Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session B7: Evolutionary Dynamics |
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Sponsoring Units: DBP GSNP Chair: Herbert Levine, University of California, San Diego Room: Portland Ballroom 254 |
Monday, March 15, 2010 11:15AM - 11:51AM |
B7.00001: Games microbes play: The game theory behind cooperative sucrose metabolism in yeast Invited Speaker: The origin of cooperation is a central challenge to our understanding of evolution. Microbial interactions can be manipulated in ways that animal interactions cannot, thus leading to growing interest in microbial models of cooperation and competition. In order for the budding yeast \textit{S. cerevisiae} to grow on sucrose, the disaccharide must first be hydrolyzed by the enzyme invertase. This hydrolysis reaction is performed outside of the cytoplasm in the periplasmic space between the plasma membrane and the cell wall. Here we demonstrate that the vast majority ($\sim $99{\%}) of the monosaccharides created by sucrose hydrolysis diffuse away before they can be imported into the cell, thus making invertase production and secretion a cooperative behavior [1]. A mutant cheater strain that does not produce invertase is able to take advantage of and invade a population of wildtype cooperator cells. However, over a wide range of conditions, the wildtype cooperator can also invade a population of cheater cells. Therefore, we observe coexistence between the two strains in well-mixed culture at steady state resulting from the fact that rare strategies outperform common strategies---the defining features of what game theorists call the snowdrift game. A simple model of the cooperative interaction incorporating nonlinear benefits explains the origin of this coexistence. Glucose repression of invertase expression in wildtype cells produces a strategy which is optimal for the snowdrift game---wildtype cells cooperate only when competing against cheater cells. In disagreement with recent theory [2], we find that spatial structure always aids the evolution of cooperation in our experimental snowdrift game. \\[4pt] [1] Gore, J., Youk, H. {\&} van Oudenaarden, A., \textit{Nature }\textbf{459}, 253 -- 256 (2009) \\[0pt] [2] Hauert, C. {\&} Doebeli, M., \textit{Nature} \textbf{428}, 643 -- 646 (2004) [Preview Abstract] |
Monday, March 15, 2010 11:51AM - 12:27PM |
B7.00002: Rates of evolution with and without sex Invited Speaker: In large populations, there can be many beneficial mutations present and new ones appearing each generation: this is typical in microbial populations and likely also for human populations. The evolutionary dynamics is then very different from the conventional picture of stochastic drift of neutral mutations with occasional selective sweeps. Even simple asexual models of the dynamics with multiple beneficial mutations are surprisingly subtle. Sexual recombination between genomes involves, in addition, the interplay between generation and elimination of diversity as well as formation and breakup of linkage between mutations that are near each other on a chromosome . After reviewing the behavior of asexual populations, new results and speculations for the effects of sexual recombination on the rate of evolution will be presented. [Preview Abstract] |
Monday, March 15, 2010 12:27PM - 1:03PM |
B7.00003: Laboratory explorations of evolution Invited Speaker: |
Monday, March 15, 2010 1:03PM - 1:39PM |
B7.00004: Evolutionary adaptation of phenotypic plasticity in a synthetic microbial system Invited Speaker: While phenotypic plasticity -the capability to respond to the environment- is vital to organisms, tests of its adaptation have remained indecisive because constraints and selection in variable environments are unknown and entangled. We show that one can determine the phenotype-fitness landscape that specifies selection on plasticity, by uncoupling the environmental cue and stress in a genetically engineered microbial system. Evolutionary trajectories revealed genetic constraints in a regulatory protein, which imposed cross-environment trade-offs that favored specialization. However, depending on the synchronicity and amplitude of the applied cue and stress variations, adaptation could break constraints, resolve trade-offs, and evolve optimal phenotypes that exhibit qualitatively altered (inverse) responses to the cue. Our results provide a first step to explain the adaptive origins of complex behavior in heterogeneous environments. [Preview Abstract] |
Monday, March 15, 2010 1:39PM - 2:15PM |
B7.00005: Alleles versus genotypes: Genetic interactions and the dynamics of selection in sexual populations Invited Speaker: Physical interactions between amino-acids are essential for protein structure and activity, while protein-protein interactions and regulatory interactions are central to cellular function. As a consequence of these interactions, the combined effect of two mutations can differ from the sum of the individual effects of the mutations. This phenomenon of genetic interaction is known as epistasis. However, the importance of epistasis and its effects on evolutionary dynamics are poorly understood, especially in sexual populations where recombination breaks up existing combinations of alleles to produce new ones. Here, we present a computational model of selection dynamics involving many epistatic loci in a recombining population. We demonstrate that a large number of polymorphic interacting loci can, despite frequent recombination, exhibit cooperative behavior that locks alleles into favorable genotypes leading to a population consisting of a set of competing clones. As the recombination rate exceeds a certain critical value this ``genotype selection'' phase disappears in an abrupt transition giving way to ``allele selection'' - the phase where different loci are only weakly correlated as expected in sexually reproducing populations. Clustering of interacting sets of genes on a chromosome leads to the emergence of an intermediate regime, where localized blocks of cooperating alleles lock into genetic modules. Large populations attain highest fitness at a recombination rate just below critical, suggesting that natural selection might tune recombination rates to balance the beneficial aspect of exploration of genotype space with the breaking up of synergistic allele combinations. [Preview Abstract] |
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