[Grey-Walter] [info] bacterias jugando a piedra, papel o tijeras

[Lluis]

hace no mucho lei en Nature algunos articulos fascinantes que superan la 
ciencia ficcion, uno el de un raton con ocho testiculos en la espalda y 
otro sobre  bacterias que juegan en proceso en paralelo a.... piedra, papel 
o tijera!!!!!!!!! ademas parece que este juego tiene mas miga de lo que 
parece, que gueno :D

como Nature son parte del jodio stablishment cientifico y tiene los 
articulos bajo codigo cerrado, pues lo pasteo aqui en abierto, el de las
bacterias concretamente.

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Biodiversity: Bacterial game dynamics

MARTIN A. NOWAK1 AND KARL SIGMUND2

1 Martin A. Nowak is at the Institute for Advanced Study, Princeton, New 
Jersey 08540, USA.
e-mail: nowak at ias.edu
2 Karl Sigmund is at the Institut für Mathematik, Universität Wien, 
Strudlhofgasse 4, A-1090 Vienna, and the Institute for Applied Systems 
Analysis, A-2361 Laxenburg, Austria.
e-mail: karl.sigmund at univie.ac.at


Studies of three bacterial strains engaged in an interaction that mimics 
the game 'rock–paper–scissors' show the importance of localized 
interactions in maintaining biodiversity.


It is not surprising that games as absorbing as bridge and chess have their 
world federations and international unions. But not everyone knows that 
even a game as lowly as rock–paper–scissors has its own society. This game, 
which must surely be very old, can be explained to any toddler. Two players 
signal, on a given cue, either rock (fist), paper (flat hand) or scissors 
(two fingers). If I display a flat hand and you show me your fist, I win, 
as 'paper wraps rock'. Similarly, scissors cuts paper, and rock smashes 
scissors. If both players make the same signal, the game ends in a draw. 
And in case you think of it as a rather simple-minded pastime, you should 
take a look at the home page of the World RPS Society1, which is a treat. 
Among other features there are links to learned papers, although you are 
advised not to visit the links in the probabilistic section, filled as it 
is with "pseudo-scholastics". No such ban appears against the link to a 
paper in Nature describing three mating strategies of the male lizard Uta 
stansburiana2. And now Nature should hit the website again, with the report 
by Kerr and colleagues on page 171 of this issue3.

Kerr et al.3 set out to investigate the mechanisms that maintain 
biodiversity in ecosystems, by studying several diverse strains of 
Escherichia coli bacteria. These strains can produce a toxin, or not; and 
they can be resistant to the toxin, or not. We may assume that the 
bacterial devices for producing both the toxin and the 'antidote' that 
confers resistance are costly in the sense that they require resources that 
could otherwise have been used by the bacteria to multiply faster.

There are four potential strains. The one producing the toxin but not the 
antidote effectively commits suicide. This strain is a non-starter, and we 
may ignore it (as did Kerr et al.). The other three are engaged in a 
rock–paper–scissors type of competition. The poison- and antidote-producing 
strain kills that which produces neither poison nor antidote. The strain 
that produces the antidote but not the poison outgrows the one that 
produces both, by economizing on the cost of an ineffective poison. And in 
the absence of the toxin-producing strain, the strain that produces no 
antidote outgrows the antidote-producing type, which is paying for an 
unneeded device.

These two-way bacterial interactions have been described previously. And 
similar rock–paper–scissors cycles of spiteful measures and costly 
countermeasures occur in other evolutionary contexts, for instance in the 
genetics of sexual species. Some chromosomes acquire mutations that prevent 
their opposite number (inherited from the other parent) from making their 
way into eggs or sperm, and so to the next generation. Some of these 
mechanisms for subverting the fair segregation of chromosomes act like the 
bacteria, by means of a poison-and-antidote-type principle4.

But what happens when the three E. coli strains are all in the same 
environment? The mere knowledge that the outcomes of pair-wise competition 
form a rock–paper–scissors cycle is not enough to predict what happens when 
all three types are present5, 6. The three competitors might co-exist 
permanently; this seems to apply, for instance, to the male lizards that 
have three different mating strategies2. Or one type might be ousted, and a 
second type outcompeted by the third, leading to just one survivor. Kerr et 
al. find that this latter outcome holds for our bacteria. If all three 
strains are equally frequent at first, the type producing antidote but no 
poison eliminates the others.

Kerr et al. also show, however, that this loss of diversity occurs only if 
the bacterial populations are well mixed. Otherwise, all three strains 
survive. This conclusion has been predicted on paper: a rush of theoretical 
investigations during the past decade has shown, in great generality, that 
if dispersal of a population is limited and its interactions with other 
populations are localized, then diversity is protected to a large degree7, 
8. This holds for an astonishing variety of scenarios, for instance in 
epidemiological models, community ecology, plant genetics, animal 
behaviour, molecular evolution9 and game theory10. Such spatial models are 
usually much harder to analyse than their homogenized 'mean field' 
counterparts. But computer simulations warn us that, in many cases, 'mean 
field' can lead to wrong conclusions.

And Kerr and colleagues are not the first to show that localized 
interactions of the rock–paper–scissors type can turn a 'one winner' 
outcome into a dynamic coexistence of all three types, endlessly chasing 
each other across the board11. The beauty of their paper is that they show 
this not only on a computer screen, but also in 'real life'. To set up the 
well-mixed case, the authors put all three strains in a flask, shake this 
cocktail, transfer a few drops to another flask, shake it again, and so on. 
Soon the flasks contain only the resistant, non-toxic strain. To ensure 
localized interactions, on the other hand, Kerr et al. spread the strains 
on a plate to let them grow, then press a cloth on the plate, transfer 
whatever clings to the cloth onto another plate, and so on. All three 
strains survive, with the boundaries between them shifting to and fro, 
reflecting the cyclic invasion and displacement of one strain by the next. 
So the outcomes on the plate and in the flask are strikingly different.

This approach opens new vistas for understanding how biological communities 
are built up — one of the most intriguing aspects of the study of 
biodiversity. The results of sequential invasions and extinctions of 
species can create complex links and webs in an ecosystem, not least 
because the outcome of an invasion depends so much on its timing and other 
contingencies. Large-scale experiments in community construction are 
generally hard to come by — not often do volcanic rocks emerge, providing 
barren ground for colonization. So ecologists have increasingly turned, 
since G. F. Gause's work in the 1930s, to manipulating mini-worlds 
inhabited by microbial species12. The paper by Kerr et al. gives a new 
impetus to such investigations, by stressing the importance of the geometry 
of neighbourhoods. Many habitats resemble the surface of a pizza more than 
a well-stirred bowl of soup.



References 1. http://www.worldrps.com/archive/index.html
2. Sinervo, B. & Lively, C. M. Nature 380, 240-243 (1996). | ISI |
3. Kerr, B., Riley, M. A., Feldman, M. W. & Bohannan, B. J. M. Nature 418, 
171-174 (2002). | Article | PubMed | ISI |
4. Hartl, D. L. The Principles of Population Genetics (Sinauer, Sunderland, 
Massachusetts, 1980).
5. Hofbauer, J. & Sigmund, K. Evolutionary Games and Population Dynamics 
(Cambridge Univ. Press, 1998).
6. Weissing, F. J. in Game Equilibrium Models I (ed. Selten, R.) 29-96 
(Springer, Berlin, 1991).
7. Durett, R. & Levine, S. Theor. Pop. Biol. 46, 363-394 (1994). | Article |
8. Dieckmann, U., Law, R. & Metz, J. A. J. (eds) The Geometry of Ecological 
Interactions: Simplifying Spatial Complexity (Cambridge Univ. Press, 2000).
9. Boerlijst, M. C. & Hogeweg, P. Physica D 48, 17-28 (1991). | ISI |
10. Nowak, M. A. & May, R. Nature 359, 826-829 (1993).
11. Nowak, M. A., Bonhoeffer, S. & May, R. M. Int. J. Bifurc. Chaos 4, 
33-56 (1994). | ISI |
12. Weatherby, A. J., Warren, P. H. & Law, R. J. Anim. Ecol. 67, 554-566 
(1998). | Article | ISI |




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