Cheating can stabilize cooperation in mutualisms

Mutualisms present a challenge for evolutionary theory. How is cooperation maintained in the face of selection for selfishness and cheating? Both theory and data suggest that partner choice, where one species preferentially directs aid to the more cooperative members of the other species, is central to cooperation in many mutualisms. However, the theory has only so far considered the evolutionary effects of partner choice on one of the species in a mutualism in isolation. Here, we investigate the co-evolution of cooperation and choice in a choosy host and its symbiont. Our model reveals that even though choice and cooperation may be initially selected, it will often be unstable. This is because choice reduces variation in the symbiont and, therefore, tends to remove the selective incentive for its own maintenance (a scenario paralleled in the lek paradox in female choice and policing in within-species cooperation). However, we also show that when variability is reintroduced into symbionts each generation, in the form of less cooperative individuals, choice is maintained. This suggests that the presence of cheaters and cheater species in many mutualisms is central to the maintenance of partner choice and, paradoxically, cooperation itself.     

Varieties of mutualistic interaction in population models

A Lotka-Volterra model of mutalism indicates eight possible cases, of which two lead to survival of both populations, two indicate inevitable extinction, and four are indeterminate, the result depending on the initial population sizes. Conventional neighborhood stability analysis is a poor indicator of the biological result expected. Modification of the Lotka-Volterra model to give non-linear isoclines is necessary to obtain a minimum of biological realism; this modified model is illustrated with an analysis of a legume-Rhizobium mutualism.     

Reactive oxygen species play a role in regulating a fungus-perennial ryegrass mutualistic interaction

Although much is known about the signals and mechanisms that lead to pathogenic interactions between plants and fungi, comparatively little is known about fungus-plant mutualistic symbioses. We describe a novel role for reactive oxygen species (ROS) in regulating the mutualistic interaction between a clavicipitaceous fungal endophyte, Epichlo? festucae, and its grass host, Lolium perenne. In wild-type associations, E. festucae grows systemically in intercellular spaces of leaves as infrequently branched hyphae parallel to the leaf axis. A screen to identify symbiotic genes isolated a fungal mutant that altered the interaction from mutualistic to antagonistic. This mutant has a single-copy plasmid insertion in the coding region of a NADPH oxidase gene, noxA. Plants infected with the noxA mutant lose apical dominance, become severely stunted, show precocious senescence, and eventually die. The fungal biomass in these associations is increased dramatically, with hyphae showing increased vacuolation. Deletion of a second NADPH oxidase gene, noxB, had no effect on the E. festucae-perennial ryegrass symbiosis. ROS accumulation was detected cytochemically in the endophyte extracellular matrix and at the interface between the extracellular matrix and host cell walls of meristematic tissue in wild-type but not in noxA mutant associations. These results demonstrate that fungal ROS production is critical in maintaining a mutualistic fungus-plant interaction.     

Trait-mediated interaction leads to structural emergence in mutualistic networks

As asymmetric structures of mutualistic networks can potentially contribute to system resilience, elucidating drivers behind the emergence of particular network architectures remains a major endeavour in ecology. Here, using an eco-evolutionary model for bipartite mutualistic networks with trait-mediated interactions, we explore how particular levels of connectance, nestedness and modularity are affected by three network assembly forces: resource accessibility, tolerance to trait difference between mutualistic pairs and competition intensity. We found that a moderate accessibility to intra-trophic resources and cross-trophic mutualistic support can result in a highly nested web, while low tolerance to trait difference between interacting pairs leads to a high level of modularity. Network-level trait complementarity leads to low connectance and high modularity, while network-level specialization can result in nested structures. Consequently, we argue that the interplay of ecological and evolutionary processes through trait-mediated interactions can explain these widely observed architectures in mutualistic networks.     

Interstrand dipole-dipole interactions can stabilize the collagen triple helix

The amino acid sequence of collagen is composed of GlyXaaYaa repeats. A prevailing paradigm maintains that stable collagen triple helices form when (2S)-proline (Pro) or Pro derivatives that prefer the C(γ)-endo ring pucker are in the Xaa position and Pro derivatives that prefer the C(γ)-exo ring pucker are in the Yaa position. Anomalously, an amino acid sequence in an invertebrate collagen has (2S,4R)-4-hydroxyproline (Hyp), a C(γ)-exo-puckered Pro derivative, in the Xaa position. In certain contexts, triple helices with Hyp in the Xaa position are now known to be hyperstable. Most intriguingly, the sequence (GlyHypHyp)(n) forms a more stable triple helix than does the sequence (GlyProHyp)(n). Competing theories exist for the physicochemical basis of the hyperstability of (GlyHypHyp)(n) triple helices. By synthesizing and analyzing triple helices with different C(γ)-exo-puckered proline derivatives in the Xaa and Yaa positions, we conclude that interstrand dipole-dipole interactions are the primary determinant of their additional stability. These findings provide a new framework for understanding collagen stability.     

Chlamydomonas reinhardtii thermal tolerance enhancement mediated by a mutualistic interaction with vitamin B12-producing bacteria

Temperature is one of the most important environmental factors affecting the growth and survival of microorganisms and in light of current global patterns is of particular interest. Here, we highlight studies revealing how vitamin B₁₂ (cobalamin)-producing bacteria increase the fitness of the unicellular alga Chlamydomonas reinhardtii following an increase in environmental temperature. Heat stress represses C. reinhardtii cobalamin-independent methionine synthase (METE) gene expression coinciding with a reduction in METE-mediated methionine synthase activity, chlorosis and cell death during heat stress. However, in the presence of cobalamin-producing bacteria or exogenous cobalamin amendments C. reinhardtii cobalamin-dependent methionine synthase METH-mediated methionine biosynthesis is functional at temperatures that result in C. reinhardtii death in the absence of cobalamin. Artificial microRNA silencing of C. reinhardtii METH expression leads to nearly complete loss of cobalamin-mediated enhancement of thermal tolerance. This suggests that methionine biosynthesis is an essential cellular mechanism for adaptation by C. reinhardtii to thermal stress. Increased fitness advantage of METH under environmentally stressful conditions could explain the selective pressure for retaining the METH gene in algae and the apparent independent loss of the METE gene in various algal species. Our results show that how an organism acclimates to a change in its abiotic environment depends critically on co-occurring species, the nature of that interaction, and how those species interactions evolve.     

Coevolution drives temporal changes in fitness and diversity across environments in a bacteria-bacteriophage interaction.

Coevolutionary interactions are thought to play a crucial role in diversification of hosts and parasitoids. Furthermore, resource availability has been shown to be a fundamental driver of species diversity. Yet, we still do not have a clear understanding of how resource availability mediates the diversity generated by coevolution between hosts and parasitoids over time. We used experiments with bacteria and bacteriophage to test how resources affect variation in the competitive ability of resistant hosts and temporal patterns of diversity in the host and parasitoid as a result of antagonistic coevolution. Bacteria and bacteriophage coevolved for over 150 bacterial generations under high and low-resource conditions. We measured relative competitive ability of the resistant hosts and phenotypic diversity of hosts and parasitoids after the initial invasion of resistant mutants and again at the end of the experiment. Variation in relative competitive ability of the hosts was both time- and environment-dependent. The diversity of resistant hosts, and the abundance of host-range mutants attacking these phenotypes, differed among environments and changed over time, but the direction of these changes differed between the host and parasitoid. Our results demonstrate that patterns of fitness and diversity resulting from coevolutionary interactions can be highly dynamic.     

Coevolution in a one predator-two prey system

Background

Our understanding of coevolution in a predator–prey system is based mostly on pair-wise interactions.

Methodology and Principal Findings

Here I analyze a one-predator–two-prey system in which the predator''s attack ability and the defense abilities of the prey all evolve. The coevolutionary consequences can differ dramatically depending on the initial trait value and the timing of the alternative prey''s invasion into the original system. If the invading prey species has relatively low defense ability when it invades, its defense is likely to evolve to a lower level, stabilizing the population dynamics. In contrast, if when it invades its defense ability is close to that of the resident prey, its defense can evolve to a higher level and that of the resident prey may suddenly cease to evolve, destabilizing the population dynamics. Destabilization due to invasion is likely when the invading prey is adaptively superior (evolution of its defense is less constrained and fast), and it can also occur in a broad condition even when the invading prey is adaptively inferior. In addition, invasion into a resident system far from equilibrium characterized by population oscillations is likely to cause further destabilization.

Conclusions and Significance

An invading prey species is thus likely to destabilize a resident community.     

Coevolution study of mitochondria respiratory chain proteins: toward the understanding of protein--protein interaction

Coevolution can be seen as the interdependency between evolutionary histories. In the context of protein evolution, functional correlation proteins are ever-present coordinated evolutionary characters without disruption of organismal integrity. As to complex system, there are two forms of protein-protein interactions in vivo, which refer to inter-complex interaction and intra-complex interaction. In this paper, we studied the difference of coevolution characters between inter-complex interaction and intra-complex interaction using "Mirror tree" method on the respiratory chain (RC) proteins. We divided the correlation coefficients of every pairwise RC proteins into two groups corresponding to the binary protein-protein interaction in intra-complex and the binary protein-protein interaction in inter-complex, respectively. A dramatical discrepancy is detected between the coevolution characters of the two sets of protein interactions (Wilcoxon test, p-value = 4.4 x 10-6). Our finding reveals some critical information on coevolutionary study and assists the mechanical investigation of protein-protein interaction.Furthermore, the results also provide some unique clue for supramolecular organization of protein complexes in the mitochondrial inner membrane. More detailed binding sites map and genome information of nuclear encoded RC proteins will be extraordinary valuable for the further mitochondria dynamics study.     

The role of auxins and cytokinins in the mutualistic interaction between Arabidopsis and Piriformospora indica

Arabidopsis growth and reproduction are stimulated by the endophytic fungus Piriformospora indica. The fungus produces low amounts of auxins, but the auxin levels and the expression of auxin-regulated genes are not altered in colonized roots. Also, mutants with reduced auxin levels (ilr1-1, nit1-3, tfl2, cyp79 b2b3) respond to P. indica. However, the fungus rescues the dwarf phenotype of the auxin overproducer sur1-1 by converting free auxin into conjugates, which also results in the downregulation of the auxin-induced IAA6 and the upregulation of the P. indica-induced LRR1 gene. The fungus produces relatively high levels of cytokinins, and the cytokinin levels are higher in colonized roots compared with the uncolonized controls. trans-Zeatin cytokinin biosynthesis and the CRE1/AHK2 receptor combination are crucial for P. indica-mediated growth stimulation, while mutants lacking cis-zeatin, impaired in other cytokinin receptor combinations, or containing reduced cytokinin levels respond to the fungus. Since root colonization is not affected in the cytokinin mutants, we propose that cytokinins are required for P. indica-induced growth promotion. Finally, a comparative analysis of the phytohormone mutants allows the conclusion that the response to P. indica is independent of the architecture and size of the roots.     

Coevolution and maladaptation

Many of the most commonly cited examples of exquisite adaptationare of coevolved symbioses. As we learn more about the coevolutionaryprocess, however, it is becoming increasingly evident that coevolutionmay also keep populations moderately maladapted much of thetime. As a result, coevolving populations may only rarely occupyadaptive peaks, because the selective landscape is under continualchange through reciprocal selection on the species themselves.These shifting patterns of coadaptation are further shaped bythe geographic structure of most species. Selection mosaicsacross landscapes and coevolutionary hotspots can favor differentevolutionary trajectories in different populations. The combinedaction of gene flow, random genetic drift, and local extinctionof populations may then continually remold these local patterns,creating a geographic mosaic in the degrees of maladaptationfound within local interactions. Recent mathematical modelsof the geographic mosaic of coevolution suggest that complexmosaics of maladaptation are a likely consequence of spatiallystructured species interactions. These models indicate thatthe spatial structure of maladaptation may depend upon the typeof coevolutionary interaction, the underlying selection mosaic,and patterns of gene flow across landscapes. By maintaininglocal polymorphisms and driving the divergence of populations,coevolution may produce spatial patterns of maladaptation thatare a source of ongoing innovation and diversification in speciesinteractions.     

Ethylene in mutualistic symbioses

Ethylene (ET) is a gaseous phytohormone that participates in various plant physiological processes and essentially contributes to plant immunity. ET conducts its functions by regulating the expression of ET-responsive genes or in crosstalk with other hormones. Several recent studies have shown the significance of ET in the establishment and development of plant-microbe interactions. Therefore, it is not surprising that pathogens and mutualistic symbionts target ET synthesis or signaling to colonize plants. This review introduces the significance of ET metabolism in plant-microbe interactions, with an emphasis on its role in mutualistic symbioses.     

Habitat destruction in mutualistic metacommunities

We investigate a mutualistic metacommunity where the strength of the mutualistic interaction between species is measured by the extent to which the presence of one species on a patch either reduces the extinction rate of the others present on the same patch or increases their ability to colonize other patches. In both cases, a strong enough mutualism enables all species to persist at habitat densities where they would all be extinct in the absence of the interaction. However, a mutualistic interaction that enhances colonization enables the species to persist at lower habitat density than one that suppresses extinction. All species abruptly go extinct (catastrophe) when the habitat density is decreased infinitesimally below a critical value. A comparison of the mean field or spatially implicit case with unrestricted dispersal and colonization to all patches in the system with a spatially explicit case where dispersal is restricted to the immediate neighbours of the original patch leads to the intriguing conclusion that restricted dispersal can be favourable for species that have a beneficial effect on each other when habitat conditions are adverse. When the mutualistic interaction is strong enough, the extinction threshold or critical amount of habitat required for the persistence of all species is lower when the dispersal is locally restricted than when unrestricted ! The persistence advantage for all species created by the mutualistic interaction increases substantially with the number of species in the metacommunity, as does the advantage for restricted dispersal over global dispersal.     

Coevolution is a short-distance force at the protein interaction level and correlates with the modular organization of protein networks

We investigated what roles coevolution plays in shaping yeast protein interaction network (PIN). We found that the extent of coevolution between two proteins decreases rapidly as their interacting distance on the PIN increases, suggesting coevolutionary constraint is a short-distance force at the molecular level. We also found that protein-protein interactions (PPIs) with strong coevolution tend to be enriched in interconnected clusters, whereas PPIs with weak coevolution are more frequently present at inter-cluster region. The findings indicate the close relationship between coevolution and modular organization of PINs, and may provide insights into evolution and modularity of cellular networks.     

Anti-Z-DNA antibody binding can stabilize Z-DNA in relaxed and linear plasmids under physiological conditions.

It is shown that anti-Z-DNA antibody binding can stabilize sequences of d(CG/GC)n and d(CA/GT)n in the Z-DNA conformation in a plasmid in the complete absence of supercoiling. This effect is quantitated by using antibody preparations of different affinities and varying concentrations. The d(CG/GC)n sequence can be stabilized under physiological conditions. This is the first demonstration that a region of Z-DNA can be stabilized by protein binding in a completely relaxed plasmid under physiological conditions. The antibody-Z-DNA complex in the relaxed plasmid is shown to be an equilibrium state and not a long-lived kinetic intermediate since specific binding of the antibody to linearized plasmids containing Z-forming sequences is observed.     

Conditional outcomes in mutualistic interactions

Interspecific interactions are traditionally displayed in a grid in which each interaction is placed according to its outcome (positive, negative or neutral) for each partner. However, recent field studies consistently find the costs and benefits that determine net effects to vary greatly in both space and time, inevitably causing outcomes within most interactions to vary as well. Interactions show 'conditionality' when costs and benefits, and thus outcomes, are affected in predictable ways by current ecological conditions. The full range of natural outcomes of a given association may reveal far more about its ecological and evolutionary dynamics than does the average outcome at a given place and time.     

Antagonistic Coevolution and Sex

One of the leading hypotheses for the maintenance of sexual reproduction is the Red Queen hypothesis. The underlying premise of the Red Queen hypothesis is that parasites rapidly evolve to infect common host genotypes. This response by parasites could result in the long-term maintenance of genetic variation and may favor sexual reproduction over asexual reproduction. The underlying ideas present a wonderful microcosm for teaching evolution. Here I present the reasons for why sex is anomalous for evolutionary theory, the rationale underlying the Red Queen hypothesis, and some empirical studies of the Red Queen hypothesis using a freshwater snail. The empirical results are consistent with the Red Queen hypothesis. In addition, the distribution of sexual and asexual reproduction in the snail leads naturally to thinking about coevolution in a geographic mosaic of parasite-mediated natural selection.