Mutation Rate and Effective Population Size of the Model Cooperative Bacterium Myxococcus xanthus.

Intrinsic rates of genetic mutation have diverged greatly across taxa and exhibit statistical associations with several other parameters and features. These include effective population size (Ne), genome size, and gametic multicellularity, with the latter being associated with both increased mutation rates and decreased effective population sizes. However, data sufficient to test for possible relationships between microbial multicellularity and mutation rate (µ) are lacking.

An inexpensive, high-throughput μPAD assay of microbial growth rate and motility on solid surfaces using Saccharomyces cerevisiae and Escherichia coli as model organisms.

Many microbial phenotypes are differentially or exclusively expressed on agar surfaces, including biofilms, motility, and sociality. However, agar-based assays are limited by their low throughput, which increases costs, lab waste, space requirements, and the time required to conduct experiments. Here, we demonstrate the use of wax-printed microfluidic paper-based analytical devices (μPADs) to measure linear growth rate of microbes on an agar growth media as a means of circumventing the aforementioned limitations.

Effect of Stressors on the Carrying Capacity of Spatially Distributed Metapopulations.

Stressors such as antibiotics, herbicides, and pollutants are becoming increasingly common in the environment. The effects of stressors on populations are typically studied in homogeneous, nonspatial settings. However, most populations in nature are spatially distributed over environmentally heterogeneous landscapes with spatially restricted dispersal.

Evolutionary Rescue from a Wave of Biological Invasion.

Evolution can potentially rescue populations from being driven extinct by biological invasions, but predictions for this occurrence are generally lacking. Here I derive theoretical predictions for evolutionary rescue of a resident population experiencing invasion from an introduced competitor that spreads over its introduced range as a traveling spatial wave that displaces residents. I compare the likelihood of evolutionary rescue from invasion for two modes of competition: exploitation and interference competition.

Carrying capacity of a spatially-structured population: Disentangling the effects of dispersal, growth parameters, habitat heterogeneity and habitat clustering.

Carrying capacity, K, is a fundamental quantity in theoretical and applied ecology. When populations are distributed over space, carrying capacity becomes a complicated function of local, global and nearby environments, dispersal rate, and the relationship between population growth parameters, e.g.

Microbial expansion-collision dynamics promote cooperation and coexistence on surfaces.

Microbes colonizing a surface often experience colony growth dynamics characterized by an initial phase of spatial clonal expansion followed by collision between neighboring colonies to form potentially genetically heterogeneous boundaries. For species with life cycles consisting of repeated surface colonization and dispersal, these spatially explicit "expansion-collision dynamics" generate periodic transitions between two distinct selective regimes, "expansion competition" and "boundary competition," each one favoring a different growth strategy. We hypothesized that this dynamic could promote stable coexistence of expansion- and boundary-competition specialists by generating time-varying, negative frequency-dependent selection that insulates both types from extinction.

Carrying capacity in a heterogeneous environment with habitat connectivity.

A large body of theory predicts that populations diffusing in heterogeneous environments reach higher total size than if non-diffusing, and, paradoxically, higher size than in a corresponding homogeneous environment. However, this theory and its assumptions have not been rigorously tested. Here, we extended previous theory to include exploitable resources, proving qualitatively novel results, which we tested experimentally using spatially diffusing laboratory populations of yeast.

Propagation and control of gene expression noise with non-linear translation kinetics.

Gene expression is a stochastic process involving small numbers of molecules. As a consequence, cells in a clonal population vary randomly and sometimes substantially from one another in the concentration of mRNA and protein species, a phenomenon known as gene expression noise. Previous theoretical models of gene expression noise assumed that translation is first-order (linear) in mRNA concentration, leading to unfiltered propagation of mRNA noise to the protein level.

Noise slows the rate of Michaelis-Menten reactions.

Microscopic randomness and the small volumes of living cells combine to generate random fluctuations in molecule concentrations called "noise". Here I investigate the effect of noise on biochemical reactions obeying Michaelis-Menten kinetics, concluding that substrate noise causes these reactions to slow. I derive a general expression for the time evolution of the joint probability density of chemical species in arbitrarily connected networks of non-linear chemical reactions in small volumes.

Crowded growth leads to the spontaneous evolution of semistable coexistence in laboratory yeast populations.

Identifying the mechanisms that create and maintain biodiversity is a central challenge in biology. Stable diversification of microbial populations often requires the evolution of differences in resource utilization. Alternatively, coexistence can be maintained by specialization to exploit spatial heterogeneity in the environment.

Nonadaptive processes can create the appearance of facultative cheating in microbes.

Adaptations to social life may take the form of facultative cheating, in which organisms cooperate with genetically similar individuals but exploit others. Consistent with this possibility, many strains of social microbes like Myxococcus bacteria and Dictyostelium amoebae have equal fitness in single-genotype social groups but outcompete other strains in mixed-genotype groups. Here we show that these observations are also consistent with an alternative, nonadaptive scenario: kin selection-mutation balance under local competition.

Spatial population expansion promotes the evolution of cooperation in an experimental Prisoner's Dilemma.

Cooperation is ubiquitous in nature, but explaining its existence remains a central interdisciplinary challenge. Cooperation is most difficult to explain in the Prisoner's Dilemma game, where cooperators always lose in direct competition with defectors despite increasing mean fitness. Here we demonstrate how spatial population expansion, a widespread natural phenomenon, promotes the evolution of cooperation.

Origins of altruism diversity II: Runaway coevolution of altruistic strategies via "reciprocal niche construction".

Understanding the evolution of altruism requires knowledge of both its constraints and its drivers. Here we show that, paradoxically, ecological constraints on altruism may ultimately be its strongest driver. We construct a two-trait, coevolutionary adaptive dynamics model of social evolution in a genetically structured population with local resource competition.

Origins of altruism diversity I: The diverse ecological roles of altruistic strategies and their evolutionary responses to local competition.

Nature abounds with a rich variety of altruistic strategies, including public resource enhancement, resource provisioning, communal foraging, alarm calling, and nest defense. Yet, despite their vastly different ecological roles, current theory typically treats diverse altruistic traits as being favored under the same general conditions. Here, we introduce greater ecological realism into social evolution theory and find evidence of at least four distinct modes of altruism.

Detecting the molecular signature of social conflict: theory and a test with bacterial quorum sensing genes.

Extending social evolution theory to the molecular level opens the door to an unparalleled abundance of data and statistical tools for testing alternative hypotheses about the long-term evolutionary dynamics of cooperation and conflict. To this end, we take a collection of known sociality genes (bacterial quorum sensing [QS] genes), model their evolution in terms of patterns that are detectable using gene sequence data, and then test model predictions using available genetic data sets. Specifically, we test two alternative hypotheses of social conflict: (1) the "adaptive" hypothesis that cheaters are maintained in natural populations by frequency-dependent balancing selection as an evolutionarily stable strategy and (2) the "evolutionary null" hypothesis that cheaters are opposed by purifying kin selection yet exist transiently because of their recurrent introduction into populations by mutation (i.

Kin selection-mutation balance: a model for the origin, maintenance, and consequences of social cheating.

Social conflict, in the form of intraspecific selfish "cheating," has been observed in a number of natural systems. However, a formal, evolutionary genetic theory of social cheating that provides an explanatory, predictive framework for these observations is lacking. Here we derive the kin selection-mutation balance, which provides an evolutionary null hypothesis for the statics and dynamics of cheating.

A generalization of Hamilton's rule for the evolution of microbial cooperation.

Hamilton's rule states that cooperation will evolve if the fitness cost to actors is less than the benefit to recipients multiplied by their genetic relatedness. This rule makes many simplifying assumptions, however, and does not accurately describe social evolution in organisms such as microbes where selection is both strong and nonadditive. We derived a generalization of Hamilton's rule and measured its parameters in Myxococcus xanthus bacteria.

The components of kin competition.

It is well known that competition among kin alters the rate and often the direction of evolution in subdivided populations. Yet much remains unclear about the ecological and demographic causes of kin competition, or what role life cycle plays in promoting or ameliorating its effects. Using the multilevel Price equation, I derive a general equation for evolution in structured populations under an arbitrary intensity of kin competition.

Multilevel and kin selection in a connected world.

Wild et al. argue that the evolution of reduced virulence can be understood from the perspective of inclusive fitness, obviating the need to evoke group selection as a contributing causal factor. Although they acknowledge the mathematical equivalence of the inclusive fitness and multilevel selection approaches, they conclude that reduced virulence can be viewed entirely as an individual-level adaptation by the parasite.

Toward a population genetic framework of developmental evolution: the costs, limits, and consequences of phenotypic plasticity.

Adaptive phenotypic plasticity allows organisms to cope with environmental variability, and yet, despite its adaptive significance, phenotypic plasticity is neither ubiquitous nor infinite. In this review, we merge developmental and population genetic perspectives to explore costs and limits on the evolution of plasticity. Specifically, we focus on the role of modularity in developmental genetic networks as a mechanism underlying phenotypic plasticity, and apply to it lessons learned from population genetic theory on the interplay between relaxed selection and mutation accumulation.

The genetic signature of conditional expression.

Conditionally expressed genes have the property that every individual in a population carries and transmits the gene, but only a fraction, , expresses the gene and exposes it to natural selection. We show that a consequence of this pattern of inheritance and expression is a weakening of the strength of natural selection, allowing deleterious mutations to accumulate within and between species and inhibiting the spread of beneficial mutations. We extend previous theory to show that conditional expression in space and time have approximately equivalent effects on relaxing the strength of selection and that the effect holds in a spatially heterogeneous environment even with low migration rates among patches.

Exposure pathway evaluations for sites that processed asbestos-contaminated vermiculite.

The Agency for Toxic Substances and Disease Registry (ATSDR) is currently evaluating the potential public health impacts associated with the processing of asbestos-contaminated vermiculite at various facilities around the country. Vermiculite ore contaminated with significant levels of asbestos was mined and milled in Libby, Montana, from the early 1920s until 1990. The majority of the Libby ore was then shipped to processing facilities for exfoliation.