Wednesday, 17 September 2014

Kin selection and horizontal gene transfer

Microbiology / kin selection peeps, I need your help understanding something.

Here is a sentence I came across in a paper from the Sterelny/Joyce/Calcott/Fraser collection on Cooperation and its Evolution;

"According to Hamilton's rule, any modification of focal relatedness of genes specifying cooperative behaviors may have an effect on the stability of cooperative behaviors between interacting individuals." (Riboli-Sasco, Taddei and Brown 2013, 281).

I realise the many perils of basing a discussion on a single amputated sentence, but I think it is fair to represent the point being made as the following;

1. Horizontal gene transfer, as occurs by transfer of plasmids in bacterial populations, will increase the relatedness of the cells that are party to the transfer, at the locus occupied by the transferred gene.
2. Higher relatedness between cells allows cooperative behaviours between those cells to be more stable, because the  behaviour is selected whenever rB > C.
3. Therefore, horizontal gene transfer can affect the stability of cooperation whenever the transferred trait is cooperative.

Now, I hope somebody will point out what I'm missing, but it seems to me that something is going wrong here. The authors take a situation in which a plasmid is carrying a gene that codes for a cooperative trait from one cell over to another cell. After the event has occurred, both cells carry that same gene. If we ask 'What is the explanation for the fact that both cells express the cooperative trait?' there seems to be an easy causal mechanical answer.......'A plasmid carried the gene that codes for the trait from one cell to the other.'

But now it is suggested that there is another answer.....'Both cells express the cooperative trait because they are related, and whenever organisms are closely related they become more likely to cooperate with each other, because of Hamilton's rule'.

There are some contexts in which it might be true that altering relatedness between two individuals might cause them to cooperate with one another. For example, in any organism that uses some system to detect kin in combination with a faculty for conditional cooperation. Bacteria are thought to do this via quorum sensing. Many organisms including humans and colonial ascidians are thought to do it via immune recognition systems. Any organism with complex cognitive faculties might do it deliberately, as part of a strategy.

However, the horizontal transfer case is not one of these contexts. The authors don't claim that the change in relatedness causes the two cells to start cooperating, rather that it affects the stability of cooperation between them. Hamilton's rule acts independently of whether the interacting partners have any capacity for recognising kin. Rather, it tells us when natural selection will influence rates of cooperation, by weeding out organisms who tend to cheat against their kin.

So an alternative explanation for cooperation that is sometimes given, an ultimate explanation, says that some set of organisms cooperate because they are related. This is not a proximate explanation, it doesn't tell us what causal-mechanical events give rise to any actual cooperation event. Nonetheless, the relatedness is taken to be explanatory in the sense of explaining the stability of the cooperation - why it is that the cooperative behaviour has not been selected against and lost out to cheating. Explanations of this kind have been called 'Distribution Explanations', in contrast to 'Origin Explanations' (Godfrey-Smith 2009). Distribution explanations refer to populations rather than to individuals, and refer to the dynamics of traits changes within those populations over time.

I feel that the authors of this paper are wrongly conflating the two explanations when they claim that horizontal transfer can influence the stability of cooperation. Horizontal transfer occurs over immediate time scales, relative to selection.  It is true that a group of cells can acquire higher relatedness, at one specific locus, as a consequence of horizontal gene transfer. And it is true that the transferred gene might be cooperative, so that cooperation also increases within that group of cells as a consequence of the transfer. But it just seems wrong to say that the horizontal transfer has increased cooperation  because it has increased relatedness. The cells are not cooperating because they are related, but only because they have been turned into cooperators.

Again, the cooperation does not occur because high relatedness has prevented selection from undermining it. We are considering something that has occurred more or less instantaneously, after all, there has been insufficient time for selection to act at all. In order to assess the stability of the cooperative behavior, the most pertinent variable is probably the likelihood of further horizontal transfer, something which the current locus-specific relatedness of the cells in the population may or may not affect.

Is there something I'm not seeing?


Tyler said...

I'm not an expert but it seems to be as you've said, they've been made cooperators by sharing the gene on the plasmid.

Ellen Clarke said...

Well see, as someone who has been devoted full time for several years to thinking about transposable elements, I'd say you're fairly close to being an expert Tyler : )

Tyler said...

Ha well thanks :) I think Arvid would have some insight into this as well