Negative frequency‐dependent selection maintains coexisting genotypes during fluctuating selection

Natural environments are rarely static; rather selection can fluctuate on timescales ranging from hours to centuries. However, it is unclear how adaptation to fluctuating environments differs from adaptation to constant environments at the genetic level. For bacteria, one key axis of environmental variation is selection for planktonic or biofilm modes of growth. We conducted an evolution experiment with Burkholderia cenocepacia, comparing the evolutionary dynamics of populations evolving under constant selection for either biofilm formation or planktonic growth with populations in which selection fluctuated between the two environments on a weekly basis. Populations evolved in the fluctuating environment shared many of the same genetic targets of selection as those evolved in constant biofilm selection, but were genetically distinct from the constant planktonic populations. In the fluctuating environment, mutations in the biofilm‐regulating genes wspA and rpfR rose to high frequency in all replicate populations. A mutation in wspA first rose rapidly and nearly fixed during the initial biofilm phase but was subsequently displaced by a collection of rpfR mutants upon the shift to the planktonic phase. The wspA and rpfR genotypes coexisted via negative frequency‐dependent selection around an equilibrium frequency that shifted between the environments. The maintenance of coexisting genotypes in the fluctuating environment was unexpected. Under temporally fluctuating environments, coexistence of two genotypes is only predicted under a narrow range of conditions, but the frequency‐dependent interactions we observed provide a mechanism that can increase the likelihood of coexistence in fluctuating environments.     

Ecological succession in long-term experimentally evolved biofilms produces synergistic communities

Many biofilm populations are known for their exceptional biodiversity, but the relative contributions of the forces that could produce this diversity are poorly understood. This uncertainty grows in the old, well-established communities found on many natural surfaces and in long-term, chronic infections. If the prevailing interactions among species within biofilms are positive, productivity should increase with diversity, but if they tend towards competition or antagonism, productivity should decrease. Here, we describe the parallel evolution of synergistic communities derived from a clone of Burkholderia cenocepacia during ∼1500 generations of biofilm selection. This long-term evolution was enabled by a new experimental method that selects for daily cycles of colonization, biofilm assembly and dispersal. Each of the six replicate biofilm populations underwent a common pattern of adaptive morphological diversification, in which three ecologically distinct morphotypes arose in the same order of succession and persisted. In two focal populations, mixed communities were more productive than any monoculture and each variant benefited from the mixture. These gains in output resulted from asymmetrical cross-feeding between ecotypes and the expansion and partitioning of biofilm space that constructed new niches. Therefore, even in the absence of starting genetic variation, prolonged selection for surface colonization generates a dynamic of ecological succession that enhances productivity.     

Character displacement promotes cooperation in bacterial biofilms

Resource competition within a group of cooperators is expected to decrease selection for cooperative behavior but can also result in diversifying selection for the use of different resources, which in turn could retard the breakdown of cooperation. Diverse groups are likely to be less susceptible to invasion by noncooperating social cheats: First, competition repression resulting from character displacement may provide less of a selective advantage to cheating; second, cheats may trade off the ability to exploit cooperators that specialize in one type of resource against cooperators that specialize in another ; third, diverse communities of any kind may have higher invasion resistance because there are fewer resources available for an invader to use . Furthermore, diverse groups are likely to be more productive than clonal groups if a wider range of total resources are being used . We addressed these issues by using the cooperative trait of biofilm formation in Pseudomonas fluorescens. Character displacement through resource competition evolved within biofilms; productivity increased with increasing character displacement, and diverse biofilms were less susceptible to invasion by cheats. These results demonstrate that diversification into different ecological niches can minimize selection against cooperation in the face of local resource competition.     

Does character displacement initiate speciation? Evidence of reduced gene flow between populations experiencing divergent selection

Character displacement – trait evolution stemming from selection to lessen resource competition or reproductive interactions between species – has long been regarded as important in finalizing speciation. By contrast, its role in initiating speciation has received less attention. Yet because selection for character displacement should act only where species co‐occur, individuals in sympatry will experience a different pattern of selection than conspecifics in allopatry. Such divergent selection might favour reduced gene flow between conspecific populations that have undergone character displacement and those that have not, thereby potentially triggering speciation. Here, we explore these ideas empirically by focusing on spadefoot toads, Spea multiplicata, which have undergone character displacement, and for which character displacement appears to cause post‐mating isolation between populations that are in sympatry with a heterospecific and those that are in allopatry. Using mitochondrial sequence data and nuclear microsatellite genotypes, we specifically asked whether gene flow is reduced between populations in different selective environments relative to that between populations in the same selective environment. We found a slight, but statistically significant, reduction in gene flow between selective environments, suggesting that reproductive isolation, and potentially ecological speciation, might indeed evolve as an indirect consequence of character displacement. Generally, character displacement may play a largely underappreciated role in instigating speciation.     

A shared coevolutionary history does not alter the outcome of coalescence in experimental populations of Pseudomonas fluorescens

Community coalescence, the mixing of multiple communities, is ubiquitous in natural microbial communities. During coalescence, theory suggests the success of a population will be enhanced by the presence of species it has coevolved with (relative to foreign species), because coevolution will result in greater resource specialization to minimize competition. Thus, more coevolved communities should dominate over less coevolved communities during coalescence events. We test these hypotheses using the bacterium Pseudomonas fluorescens which diversifies into coexisting niche‐specialist morphotypes. We first evolved replicate populations for ~40 generations and then isolated evolved genotypes. In a series of competition trials, we determined if using coevolved versus random genotypes affected the relative performance of “communities” of single and multiple genotypes. We found no effect of coevolutionary history on either genotype fitness or community performance, which suggests parallel (co)evolution between communities. However, fitness was enhanced by the presence of other genotypes of the same strain type (wild‐type or an isogenic strain with a LacZ marker; the inclusion of the latter necessary to distinguish genotypes during competition), indicative of local adaptation with respect to genetic background. Our results are the first to investigate the effect of (co)evolution on the outcome of coalescence and suggest that when input populations are functionally similar and added at equal mixing ratios, the outcome community may not be asymmetrically dominated by either input population.     

Character displacement and the origins of diversity

In The Origin of Species, Darwin proposed his principle of divergence of character (a process now termed "character displacement") to explain how new species arise and why they differ from each other phenotypically. Darwin maintained that the origin of species and the evolution of differences between them is ultimately caused by divergent selection acting to minimize competitive interactions between initially similar individuals, populations, and species. Here, we examine the empirical support for the various claims that constitute Darwin's principle, specifically that (1) competition promotes divergent trait evolution, (2) the strength of competitively mediated divergent selection increases with increasing phenotypic similarity between competitors, (3) divergence can occur within species, and (4) competitively mediated divergence can trigger speciation. We also explore aspects that Darwin failed to consider. In particular, we describe how (1) divergence can arise from selection acting to lessen reproductive interactions, (2) divergence is fueled by the intersection of character displacement and sexual selection, and (3) phenotypic plasticity may play a key role in promoting character displacement. Generally, character displacement is well supported empirically, and it remains a vital explanation for how new species arise and diversify.     

Superinfection drives virulence evolution in experimental populations of bacteria and plasmids

A prominent hypothesis proposes that pathogen virulence evolves in large part due to a trade‐off between infectiousness and damage to hosts. Other explanations emphasize how virulence evolves in response to competition among pathogens within hosts. Given the proliferation of theoretical possibilities, what best predicts how virulence evolves in real biological systems? Here, I show that virulence evolution in experimental populations of bacteria and self‐transmissible plasmids is best explained by within‐host competition. Plasmids evolved to severely reduce the fitness of their hosts even in the absence of uninfected cells. This result is inconsistent with the trade‐off hypothesis, which predicts that under these conditions vertically transmitted pathogens would evolve to be less virulent. Plasmid virulence was strongly correlated with the ability to superinfect cells containing competing plasmid genotypes, suggesting a key role for within‐host competition. When virulent genotypes became common, hosts evolved resistance to plasmid infection. These results show that the trade‐off hypothesis can incorrectly predict virulence evolution when within‐host interactions are neglected. They also show that symbioses between bacteria and plasmids can evolve to be surprisingly antagonistic.     

Experimental comparison of competition and facilitation in alpine communities varying in productivity

Question. Competitive and facilitative interactions among plant species in different abiotic environments potentially link productivity, vegetation structure, species composition and functional diversity. We investigated these interactions among four alpine communities along an environmental productivity gradient in a generally harsh climate. We hypothesised that the importance of competition would be higher in more productive sites. Location. Mt. M. Khatipara (43°27′N, 41°41′E, altitude 2750 m), NW Caucasus, Russia. Communities ranged from low‐productivity alpine lichen heath (ALH) and snowbed communities (SBC), to intermediate productivity Festuca grassland (FVG), and high‐productivity Geranium‐Hedysarum meadow (GHM). Methods. We quantified the relative influence of competition and facilitation on community structure by expressing biomass of target species within each natural community proportionally to biomass of the species in a “null community” with experimental release from interspecific competition by removing all other species (for 6 years). An overall index of change in community composition due to interspecific interactions was calculated as the sum of absolute or proportional differences of the component species. Results. Species responses to neighbour removal ranged from positive to neutral. There was no evidence of facilitation among the selected dominant species. As expected, competition was generally most important in the most productive alpine community (GHM). The intermediate position for low‐productivity communities of stressful environments (ALH, SBC) and the last position of intermediately productive FVG were unexpected. Conclusions. Our results appear to support the Fretwell‐Oksanen hypothesis in that competition in communities of intermediate productivity was less intense than in low‐ or high‐productive communities. However, the zero net effect of competition and facilitation in FVG might be the result of abiotic stress due to strong sun exposure and high soil temperatures after neighbour removal. Thus, non‐linear relationships between soil fertility, productivity and different abiotic stresses may also determine the balance between competition and facilitation.     

Competition drives trait evolution and character displacement between Mimulus species along an environmental gradient

Closely related species may evolve to coexist stably in sympatry through niche differentiation driven by in situ competition, a process termed character displacement. Alternatively, past evolution in allopatry may have already sufficiently reduced niche overlap to permit establishment in sympatry, a process called ecological sorting. The relative importance of each process to niche differentiation is contentious even though they are not mutually exclusive and are both mediated via multivariate trait evolution. We explore how competition has impacted niche differentiation in two monkeyflowers, Mimulus alsinoides and M. guttatus, which often co‐occur. Through field observations, common gardens, and competition experiments, we demonstrate that M. alsinoides is restricted to marginal habitats in sympatry and that the impacts of character displacement on niche differentiation are complex. Competition with M. guttatus alters selection gradients and has favored taller M. alsinoides with earlier seasonal flowering at low elevation and floral shape divergence at high elevation. However, no trait exhibits the pattern typically associated with character displacement, higher divergence between species in sympatry than allopatry. Thus, although character displacement was unlikely the process driving initial divergence along niche axes necessary for coexistence, we conclude that competition in sympatry has likely driven trait evolution along additional niche axes.     

Field data do not support a textbook example of convergent character displacement

Experimental evidence supporting convergent character displacement is rare; only one example exists and it is in the form of orientation and territory competition experiments performed in the laboratory. However, outcomes of laboratory experiments involving behaviour or competition can be artefacts of unnatural conditions and, therefore, the results of the previous experiments supporting convergent character displacement are equivocal. In this study, we re-examine the evolution of melanic nuptial coloration in male three-spined stickleback (Gasterosteus aculeatus) inhabiting the Chehalis River drainage in Washington State. This novel nuptial coloration has been thought to have evolved in response to competition for nesting territories with the co-distributed Olympic mudminnow (Norzumbra hubbsi), which is also melanic and breeds at the same time. I found that melanic stickleback males did not have an advantage over their red counterparts from typical populations when competing for nesting territories with Olympic mudminnows. Additionally competitive interactions between sticklebacks and mudminnows were rare in both cage experiments and naturally breeding sticklebacks. Finally, melanic coloration in the Chehalis populations did not develop until males were parental, well after the hypothesized territory establishment period. These results refute the only experimental support for convergent character displacement and emphasize the importance of conducting behavioural experiments and observations under natural conditions.     

Ecological character displacement caused by reproductive interference

We carried out a theoretical investigation of whether ecological character displacement can be caused by reproductive interference. Our model assumes that a quantitative character is associated with both resource use and species recognition, and that heterospecific mating incurs costs. The model shows that ecological character displacement can occur as a consequence of evolution of premating isolation; this conclusion is based on the premise that resource competition is less intense between species than within species and that the ecological character also contributes to premating isolation. When resource competition between species is intense, extinction of either species may occur by competitive exclusion before ecological character divergence. Some observational studies have shown that character displacement in body size is associated with not only resources use but also species recognition. We propose that body size displacement can occur as a consequence of evolution of premating isolation. Our results suggest that ecological character displacement results from reproductive character displacement.     

EXPLORING THE SOCIOBIOLOGY OF PYOVERDIN‐PRODUCING PSEUDOMONAS

The idea that bacteria are social is a popular concept with implications for understanding the ecology and evolution of microbes. The view arises predominately from reasoning regarding extracellular products, which, it has been argued, can be considered “public goods.” Among the best studied is pyoverdin—a diffusible iron‐chelating agent produced by bacteria of the genus Pseudomonas. Here we report the de novo evolution of pyoverdin nonproducing mutants, genetically characterize these types and then test the appropriateness of the sociobiology framework by performing growth and fitness assays in the same environment in which the nonproducing mutants evolved. Our data draw attention to discordance in the fit between social evolution theory and biological reality. We show that pyoverdin‐defective genotypes can gain advantage by avoiding the cost of production under conditions where the molecule is not required; in some environments pyoverdin is personalized. By exploring the fitness consequences of nonproducing types under a range of conditions, we show complex genotype‐by‐environment interactions with outcomes that range from social to asocial. Together these findings give reason to question the generality of the conclusion that pyoverdin is a social trait.     

Evolution of species interactions determines microbial community productivity in new environments

Diversity generally increases ecosystem productivity over short timescales. Over longer timescales, both ecological and evolutionary responses to new environments could alter productivity and diversity–productivity relationships. In turn, diversity might affect how component species adapt to new conditions. We tested these ideas by culturing artificial microbial communities containing between 1 and 12 species in three different environments for ∼60 generations. The relationship between community yields and diversity became steeper over time in one environment. This occurred despite a general tendency for the separate yields of isolates of constituent species to be lower at the end if they had evolved in a more diverse community. Statistical comparisons of community and species yields showed that species interactions had evolved to be less negative over time, especially in more diverse communities. Diversity and evolution therefore interacted to enhance community productivity in a new environment.     

Competition intensity and its importance: results of field experiments with<Emphasis Type="Italic"> Anthoxanthum odoratum</Emphasis>

The effect of community productivity on competition was studied in 82 permanent plots using two removal experiments with the rhizomatous perennial grass Anthoxanthum odoratum. The removal of neighbouring plants had a positive effect on the number of shoots and total above-ground biomass of Anthoxanthum but no significant effect on mean shoot biomass. The relative competition intensity coefficient (RCI) calculated from these data showed that competition intensity increased with increasing community productivity. Similarly, the importance of competition and the difference between local maximum and local average population density increased with increasing community productivity. We concluded that for Anthoxanthum the impact of competition is greater in high-productivity areas and that competition reduces population density. No evidence was found supporting the importance of positive interactions between plants in tundra areas. Received: 22 June 1999 / Accepted: 3 April 2000     

Competition for canopy cover between accessions of Phalaris minor that are susceptible and resistant to ACCase inhibiting herbicides

Herbicide resistance has the potential to impose fitness costs in plants. In Mexico, Phalaris minor Retz. has evolved resistance to acetyl‐coa carboxylase (ACCase) inhibiting herbicides. However, it is unknown whether these changes may involve other ecological costs for resistant individuals under non‐selective conditions. The aim of this study was to determine whether the evolution of resistance has reduced the ability of P. minor to compete for resources at the seedling stage. De Wit's replacement series experiments (susceptible:resistant 100:0, 50:50, 0:100) were performed under two initial conditions. In the first experiment, we examined the effect of differences in germination time between resistant and susceptible genotypes on canopy cover. In the second experiment, germination was synchronized, and all the replacement series began at the same time. Delayed germination of resistant accessions reduced their ability to compete for canopy space, competition intensity index, and relative productivity in relation to susceptible accessions. When germination was synchronized, the resistant accessions had the same canopy cover and productivity as susceptible accessions. The delayed germination may cause displacement of resistant individuals in the absence of the selective factor (herbicide).     

Intraspecific competitive interactions rapidly evolve via spontaneous mutations

Using a mechanistic spatially explicit trait-based neighborhood-model, we quantify the impact of mutations on intraspecific spatial interactions to better understand mechanisms underlying the maintenance of genetic variation and the potential effects of these evolved interactions on the population dynamics of Arabidopsis thaliana. We use 100 twenty-fifth generation mutation accumulation (MA) lines (genotypes) derived from one founder genotype to study mutational effects on neighbor responses in a field experiment. We created individual-based maps (15,000 individuals), including phenotypic variation, to quantify mutational effects within genotypes versus between genotypes on reproduction and survival. At small-scale (within 80 cm of the focal plant), survival is enhanced but seed-set is decreased when a genotype is surrounded by different genotypes. At large-scale (within 200 cm of the focal plant), seed set is facilitated by different genotypes while the same genotype has either no effect or negative effects. The direction of the interactions among MA lines suggests that at small scale these interactions may contribute to the maintenance of genetic variation and at large scale contribute to the survival of the population. This may suggest, that, mutations potentially have immediate effects on population and community dynamics by influencing the outcome of competitive and faciliatory interactions among conspecifics.

     

Structural Stability of Burkholderia cenocepacia Biofilms Is Reliant on eDNA Structure and Presence of a Bacterial Nucleic Acid Binding Protein

Cystic fibrosis (CF) is the most common lethal inherited genetic disorder affection Caucasians. Even with medical advances, CF is life-shortening with patients typically surviving only to age 38. Infection of the CF lung by Burkholderia cenocepacia presents exceptional challenges to medical management of these patients as clinically this microbe is resistant to virtually all antibiotics, is highly transmissible and infection of CF patients with this microbe renders them ineligible for lung transplant, often the last lifesaving option. Here we have targeted two abundant components of the B. cenocepacia biofilm for immune intervention: extracellular DNA and DNABII proteins, the latter of which are bacterial nucleic acid binding proteins. Treatment of B. cenocepacia biofilms with antiserum directed at one of these DNABII proteins (integration host factor or IHF) resulted in significant disruption of the biofilm. Moreover, when anti-IHF mediated destabilization of a B. cenocepacia biofilm was combined with exposure to traditional antibiotics, B. cenocepacia resident within the biofilm and thereby typically highly resistant to the action of antibiotics, were now rendered susceptible to killing. Pre-incubation of B. cenocepacia with anti-IHF serum prior to exposure to murine CF macrophages, which are normally unable to effectively degrade ingested B. cenocepacia, resulted in a statistically significant increase in killing of phagocytized B. cenocepacia. Collectively, these findings support further development of strategies that target DNABII proteins as a novel approach for treatment of CF patients, particularly those whose lungs are infected with B. cenocepacia.