development and decoupling did not occur when groups reproduced via a cheat-purging regime. Our findings capture key events in the evolution of Darwinian individuality during the transition from single cells to multicellularity.
[big snip]
Perspective

Multicellular organisms are descendants of once free-living cells1, 3, 4. By virtue of their capacity for differential reproduction, ancestral free-living cells were units of selection33. During the transition to multicellularity, collectives of cells emerged that came to participate in Darwinian processes in their own right2, 5, 11, 19. The essential ingredient was a means of collective reproduction5, 11. This most seminal of Darwinian properties emerges afresh at each transition and requires explanation44. Here we have shown that cheating cellsthose types seemingly most detrimental to the persistence of newly formed cooperative entitiescan function as a germ line within a life cycle that facilitates the reproduction of collectives. Moreover, the two-phase life cycle presents selection with an altogether new kind of biological entity: each state becomes a different attribute of a single organism whose evolution is unified through a developmental programme45. When reproduction of collectives is via fragmentation (a single-phase life cycle), the traits that yield success at the higher level are largely those that determine success of single cells. This offers limited opportunity for the emergence of new kinds of biological individuality because properties of higher and lower levels remain aligned19, 38.
Direct observation of early stages in an evolutionary transition requires that issues surrounding levels of selection be considered2, 5, 19. This necessarily leads to territory in which a range of perspectives is possible (see Supplementary Discussion). Our experimental design incorporates an ecology that is explicitly multi-level: both individual cells (that reproduce once every hour), and individual lineages (that reproduce once every 9*days) can be units of selection; however, selection operates on cells and lineages over different timescales. While selection on individual cells favours short-term success, short-term success is unlikely to facilitate persistence of lineages. Indeed, persistence requires more than simply switching between phenotypes: it involves a developmental programme that underpins expression of a collective phase in which a soma-like body is constructed from germ-like cells. Cells of the body must simultaneously play an ecological role (maintaining the body near oxygen via a robust mat phenotype) while producing the seeds of the next generation of bodies (the germ-like cells). 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) levelwith the lower level beginning to function for the good of the collective.
The research has to do with using a bacterium named Pseudomonas fluorescens as a means for exploring the origins of multicellularity. This organism is well established as a test bed for these kinds of experiments because what it can do is go from single celled organisms to clusters of cooperating organisms (called mats) fairly readily. But to go from single cells to mats of cells, the individual cells have to give up their individualistic identity and cooperate. By doing this, they reap benefits not open to the cells if they remain as individuals. As individuals, the cells remain immersed in a broth that can be toxic, thanks to the accumulation of waste products which deprive the cells of oxygen. As cooperators forming mats though, things change because the mats float, inhabiting the fluid/air interface and thus allowing the cells in the mat to gain access to oxygen.


Here is where it really starts to get interesting however.


The cells get access to oxygen by forming mats. In doing so, they have to expend food and energy producing a glue that binds them together. So, if cheats can learn to live with the cooperators, then the cheats also access the oxygen but dont have to give anything to the group to allow it to continue.


That is, cheats in the mat gain the benefits of being in the mat, without having to expend food and energy that allow the other cells to participate in mat formation.

As a result, the cheats outcompete the cooperators and soon overwhelm them and the mat dies. It disintegrates.
So mats only last for a while before dying.


As a result, experiments on the origins of multicellularity have focused on systems which purge the cheats from the habitat. It seemed an intuitive thing to do.

However, the results were of limited success.


Now here is where it begins to get even more interesting. The researchers wondered if there were ways in which the cheats may actually help the mats, not by preventing their deaths, but rather by acting as a kind of sex cell and on the death of the mat, the cheats being able to act as the nucleus for the next generation of mat, simply by virtue of the fact that a cheat cell also has the genome of the cooperating cell which it carries on into the future as the beginning of the next generation.


The experiment was able to show that there is a condition in which this is what happens. The cooperating mat cells behave as our somites - the non sex cells (skin, liver, hair, brain, eye cells) which ultimately die. The cheats in the mat behave as our sex cells, propagating the genome of the mat into the next generation, forming the nucleus of the new mat.


Like the cheats, our sex cells really dont contribute to the daily continuation of the body. They kind of bludge off it, using the food and energy produced by the rest of the body. But where they come into their own is on the death of the organism. When the organism dies, its the sex cells that get the new generation underway. Thus it was with the cheats.


Their research was not so much about the origins of sex, but rather it was about the origins of multicellularity.

The other fascinating thing was this. What is good for the individual cells in the mat, is not necessarily good for the mat itself. That is, if selection acts on the mat in a positive way, then it may at the same time, act on the individual cells in a negative manner, and vice versa. And certainly selection acts on cells over the life time of a cell, with is very much less than selection acting on a mat over the life time of a mat. Mats have a life time measured in days. Cells have a life time measured in hours.
Yet in this experiment, the mat and the cells became decoupled, such that selection at one level did not overly affect selection at the other level.
Hopefully I can describe the experiment to you, and give you an idea about what they found out, and what the genetics were that allowed this system of cooperating mat cells + cheat cells was all about, and how the cheats played a crucial role in bringing about the next generation of mat.


This set of essays may be slow in coming. I dont know. But it is that hectic time of the year and besides, Im busy digging up the back yard, remodeling it. Nevertheless, reading and learning some interesting idea is always a lot more fun than hot sweaty work in dust under a blazing sun or in mud and slush when a storm comes.  







To be continued ....