Adaptation varies through space and time in a coevolving host-parasitoid interaction

One of the central challenges of evolutionary biology is to understand how coevolution organizes biodiversity over complex geographic landscapes. Most species are collections of genetically differentiated populations, and these populations have the potential to become adapted to their local environments in different ways. The geographic mosaic theory of coevolution incorporates this idea by proposing that spatial variation in natural selection and gene flow across a landscape can shape local coevolutionary dynamics. These effects may be particularly strong when populations differ across productivity gradients, where gene flow will often be asymmetric among populations. Conclusive empirical tests of this theory have been particularly difficult to perform because they require knowledge of patterns of gene flow, historical population relationships and local selection pressures. We have tested these predictions empirically using a model community of bacteria and bacteriophage (viral parasitoids of bacteria). We show that gene flow across a spatially structured landscape alters coevolution of parasitoids and their hosts and that the resulting patterns of adaptation can fluctuate in both space and time.     

Local dispersal promotes biodiversity in a real-life game of rock-paper-scissors

One of the central aims of ecology is to identify mechanisms that maintain biodiversity. Numerous theoretical models have shown that competing species can coexist if ecological processes such as dispersal, movement, and interaction occur over small spatial scales. In particular, this may be the case for non-transitive communities, that is, those without strict competitive hierarchies. The classic non-transitive system involves a community of three competing species satisfying a relationship similar to the children's game rock-paper-scissors, where rock crushes scissors, scissors cuts paper, and paper covers rock. Such relationships have been demonstrated in several natural systems. Some models predict that local interaction and dispersal are sufficient to ensure coexistence of all three species in such a community, whereas diversity is lost when ecological processes occur over larger scales. Here, we test these predictions empirically using a non-transitive model community containing three populations of Escherichia coli. We find that diversity is rapidly lost in our experimental community when dispersal and interaction occur over relatively large spatial scales, whereas all populations coexist when ecological processes are localized.     

Coulomb-blockade transport in single-crystal organic thin-film transistors

Coulomb-blockade transport--whereby the Coulomb interaction between electrons can prohibit their transport around a circuit--occurs in systems in which both the tunnel resistance, Rb between neighbouring sites is large (>h/e2) and the charging energy, E(C) (E(C) = e2/2C, where C is the capacitance of the site), of an excess electron on a site is large compared to kT. (Here e is the charge of an electron, k is Boltzmann's constant, and h is Planck's constant.) The nature of the individual sites--metallic, superconducting, semiconducting or quantum dot--is to first order irrelevant for this phenomenon to be observed. Coulomb blockade has also been observed in two-dimensional arrays of normal-metal tunnel junctions, but the relatively large capacitances of these micrometre-sized metal islands results in a small charging energy, and so the effect can be seen only at extremely low temperatures. Here we demonstrate that organic thin-film transistors based on highly ordered molecular materials can, to first order, also be considered as an array of sites separated by tunnel resistances. And as a result of the sub-nanometre sizes of the sites (the individual molecules), and hence their small capacitances, the charging energy dominates at room temperature. Conductivity measurements as a function of both gate bias and temperature reveal the presence of thermally activated transport, consistent with the conventional model of Coulomb blockade.     

Enantiospecific electrodeposition of a chiral catalyst

Many biomolecules are chiral--they can exist in one of two enantiomeric forms that only differ in that their structures are mirror images of each other. Because only one enantiomer tends to be physiologically active while the other is inactive or even toxic, drug compounds are increasingly produced in an enantiomerically pure form using solution-phase homogeneous catalysts and enzymes. Chiral surfaces offer the possibility of developing heterogeneous enantioselective catalysts that can more readily be separated from the products and reused. In addition, such surfaces might serve as electrochemical sensors for chiral molecules. To date, chiral surfaces have been obtained by adsorbing chiral molecules or slicing single crystals so that they exhibit high-index faces, and some of these surfaces act as enantioselective heterogeneous catalysts. Here we show that chiral surfaces can also be produced through electrodeposition, a relatively simple solution-based process that resembles biomineralization in that organic molecules adsorbed on surfaces have profound effects on the morphology of the inorganic deposits. When electrodepositing a copper oxide film on an achiral gold surface in the presence of tartrate ion in the deposition solution, the chirality of the ion determines the chirality of the deposited film, which in turn determines the film's enantiospecificity during subsequent electrochemical oxidation reactions.     

Local migration promotes competitive restraint in a host-pathogen 'tragedy of the commons'

Fragmented populations possess an intriguing duplicity: even if subpopulations are reliably extinction-prone, asynchrony in local extinctions and recolonizations makes global persistence possible. Migration is a double-edged sword in such cases: too little migration prevents recolonization of extinct patches, whereas too much synchronizes subpopulations, raising the likelihood of global extinction. Both edges of this proverbial sword have been explored by manipulating the rate of migration within experimental populations. However, few experiments have examined how the evolutionary ecology of fragmented populations depends on the pattern of migration. Here, we show that the migration pattern affects both coexistence and evolution within a community of bacterial hosts (Escherichia coli) and viral pathogens (T4 coliphage) distributed across a large network of subpopulations. In particular, different patterns of migration select for distinct pathogen strategies, which we term 'rapacious' and 'prudent'. These strategies define a 'tragedy of the commons': rapacious phage displace prudent variants for shared host resources, but prudent phage are more productive when alone. We find that prudent phage dominate when migration is spatially restricted, while rapacious phage evolve under unrestricted migration. Thus, migration pattern alone can determine whether a de novo tragedy of the commons is resolved in favour of restraint.     

A taxa-area relationship for bacteria

A positive power-law relationship between the number of species in an area and the size of that area has been observed repeatedly in plant and animal communities. This species-area relationship, thought to be one of the few laws in ecology, is fundamental to our understanding of the distribution of global biodiversity. However, such a relationship has not been reported for bacteria, and little is known regarding the spatial distribution of bacteria, relative to what is known of plants and animals. Here we describe a taxa-area relationship for bacteria over a scale of centimetres to hundreds of metres in salt marsh sediments. We found that bacterial communities located close together were more similar in composition than communities located farther apart, and we used the decay of community similarity with distance to show that bacteria can exhibit a taxa-area relationship. This relationship was driven primarily by environmental heterogeneity rather than geographic distance or plant composition.