All,
Nice job on choosing papers. Here is a summary of what I heard. Please comment on this post to add to, correct, or question my brief overview.
All readings can be found at:
In chronological order of publication,
1) Dave, who chose Bull JJ Molineaux IJ, Rice WR. 1991. Selection of benevolence in a host-parasite system. Evolution 45:875-82.
They evolved a plasmid pBR322-phage M13 chimera under high-fidelity (had to stick with E coli host) and low fidelity (were allowed to infect multiple partners) conditions. Found that each became more fit in the selective environment, and that high-fidelity phagemids lost the ability to infect horizontally as a consequence of a reduced phage genome.
2) Sam, who chose: Tamas Czaran, Rolf F. Hoekstra, and Ludo Pagie. 2002. Chemical warfare between microbes promotes biodiversity. PNAS.
Simulated the same basic rock-paper-scissors dynamic of Kerr et al, but allowed as many as 14 different toxin/antitoxin/susceptible combinations to evolve. Studied rates and influences of mutation and recombination in a spatial matrix. Found that a frozen state of coexistence of “hyperimmunity” is likely but can be disrupted by either recombination or asynchronous temporal scales of dominance between players (eg toxin killing is faster than resource competition). Very interesting discussion of their findings — see the paper.
3) Dan, who chose: R. Craig MacLean & Ivana Gudelj. 2006. Resource competition and social conflict in experimental populations of yeast. Nature.
Used two strains of yeast that were 1) capable of respiration and fermentation or 2) capable only of respiration. The latter was superior in head-to-head mixed competition under chemostat conditions in which glucose was not limiting. The 2 stably coexisted in batch culture owing to the need to ferment in stationary phase and because the cheater (no fermentation) was sensitive to toxic metabolic intermediates produced by its own growth. Also, from the article, “In agreement with our hypothesis, the itness of the cooperator at the level of the metapopulation is both positive frequency-dependent (F1,7 ¼ 55, P , 0.0001; Fig. 4) and negative density-dependent (F1,7 ¼ 41, P , 0.0001; Fig. 4), implying that a metapopulation of cooperators can resist invasion by a rare cheater provided that the number of cells that colonize each patch is below a critical threshold. Positive frequency-dependent selection for cooperation also implies that a rare cooperator cannot invade a
population of cheaters.”
4) Laura, who chose: Federico Prado. and Benjamin Kerr. 2007. THE EVOLUTION OF RESTRAINT IN BACTERIAL BIOFILMS UNDER NONTRANSITIVE COMPETITION. Evolution.
The authors modeled a system similar to the original Kerr paper involving colicin production, resistance, and sensitivity, and found that in structured communities containing these players, the players neither evolve maximum toxicity or resistance. That is, the conditions favor an intermediate state of restraint.
5) Abe, who chose: Brockhurst, MA. Population Bottlenecks Promote Cooperation in Bacterial Biofilms.
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0000634/trackback
Reasoning that kin selection is strongest in smaller populations where the probability of genetic relatedness among individuals is highest, the author tested effects of bottleneck size on the evolution and stability of cooperative traits in the Ps. fluorescens system of Rainey, Travisano, et al. From the text: “In line with predictions, the frequency of evolved cheats within the populations increased with increasing bottleneck size. This result highlights the importance of ecologically mediated population bottlenecks in the maintenance of social traits in microbes.”
Sanjuro, I welcome your addition, and others, please feel free to comment!
“The Evolution of Restraint in Bacterial Biofilms Under Nontransitive Competition”
Notes
In this paper, the authors investigate evolution within toxic bacterial biofilms using a computer simulation representing a nontransitive (rock-paper-scissors) community of 3 E. coli populations (colecin producer, colecin sensitive, colecin resistor)
Using their simulations, they explore how factors known to vary among isolates (such as cost of resistance and level of toxicity) evolve in space. They also explored cases where the level of toxicity is a function of the cost of it. These investigations were done to shed light on how evolutionary changes in the nature of these interactions influenced the behavior of the bacterial community.
Simulation
-Virtual bacteria ‘embedded’ in a square lattice of L x L nodes, individually assigned a state (S:sensitive, P:producer, R:resistant, E:Empty node)
-Randomly nodes were selected and its state changed due to probability (neighborhood is a key factor in this probability)
-Neighborhood: the set of nodes around the focal node (one being changed) that influences the probability of change
-Small neighborhood=natal dispersal, competition for space, toxicity effects spatially restricted
-Large neighborhood=same interactions over greater distances
-Local neighborhood=8 lattice points surrounding focal node, structured community
Findings:
-There is restraint in the average death rate of R guys in structured communities with all three phenotypes present, death rate does not evolve to its minimum, resistant population evolves competitive restraint
*My complaint here is that they ‘re-seeded’ the populations to keep all three strains, otherwise 1 or 2 were quickly lost. This is kind of stacking the deck to get the outcome they want
-They also found that significantly restrained growth rate evolves in the complete structured community only.
-“In a structured environment, toxicity has the effect of “clearing” sensitive cells and allowing (P cells)…prime access to the cleared real estate.” Again, they re-seeded. R cells in this instance evolved toxicity to a high level, but not their maximum value
this = restraint
TAKE HOME MESSAGE:
-In structured communities with all three strains present, in a nontransitive dynamic, the strains don’t evolve the best competitive strategy. They are restrained.
-They argue that selection favors “survival of the weaker.” The weakest die early and the strongest “improve themselves to death.”
Restraint allows the middle guy to live longer and is an evolutionary stable strategy.
Thanks Laura for a clear summary, and I agree with the problems of re-seeding populations to maintain diversity. Sometimes models must be forced to maintain ecology and disallow evolution.
Just a small additive to the blurb Vaughn wrote:
There is the “frozen state” Quasi-equilibrium that he mentions, but there is also a Hyperimmunity Quasi-equilibrium. Basically in this situation the patches (cubes of the grid) survive long enough to collect more toxin + immunity genes by mutation which leads to multiple toxicity. Once there is an accumulation of these toxin + immunity genes the loss of toxin-producing genes and the maitanence of the resistance becames advantagous due to a lower metabolic cost.
Sam — really good addition and an important point.
VC