Parallel and divergent genotypic evolution in experimental populations of Ralstonia sp

Convergent genetic adaptation of Escherichia coli in minimal media leads to pleiotropic divergence

Adaptation in an environment can either be beneficial, neutral or disadvantageous in another. To test the genetic basis of pleiotropic behaviour, we evolved six lines of E. coli independently in environments where glucose and galactose were the sole carbon sources, for 300 generations. All six lines in each environment exhibit convergent adaptation in the environment in which they were evolved. However, pleiotropic behaviour was observed in several environmental contexts, including other carbon environments. Genome sequencing reveals that mutations in global regulators rpoB and rpoC cause this pleiotropy. We report three new alleles of the rpoB gene, and one new allele of the rpoC gene. The novel rpoB alleles confer resistance to Rifampicin, and alter motility. Our results show how single nucleotide changes in the process of adaptation in minimal media can lead to wide-scale pleiotropy, resulting in changes in traits that are not under direct selection.

Conserved Metabolic and Evolutionary Themes in Microbial Degradation of Carbamate Pesticides

Carbamate pesticides are widely used as insecticides, nematicides, acaricides, herbicides and fungicides in the agriculture, food and public health sector. However, only a minor fraction of the applied quantity reaches the target organisms. The majority of it persists in the environment, impacting the non-target biota, leading to ecological disturbance. The toxicity of these compounds to biota is mediated through cholinergic and non-cholinergic routes, thereby making their clean-up cardinal. Microbes, specifically bacteria, have adapted to the presence of these compounds by evolving degradation pathways and thus play a major role in their removal from the biosphere. Over the past few decades, various genetic, metabolic and biochemical analyses exploring carbamate degradation in bacteria have revealed certain conserved themes in metabolic pathways like the enzymatic hydrolysis of the carbamate ester or amide linkage, funnelling of aryl carbamates into respective dihydroxy aromatic intermediates, C1 metabolism and nitrogen assimilation. Further, genomic and functional analyses have provided insights on mechanisms like horizontal gene transfer and enzyme promiscuity, which drive the evolution of degradation phenotype. Compartmentalisation of metabolic pathway enzymes serves as an additional strategy that further aids in optimising the degradation efficiency. This review highlights and discusses the conclusions drawn from various analyses over the past few decades; and provides a comprehensive view of the environmental fate, toxicity, metabolic routes, related genes and enzymes as well as evolutionary mechanisms associated with the degradation of widely employed carbamate pesticides. Additionally, various strategies like application of consortia for efficient degradation, metabolic engineering and adaptive laboratory evolution, which aid in improvising remediation efficiency and overcoming the challenges associated with in situ bioremediation are discussed.

The evolution of trait correlations constrains phenotypic adaptation to high CO2 in a eukaryotic alga

Microbes form the base of food webs and drive biogeochemical cycling. Predicting the effects of microbial evolution on global elemental cycles remains a significant challenge due to the sheer number of interacting environmental and trait combinations. Here, we present an approach for integrating multivariate trait data into a predictive model of trait evolution. We investigated the outcome of thousands of possible adaptive walks parameterized using empirical evolution data from the alga Chlamydomonas exposed to high CO₂. We found that the direction of historical bias (existing trait correlations) influenced both the rate of adaptation and the evolved phenotypes (trait combinations). Critically, we use fitness landscapes derived directly from empirical trait values to capture known evolutionary phenomena. This work demonstrates that ecological models need to represent both changes in traits and changes in the correlation between traits in order to accurately capture phytoplankton evolution and predict future shifts in elemental cycling.

Trophic network architecture of root-associated bacterial communities determines pathogen invasion and plant health

Host-associated bacterial communities can function as an important line of defence against pathogens in animals and plants. Empirical evidence and theoretical predictions suggest that species-rich communities are more resistant to pathogen invasions. Yet, the underlying mechanisms are unclear. Here, we experimentally test how the underlying resource competition networks of resident bacterial communities affect invasion resistance to the plant pathogen Ralstonia solanacearum in microcosms and in tomato plant rhizosphere. We find that bipartite resource competition networks are better predictors of invasion resistance compared with resident community diversity. Specifically, communities with a combination of stabilizing configurations (low nestedness and high connectance), and a clear niche overlap with the pathogen, reduce pathogen invasion success, constrain pathogen growth within invaded communities and have lower levels of diseased plants in greenhouse experiments. Bacterial resource competition network characteristics can thus be important in explaining positive diversity-invasion resistance relationships in bacterial rhizosphere communities.

Bacterial genome evolution within a clonal population: from in vitro investigations to in vivo observations

Bacteria are faced with a diversity of environmental stresses that include high salt concentrations, heavy metals and pH fluctuations. Adaptation to resist such stresses is a complex phenomenon that involves global pathways and simultaneous acquisition of multiple unrelated properties. During the last 3 years, the development of new technologies in the field of molecular biology has led to numerous fundamental and quantitative in vitro and in vivo evolutionary studies that have improved our understanding of the principles underlying bacterial adaptations, and helped us develop strategies to cope with the health burden of bacterial virulence. In this review, the authors discuss the evolution of bacteria in the laboratory and in human patients.

Adaptation and heterogeneity of Escherichia coli MC1000 growing in complex environments

In a study aiming to assess bacterial evolution in complex growth media, we evaluated the long-term adaptive response of Escherichia coli MC1000 in Luria-Bertani (LB) medium. Seven parallel populations were founded and followed over 150 days in sequential batch cultures under three different oxygen conditions (defined environments), and 19 evolved forms were isolated. The emergence of forms with enhanced fitness was evident in competition experiments of all evolved forms versus the ancestral strain. The evolved forms were then subjected to phenotypic and genomic analyses relative to the ancestor. Profound changes were found in their phenotypes as well as whole-genome sequences. Interestingly, considerable heterogeneity was found at the intrapopulational level. However, consistently occurring parallel adaptive responses were found across all populations. The evolved forms all contained a mutation in galR, a repressor of the galactose operon. Concomitantly, the new forms revealed enhanced growth on galactose as well as galactose-containing disaccharides. This response was likely driven by the LB medium.

Replaying the tape of life: quantification of the predictability of evolution

The question whether adaptation follows a deterministic route largely prescribed by the environment or can proceed along a large number of alternative trajectories has engaged extensive research over the recent years. Experimental evolution studies enabled by advances in high throughput techniques for genome sequencing and manipulation, along with increasingly detailed mathematical modeling of fitness landscapes, are beginning to allow quantitative exploration of the repeatability of evolutionary trajectories. It is becoming clear that evolutionary trajectories in static correlated fitness landscapes are substantially non-random but the relative contributions of determinism and stochasticity in the evolution of specific phenotypes strongly depend on the specific conditions, particularly the magnitude of the selective pressure and the number of available beneficial mutations.

Real time forecasting of near-future evolution

A metaphor for adaptation that informs much evolutionary thinking today is that of mountain climbing, where horizontal displacement represents change in genotype, and vertical displacement represents change in fitness. If it were known a priori what the 'fitness landscape' looked like, that is, how the myriad possible genotypes mapped onto fitness, then the possible paths up the fitness mountain could each be assigned a probability, thus providing a dynamical theory with long-term predictive power. Such detailed genotype-fitness data, however, are rarely available and are subject to change with each change in the organism or in the environment. Here, we take a very different approach that depends only on fitness or phenotype-fitness data obtained in real time and requires no a priori information about the fitness landscape. Our general statistical model of adaptive evolution builds on classical theory and gives reasonable predictions of fitness and phenotype evolution many generations into the future.

The behavior and significance of degradative plasmids belonging to Inc groups in Pseudomonas within natural environments and microcosms

Over the past few decades, degradative plasmids have been isolated from bacteria capable of degrading a variety of both natural and man-made compounds. Degradative plasmids belonging to three incompatibility (Inc) groups in Pseudomonas (IncP-1, P-7, and P-9) have been well studied in terms of their replication, maintenance, and capacity for conjugative transfer. The host ranges of these plasmids are determined by replication or conjugative transfer systems. The host range of IncP-1 is broad, that of IncP-9 is intermediate, and that of IncP-7 is narrow. To understand the behavior of these plasmids and their hosts in various environments, the survivability of inocula, stability or transferability, and efficiency of biodegradation in environments and microcosms have been monitored. The biodegradation and plasmid transfer in various environments have been observed for all three groups, although the kinds of transconjugants differed with the Inc groups. In some cases, the deletion and amplification of catabolic genes acted to reduce the production of toxic catabolic intermediates, or to increase the activity on a particular catabolic pathway. The combination of degradative genes, the plasmid backbone of each Inc group, and the host of the plasmids is key to the degraders adapting to various hosts or to heterogeneous environments.

Adaptive landscapes in evolving populations of Pseudomonas fluorescens

The repeatability of adaptive evolution depends on the ruggedness of the underlying adaptive landscape. We contrasted the relative ruggedness of two adaptive landscapes by measuring the variance in fitness and metabolic phenotype within and among genetically distinct strains of Pseudomonas fluorescens in two environments differing only in the carbon source provided (glucose vs. xylose). Fitness increased in all lines, plateauing in one environment but not the other. The pattern of variance in fitness among replicate lines was unique to the selection environment; it increased over the course of the experiment in xylose but not in glucose. Metabolic phenotypes displayed two results: (1) populations adapted via changes that were distinctive to their selection environment, and (2) endpoint phenotypes were less variable in glucose than in xylose. These results indicate that although the response to selection is highly repeatable at the level of fitness, the underlying genetic routes taken were different for each environment and more variable in xylose. We suggest that this reflects a more rugged adaptive landscape in xylose compared to glucose. Our study demonstrates the utility of using replicate selection lines with different evolutionary starting points to try and quantify the relative ruggedness of adaptive landscapes.

Experimental evolution with E. coli in diverse resource environments. I. Fluctuating environments promote divergence of replicate populations

BACKGROUND

Environmental conditions affect the topology of the adaptive landscape and thus the trajectories followed by evolving populations. For example, a heterogeneous environment might lead to a more rugged adaptive landscape, making it more likely that replicate populations would evolve toward distinct adaptive peaks, relative to a uniform environment. To date, the influence of environmental variability on evolutionary dynamics has received relatively little experimental study.

RESULTS

We report findings from an experiment designed to test the effects of environmental variability on the adaptation and divergence of replicate populations of E. coli. A total of 42 populations evolved for 2000 generations in 7 environmental regimes that differed in the number, identity, and presentation of the limiting resource. Regimes were organized in two sets, having the sugars glucose and maltose singly and in combination, or glucose and lactose singly and in combination. Combinations of sugars were presented either simultaneously or as temporally fluctuating resource regimes. This design allowed us to compare the effects of resource identity and presentation on the evolutionary trajectories followed by replicate populations. After 2000 generations, the fitness of all populations had increased relative to the common ancestor, but to different extents. Populations evolved in glucose improved the least, whereas populations evolving in maltose or lactose increased the most in their respective sets. Among-population divergence also differed across regimes, with variation higher in those groups that evolved in fluctuating environments than in those that faced constant resource regimens. This divergence under the fluctuating conditions increased between 1000 and 2000 generations, consistent with replicate populations evolving toward distinct adaptive peaks.

CONCLUSIONS

These results support the hypothesis that environmental heterogeneity can give rise to more rugged adaptive landscapes, which in turn promote evolutionary diversification. These results also demonstrate that this effect depends on the form of environmental heterogeneity, with greater divergence when the pairs of resources fluctuated temporally rather than being presented simultaneously.

Evolution of efficient pathways for degradation of anthropogenic chemicals

Anthropogenic compounds used as pesticides, solvents and explosives often persist in the environment and can cause toxicity to humans and wildlife. The persistence of anthropogenic compounds is due to their recent introduction into the environment; microbes in soil and water have had relatively little time to evolve efficient mechanisms for degradation of these new compounds. Some anthropogenic compounds are easily degraded, whereas others are degraded very slowly or only partially, leading to accumulation of toxic products. This review examines the factors that affect the ability of microbes to degrade anthropogenic compounds and the mechanisms by which new pathways emerge in nature. New approaches for engineering microbes with enhanced degradative abilities include assembly of pathways using enzymes from multiple organisms, directed evolution of inefficient enzymes, and genome shuffling to improve microbial fitness under the challenging conditions posed by contaminated environments.

Short-term genome evolution of Listeria monocytogenes in a non-controlled environment

Background

While increasing data on bacterial evolution in controlled environments are available, our understanding of bacterial genome evolution in natural environments is limited. We thus performed full genome analyses on four Listeria monocytogenes, including human and food isolates from both a 1988 case of sporadic listeriosis and a 2000 listeriosis outbreak, which had been linked to contaminated food from a single processing facility. All four isolates had been shown to have identical subtypes, suggesting that a specific L. monocytogenes strain persisted in this processing plant over at least 12 years. While a genome sequence for the 1988 food isolate has been reported, we sequenced the genomes of the 1988 human isolate as well as a human and a food isolate from the 2000 outbreak to allow for comparative genome analyses.

Results

The two L. monocytogenes isolates from 1988 and the two isolates from 2000 had highly similar genome backbone sequences with very few single nucleotide (nt) polymorphisms (1 – 8 SNPs/isolate; confirmed by re-sequencing). While no genome rearrangements were identified in the backbone genome of the four isolates, a 42 kb prophage inserted in the chromosomal comK gene showed evidence for major genome rearrangements. The human-food isolate pair from each 1988 and 2000 had identical prophage sequence; however, there were significant differences in the prophage sequences between the 1988 and 2000 isolates. Diversification of this prophage appears to have been caused by multiple homologous recombination events or possibly prophage replacement. In addition, only the 2000 human isolate contained a plasmid, suggesting plasmid loss or acquisition events. Surprisingly, besides the polymorphisms found in the comK prophage, a single SNP in the tRNA Thr-4 prophage represents the only SNP that differentiates the 1988 isolates from the 2000 isolates.

Conclusion

Our data support the hypothesis that the 2000 human listeriosis outbreak was caused by a L. monocytogenes strain that persisted in a food processing facility over 12 years and show that genome sequencing is a valuable and feasible tool for retrospective epidemiological analyses. Short-term evolution of L. monocytogenes in non-controlled environments appears to involve limited diversification beyond plasmid gain or loss and prophage diversification, highlighting the importance of phages in bacterial evolution.

Preliminary characterization of the normal microbiota of the human vulva using cultivation-independent methods

The objective of this study was to perform a preliminary characterization of the microbial populations of the normal human vulva. Genomic DNA was isolated from samples of the labia majora and labia minora from four healthy women, and sequences of bacterial 16S rRNA genes in each were determined. The sequences were compared with those of known bacterial species to classify the numerically abundant populations in these communities. Even among this limited number of individuals, the microbiota of the human vulva was found to be quite diverse. Each woman had a distinctive microbiota and no single species was common to all women. The microbiota of the labia majora and labia minora differed, although both had appreciable numbers of lactobacilli and strict anaerobes. A greater diversity of populations inhabited the labia majora compared with the labia minora. The results indicated that the microbiota of the vulva includes populations known to be commensals of the microbiota of the skin, colon and vagina, and is much more complex than previously thought, suggesting that more extensive investigations are warranted.

Microbial experiments on adaptive landscapes

The adaptive landscape is one of the most widely used metaphors in evolutionary biology. It is created by plotting fitness against phenotypes or genotypes in a given environment. The shape of the landscape is crucial in predicting the outcome of evolution: whether evolution will result in populations reaching predictable end points, or whether multiple evolutionary outcomes are more likely. In a more applied sense, the landscape will determine whether organisms will evolve to lose 'costly' resistance to antibiotics, herbicides or pesticides when the use of the control agent is stopped. Laboratory populations of microbes allow evolution to be observed in real time and, as such, provide key insights into the topology of adaptive landscapes.

Role of eukaryotic microbiota in soil survival and catabolic performance of the 2,4-D herbicide degrading bacteria Cupriavidus necator JMP134

Cupriavidus necator (formerly Ralstonia eutropha) JMP134, harbouring the catabolic plasmid pJP4, is the best-studied 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide degrading bacterium. A study of the survival and catabolic performance of strain JMP134 in agricultural soil microcosms exposed to high levels of 2,4-D was carried out. When C. necator JMP134 was introduced into soil microcosms, the rate of 2,4-D removal increased only slightly. This correlated with the poor survival of the strain, as judged by 16S rRNA gene terminal restriction fragment length polymorphism (T-RFLP) profiles, and the semi-quantitative detection of the pJP4-borne tfdA gene sequence, encoding the first step in 2,4-D degradation. After 3 days of incubation in irradiated soil microcosms, the survival of strain JMP134 dramatically improved and the herbicide was completely removed. The introduction of strain JMP134 into native soil microcosms did not produce detectable changes in the structure of the bacterial community, as judged by 16S rRNA gene T-RFLP profiles, but provoked a transient increase of signals putatively corresponding to protozoa, as indicated by 18S rRNA gene T-RFLP profiling. Accordingly, a ciliate able to feed on C. necator JMP134 could be isolated after soil enrichment. In native soil microcosms, C. necator JMP134 survived better than Escherichia coli DH5alpha (pJP4) and similarly to Pseudomonas putida KT2442 (pJP4), indicating that species specific factors control the survival of strains harbouring pJP4. The addition of cycloheximide to soil microcosms strongly improved survival of these three strains, indicating that the eukaryotic microbiota has a strong negative effect in bioaugmentation with catabolic bacteria.

Loss of expression of cspC, a cold shock family gene, confers a gain of fitness in Escherichia coli K-12 strains

The CspA family of cold shock genes in Escherichia coli K-12 includes nine paralogs, cspA to cspI. Some of them have been implicated in cold stress adaptation. Screening for mutations among common laboratory E. coli strains showed a high degree of genetic diversity in cspC but not in cspA and cspE. This diversity in cspC was due to a wide spectrum of variations including insertions of IS elements, deletion, and point mutation. Northern analysis of these mutants showed loss of cspC expression in all but one case. Further analysis of the loss-of-function cspC mutants showed that they have a fitness advantage in broth culture after 24 h over their isogenic wild-type derivatives. Conversely, introduction of mutated cspC alleles conferred a competitive fitness advantage to AB1157, a commonly used laboratory strain. This provides the evidence that loss of cspC expression is both necessary and sufficient to confer a gain of fitness as seen in broth culture over 24 h. Together, these results ascribe a novel role in cellular growth at 37 degrees C for CspC, a member of the cold shock domain-containing protein family.

The host acts as a genetic bottleneck during serial infections: an insect-fungal model system

The genetic variation of a pathogen population is a pivotal component of pathogen evolution, having important implications for emerging diseases, nosocomial infections, and laboratory subculturing practices. Furthermore, it is undoubtedly altered during infection of a host. We address this issue using an insect-fungal model system to examine the influence of serial host passage on the genetic variation of a pathogen population. Using amplified fragment length polymorphism, a strain of the opportunistic fungus, Aspergillus flavus, showing initially 98% genetic similarity, was assessed for changes in genetic diversity during repeated passage through Galleria mellonella larvae and compared to that of a parallel population serially subcultured on artificial media. In two independent trials, the genetic diversity of the population passed through the insect dropped significantly, while the genetic variation of the population subcultured on media increased or remained unchanged. However, there were no changes in virulence or the production of protease or aflatoxin, indicating an apparent lack of selection. We suggest that the insect acted as a genetic bottleneck, reducing the genetic diversity of the A. flavus population. The ability of a host to produce a genetic bottleneck in a pathogen population impacts our understanding of emerging diseases, nosocomial infections, and laboratory subculturing practices.

The argRB of Escherichia coli is rare in isolates obtained from natural sources

A single nucleotide polymorphism between Escherichia coli strains K12 and B is known to alter the mechanism by which the arginine repressor regulates arginine biosynthesis, from a regulated system in E. coli K12 to a deregulated system in E. coli B. Laboratory experiments have demonstrated that the different regulatory strategies are selectively favored under different environmental conditions. In this study we analyzed 537 E. coli strains and show that the argR allele in E. coli B, which causes deregulation, is rare in isolates obtained from natural sources. Moreover, sequence analysis of 85 strains shows no evidence of selection at the arginine repressor locus. This illustrates that analysis of sequence data is insufficient to detect selection of uncommon alleles in rare environments.

Parallel adaptive evolution cultures of Escherichia coli lead to convergent growth phenotypes with different gene expression states

Laboratory evolution can be used to address fundamental questions about adaptation to selection pressures and, ultimately, the process of evolution. In this study, we investigated the reproducibility of growth phenotypes and global gene expression states during adaptive evolution. The results from parallel, replicate adaptive evolution experiments of Escherichia coli K-12 MG1655 grown on either lactate or glycerol minimal media showed that (1) growth phenotypes at the endpoint of evolution are convergent and reproducible; (2) endpoints of evolution have different underlying gene expression states; and (3) the evolutionary gene expression response involves a large number of compensatory expression changes and a smaller number of adaptively beneficial expression changes common across evolution strains. Gene expression changes initially showed a large number of differentially expressed genes in response to an environmental change followed by a return of most genes to a baseline expression level, leaving a relatively small set of differentially expressed genes at the endpoint that varied between evolved populations.

Alternative Designs and the Evolution of Functional Diversity

According to conventional wisdom, functional diversity is exclusively a consequence of species having evolved adaptations to fill different niches within a heterogeneous environment. This view anticipates only one optimal combination of trait values in a given environment, but it is also conceivable that alternative designs of equal fitness in the same environment might evolve. To investigate that possibility, we use a genetic algorithm to search for optimal combinations of 34 functional traits in a realistic model of tree seedling growth and survival. We show that separate lineages of seedlings evolving in identical environments result in many alternative functional designs of approximately equal fitness.

Deletion of the yiaMNO transporter genes affects the growth characteristics of Escherichia coli K-12

Binding-protein-dependent secondary transporters make up a unique transport protein family. They use a solute-binding protein in proton-motive-force-driven transport. Only a few systems have been functionally analysed. TheyiaMNOgenes ofEscherichia coliK-12 encode one family member that transports the rare pentosel-xylulose. Its physiological role is unknown, since wild-typeE. coliK-12 does not utilizel-xylulose as sole carbon source. Deletion of theyiaMNOgenes inE. coliK-12 strain MC4100 resulted in remarkable changes in the transition from exponential growth to the stationary phase, high-salt survival and biofilm formation.

Molecular analysis of bacterial community based on 16S rDNA and functional genes in activated sludge enriched with 2,4-dichlorophenoxyacetic acid (2,4-D) under different cultural conditions

Differential emergence and diversity of bacterial communities from activated sludge in response to varied cultural conditions using 2,4-dichlorophenoxyacetic acid (2,4-D) were investigated by coupling molecular analyses based on 16S rDNA with functional genes. We employed three different cultural conditions: (1) a culture sequentially fed a high concentration (300 mg/L) of 2,4-D (HS); (2) a culture continuously fed a low concentration (10 mg/L) of 2,4-D (LC); and (3) a serial batch culture in which 1% (v/v) of culture was transferred to a fresh medium containing a high concentration (300 mg/L) of 2,4-D (HB). The HS and LC bioreactors were operated for 3 months and HB was repeatedly transferred for 1 month. The 2,4-D was stably degraded under all the cultural conditions tested. PCR amplification and cloning-based analysis of functional genes using community DNAs from the cultures revealed five different oxygenase genes that may be involved in the initial step of 2,4-D degradation. All five gene-types were present in HS, while one of the five genes, type V (tftA) was not detected in LC. Quantitative PCR analysis showed that in HS, Ralstonia eutropha JMP 134 type-tfdA4 (type I) was the most abundant in copy number (2.0 +/- 0.1 x 10(7) copies/microg DNA) followed by RASC type-tfdA (type II) (1.8 +/- 1.0 x 10(6) copies/microg DNA), putative cadA-like gene (type IV) (2.6 +/- 0.8 x 10(5) copies/microg DNA), cadA gene (type III) (1.3 +/- 1.0 x 10(4) copies/microg DNA), and tftA gene (type V) (3.5 +/- 1.1 x 10(3) copies/microg DNA). Similar results were obtained in LC. In contrast, HB contained only type I and type III genes, and the type I gene was five orders of magnitude greater in copy number than the type III gene. Denaturing gel gradient electrophoresis (DGGE) analysis of PCR, amplified 16S rDNA fragments of bacterial communities in the three different cultures showed low similarity coefficient values (< or =0.35) when compared to the original activated sludge, suggesting that 2,4-D amendment caused a drastic change in the bacterial community. Particularly, HB showed only six bands (16-18 bands in the other cultures) and very low similarity coefficient values when compared to the other communities (0.10 to HS, 0.17 to LC, and 0.0 to original sludge). These results indicated that serial batch culturing (HB) resulted in a phylogenetically limited number of 2,4-D degrading bacteria carrying limited catabolic genes whereas more diverse 2,4-D degraders and catabolic genes were present in HS and LC. Therefore, the approach used for monitoring should be taken into account when one evaluates the population dynamics of contaminant-degrading bacteria at bioremediation sites.

Evolutionary genomics of ecological specialization

We used a combination of genomic techniques to monitor chromosomal evolution across hundreds of generations as Escherichia coli adapted to growth-limiting concentrations of either lactulose, methyl-galactoside, or a 72:28 mixture of the two. DNA microarrays identified 8 unique duplications and 16 unique deletions among 42 evolvants from 23 chemostat experiments. Each mutation was confirmed by sequencing PCR-amplified flanking genomic DNA and, except for one deletion, an insertion sequence was found at the break point. vPCR of insertion sequences identified these same mutations and 16 additional insertions (all confirmed by sequencing). The pattern of genomic evolution is highly reproducible. Statistical analyses show that duplications at lac and mutations in mgl are adaptations specific to lactulose and to methyl-galactoside, respectively. Adaptation to mixed sugars is characterized by similar mutations, but lac duplications and mgl mutations usually arise in different backgrounds, producing ecological specialists for each sugar. This suggests that an antagonistic pleiotropic tradeoff between duplications at lac and mutations in mgl retards the evolution of generalists. Other mutations that repeatedly appear in replicate experiments are adaptations to the chemostat environment and are not specific to one or the other sugar.

Microbial astronauts: assembling microbial communities for advanced life support systems

Extension of human habitation into space requires that humans carry with them many of the microorganisms with which they coexist on Earth. The ubiquity of microorganisms in close association with all living things and biogeochemical processes on Earth predicates that they must also play a critical role in maintaining the viability of human life in space. Even though bacterial populations exist as locally adapted ecotypes, the abundance of individuals in microbial species is so large that dispersal is unlikely to be limited by geographical barriers on Earth (i.e., for most environments "everything is everywhere" given enough time). This will not be true for microbial communities in space where local species richness will be relatively low because of sterilization protocols prior to launch and physical barriers between Earth and spacecraft after launch. Although community diversity will be sufficient to sustain ecosystem function at the onset, richness and evenness may decline over time such that biological systems either lose functional potential (e.g., bioreactors may fail to reduce BOD or nitrogen load) or become susceptible to invasion by human-associated microorganisms (pathogens) over time. Research at the John F. Kennedy Space Center has evaluated fundamental properties of microbial diversity and community assembly in prototype bioregenerative systems for NASA Advanced Life Support. Successional trends related to increased niche specialization, including an apparent increase in the proportion of nonculturable types of organisms, have been consistently observed. In addition, the stability of the microbial communities, as defined by their resistance to invasion by human-associated microorganisms, has been correlated to their diversity. Overall, these results reflect the significant challenges ahead for the assembly of stable, functional communities using gnotobiotic approaches, and the need to better define the basic biological principles that define ecosystem processes in the space environment.

Macroevolution simulated with autonomously replicating computer programs

The process of adaptation occurs on two timescales. In the short term, natural selection merely sorts the variation already present in a population, whereas in the longer term genotypes quite different from any that were initially present evolve through the cumulation of new mutations. The first process is described by the mathematical theory of population genetics. However, this theory begins by defining a fixed set of genotypes and cannot provide a satisfactory analysis of the second process because it does not permit any genuinely new type to arise. The evolutionary outcome of selection acting on novel variation arising over long periods is therefore difficult to predict. The classical problem of this kind is whether 'replaying the tape of life' would invariably lead to the familiar organisms of the modern biota. Here we study the long-term behaviour of populations of autonomously replicating computer programs and find that the same type, introduced into the same simple environment, evolves on any given occasion along a unique trajectory towards one of many well-adapted end points.

Dynamics of genome architecture in Rhizobium sp. strain NGR234

Bacterial genomes are usually partitioned in several replicons, which are dynamic structures prone to mutation and genomic rearrangements, thus contributing to genome evolution. Nevertheless, much remains to be learned about the origins and dynamics of the formation of bacterial alternative genomic states and their possible biological consequences. To address these issues, we have studied the dynamics of the genome architecture in Rhizobium sp. strain NGR234 and analyzed its biological significance. NGR234 genome consists of three replicons: the symbiotic plasmid pNGR234a (536,165 bp), the megaplasmid pNGR234b (>2,000 kb), and the chromosome (>3,700 kb). Here we report that genome analyses of cell siblings showed the occurrence of large-scale DNA rearrangements consisting of cointegrations and excisions between the three replicons. As a result, four new genomic architectures have emerged. Three consisted of the cointegrates between two replicons: chromosome-pNGR234a, chromosome-pNGR234b, and pNGR234a-pNGR234b. The other consisted of a cointegrate of the three replicons (chromosome-pNGR234a-pNGR234b). Cointegration and excision of pNGR234a with either the chromosome or pNGR234b were studied and found to proceed via a Campbell-type mechanism, mediated by insertion sequence elements. We provide evidence showing that changes in the genome architecture did not alter the growth and symbiotic proficiency of Rhizobium derivatives.

Mechanisms causing rapid and parallel losses of ribose catabolism in evolving populations of Escherichia coli B

Twelve populations of Escherichia coli B all lost D-ribose catabolic function during 2,000 generations of evolution in glucose minimal medium. We sought to identify the population genetic processes and molecular genetic events that caused these rapid and parallel losses. Seven independent Rbs(-) mutants were isolated, and their competitive fitnesses were measured relative to that of their Rbs(+) progenitor. These Rbs(-) mutants were all about 1 to 2% more fit than the progenitor. A fluctuation test revealed an unusually high rate, about 5 x 10(-5) per cell generation, of mutation from Rbs(+) to Rbs(-), which contributed to rapid fixation. At the molecular level, the loss of ribose catabolic function involved the deletion of part or all of the ribose operon (rbs genes). The physical extent of the deletion varied between mutants, but each deletion was associated with an IS150 element located immediately upstream of the rbs operon. The deletions apparently involved transposition into various locations within the rbs operon; recombination between the new IS150 copy and the one upstream of the rbs operon then led to the deletion of the intervening sequence. To confirm that the beneficial fitness effect was caused by deletion of the rbs operon (and not some undetected mutation elsewhere), we used P1 transduction to restore the functional rbs operon to two Rbs(-) mutants, and we constructed another Rbs(-) strain by gene replacement with a deletion not involving IS150. All three of these new constructs confirmed that Rbs(-) mutants have a competitive advantage relative to their Rbs(+) counterparts in glucose minimal medium. The rapid and parallel evolutionary losses of ribose catabolic function thus involved both (i) an unusually high mutation rate, such that Rbs(-) mutants appeared repeatedly in all populations, and (ii) a selective advantage in glucose minimal medium that drove these mutants to fixation.

Rapid phenotypic change and diversification of a soil bacterium during 1000 generations of experimental evolution

Evolutionary pathways open to even relatively simple organisms, such as bacteria, may lead to complex and unpredictable phenotypic changes, both adaptive and non-adaptive. The evolutionary pathways taken by 18 populations of Ralstonia strain TFD41 while they evolved in defined environments for 1000 generations were examined. Twelve populations evolved in liquid media, while six others evolved on agar surfaces. Phenotypic analyses of these derived populations identified some changes that were consistent across all populations and others that differed among them. The evolved populations all exhibited morphological changes in their cell envelopes, including reductions of the capsule in each population and reduced prostheca-like surface structures in most populations. Mean cell length increased in most populations (in one case by more than fourfold), although a few populations evolved shorter cells. Carbon utilization profiles were variable among the evolved populations, but two distinct patterns were correlated with genetic markers introduced at the outset of the experiment. Fatty acid methyl ester composition was less variable across populations, but distinct patterns were correlated with the two physical environments. All 18 populations evolved greatly increased sensitivity to bile salts, and all but one had increased adhesion to sand; both patterns consistent with changes in the outer envelope. This phenotypic diversity contrasts with the fairly uniform increases in competitive fitness observed in all populations. This diversity may represent a set of equally probable adaptive solutions to the selective environment; it may also arise from the chance fixation of non-adaptive mutations that hitchhiked with a more limited set of beneficial mutations.

Studies of Adaptive Radiation Using Model Microbial Systems

Adaptive radiation is a fundamental process in the evolution of biodiversity. The effects of seasonality, resource partitioning, and spatial heterogeneity have been examined experimentally using evolving populations of microbes. In all environmental conditions examined, ecological interactions have large effect on the likelihood and outcome of adaptive radiation. Adaptive radiation in seasonal environments can arise because of demographic trade-offs and excretion of metabolites. Resource partitioning can occur even in the absence of temporal or spatial heterogeneity. Spatial heterogeneity arising via growth is sufficient to generate microenvironments and subsequent adaptive radiation. However, ecological interactions maintaining diversity can be sensitive to slight alterations of the environmental conditions. Laboratory populations of microbes are ideal model systems to test the factors essential for adaptive radiation.

Replicability and recurrence in the experimental evolution of a group I ribozyme

In order to explore the variety of possible responses available to a ribozyme population evolving a novel phenotype, five Tetrahymena thermophila group I intron ribozyme pools were evolved in parallel for cleavage of a DNA oligonucleotide. These ribozyme populations were propagated under identical conditions and characterized when they reached apparent phenotypic plateaus; the populations that reached the highest plateau showed a near 100-fold improvement in DNA cleavage activity. A detailed characterization of the evolved response in these populations reveals at least two distinct phenotypic trajectories emerging as a result of the imposed selection. Not only do these distinct solutions exhibit differential DNA cleavage activity, but they also exhibit a very different correlation with a related, but unselected, phenotype: RNA cleavage activity. In turn, each of these trajectories is underwritten by differing genotypic profiles. This study underscores the complex network of possible trajectories through sequence space available to an evolving population and uncovers the diversity of solutions that result when the process of experimental evolution is repeated multiple times in a simple, engineered system.

TRAP transporters: an ancient family of extracytoplasmic solute-receptor-dependent secondary active transporters

Tripartite ATP-independent periplasmic transporters (TRAP-T) represent a novel type of secondary active transporter that functions in conjunction with an extracytoplasmic solute-binding receptor. The best characterized TRAP-T family member is from Rhodobacter capsulatus and is specific for C4-dicarboxylates [Forward, J. A., Behrendt, M. C., Wyborn, N. R., Cross, R. & Kelly, D. J. (1997). J Bacteriol 179, 5482-5493]. It consists of three essential proteins, DctP, a periplasmic C4-dicarboxylate-binding receptor, and two integral membrane proteins, DctM and DctQ, which probably span the membrane 12 and 4 times, respectively. Homologues of DctM, DctP and DctQ were identified in all major bacterial subdivisions as well as in archaea. An orphan DctP homologue in the Gram-positive bacterium Bacillus subtilis may serve as a receptor for a two-component transcriptional regulatory system rather than as a constituent of a TRAP-T system. Phylogenetic data suggest that all present day TRAP-T systems probably evolved from a single ancestral transporter with minimal shuffling of constituents between systems. Homologous TRAP-T constituents exhibit decreasing degrees of sequence identity in the order DctM > DctP > DctQ. DctM appears to belong to a large superfamily of transporters, the ion transporter (IT) superfamily, one member of which can function by either protonmotive force- or ATP-dependent energization. It is proposed that IT superfamily members exhibit the unusual capacity to function in conjunction with auxiliary proteins that modify the transport process by providing (i) high-affinity solute reception, (ii) altered energy coupling and (iii) additional yet to be defined functions.

Substitution, insertion, deletion, suppression, and altered substrate specificity in functional protocatechuate 3,4-dioxygenases

Protocatechuate 3,4-dioxygenase is a member of a family of bacterial enzymes that cleave the aromatic rings of their substrates between two adjacent hydroxyl groups, a key reaction in microbial metabolism of varied environmental chemicals. In an appropriate genetic background, it is possible to select for Acinetobacter strains containing spontaneous mutations blocking expression of pcaH or -G, genes encoding the alpha and beta subunits of protocatechuate 3, 4-dioxygenase. The crystal structure of the Acinetobacter oxygenase has been determined, and this knowledge affords us the opportunity to understand how mutations alter function in the enzyme. An earlier investigation had shown that a large fraction of spontaneous mutations inactivating Acinetobacter protocatechuate oxygenase are either insertions or large deletions. Therefore, the prior procedure of mutant selection was modified to isolate Acinetobacter strains in which mutations within pcaH or -G cause a heat-sensitive phenotype. These mutations affected residues distributed throughout the linear amino acid sequences of PcaH and PcaG and impaired the dioxygenase to various degrees. Four of 16 mutants had insertions or deletions in the enzyme ranging in size from 1 to 10 amino acid residues, highlighting areas of the protein where large structural changes can be tolerated. To further understand how protein structure influences function, we isolated strains in which the phenotypes of three different deletion mutations in pcaH or -G were suppressed either by a spontaneous mutation or by a PCR-generated random mutation introduced into the Acinetobacter chromosome by natural transformation. The latter procedure was also used to identify a single amino acid substitution in PcaG that conferred activity towards catechol sufficient for growth with benzoate in a strain in which catechol 1,2-dioxygenase was inactivated.