To trust or

Guess what is common to viral strains that infect bacteria and two supposed partners in crime

Published: Friday 15 October 1999

the dilemma facing two co-accused -- of whether to turn approver or not -- has been used in evolutionary biology to term a well-known puzzle. But to understand the biological implications of the 'prisoner's dilemma', it is important to explain the theoretical dilemma first.

Suppose two prisoners are held separately in a jail on the suspicion of having committed a burglary. Lacking hard evidence, the police makes each of them an offer of a reduced charge in exchange for helpful information implicating the other. This leaves the duo with the following set of options: The first prisoner can refuse to help the police; if his partner also refuses, both get to spend six months in prison. But if he/she cooperates with the police and the partner does not, he/she gets off relatively lightly and has to spend just 3 months in prison. On the other hand, if the first prisoner refuses to help the police and the partner collaborates with the police, he/she is sentenced for two years. Finally, if he/she cooperates with the police and the partner does so too, even though they implicate each other, both get some remission and end up spending just one year in jail.

The conclusion is inescapable: regardless of whether the companion-in-crime chooses to remain faithful to their friendship or not, it always pays to cooperate with the police and implicate the other. Prisoner number two, going through the same steps of reasoning, also reaches the same conclusion. The end result is that they implicate each other and each goes to jail for one year. The puzzle is that logical analysis tells them that this is the only 'fail-safe' option that they can adopt -- nothing that the other person does can make matters worse for them, even though refusing to cooperate with the police is a better option. This, then, is the prisoner's dilemma. One betrays the fellow prisoner because one does not want to take the risk of trusting the other.

Many scientists have speculated that when individuals of a species interact with each other under conditions in which trusting the other is advantageous but at the same time fraught with risk, the logic of the prisoner's dilemma comes into play. There has been little evidences in support of this contention. The latest is the work on bacterial viruses, called phages, by P E Turner and Lin Chao of the University of Maryland, usa ( Nature , Vol 398, p441-443).

The situation studied by Turner and Chao involves a virus of the parental strain ( P ) and a mutant derived from the same ( M ). After infecting a bacterium, each virus needs to manufacture a number of proteins in order to enable it to make copies of itself. The researchers wanted to know how efficiently a virus make these products when it is by itself or when it is with the mutant. The results were striking. Let us measure all growth rates relative to that found when two P-type strains infect a bacterium. Therefore the PP situation yields a growth rate of 1.0 units, for instance. However, if one of the two P strains behaves like an M strain, its growth rate jumps to 1.99 units. Two M strains in a cell result in both having a growth rate of 0.83 units, and in this situation, behaving like a P strain does not help. This choice results in a growth rate of just 0.65 units. The consequence is that it helps to behave like the mutant M and be guaranteed of a 'return' of at least 0.83 units. This is so even though the ideal situation would demand that all the viruses that infect a bacterium behaved like P and obtained a 'return' of 1.0 units. And the reason for deciding to behave like M is that no other virus can exploit you.

However, beyond this conclusion is a puzzle. The puzzle is that M is a mutant, and therefore rare. The strain P is the common variety. One explanation for this surprising example of cooperative behaviour is that it is rare for viruses belonging to different strains to infect the same bacterium. Therefore, finding themselves inevitably in the company of their genetically identical cohorts, they can afford the luxury of trust. But can the two prisoners do the same?

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