COEVOLUTIONARY ALTERNATION IN ANTAGONISTIC INTERACTIONS

Relationships between functional alpha and beta diversities of flea parasites and their small mammalian hosts

We studied the relationships between functional alpha and beta diversities of fleas and their small mammalian hosts in 4 biogeographic realms (the Afrotropics, the Nearctic, the Neotropics and the Palearctic), considering 3 components of alpha diversity (functional richness, divergence and regularity). We asked whether (a) flea alpha and beta diversities are driven by host alpha and beta diversities; (b) the variation in the off-host environment affects variation in flea alpha and beta diversities; and (c) the pattern of the relationship between flea and host alpha or beta diversities differs between geographic realms. We analysed alpha diversity using modified phylogenetic generalized least squares and beta diversity using modified phylogenetic generalized dissimilarity modelling. In all realms, flea functional richness and regularity increased with an increase in host functional richness and regularity, respectively, whereas flea functional divergence correlated positively with host functional divergence in the Nearctic only. Environmental effects on the components of flea alpha diversity were found only in the Holarctic realms. Host functional beta diversity was invariantly the best predictor of flea functional beta diversity in all realms, whereas the effects of environmental variables on flea functional beta diversity were much weaker and differed between realms. We conclude that flea functional diversity is mostly driven by host functional diversity, whereas the environmental effects on flea functional diversity vary (a) geographically and (b) between components of functional alpha diversity.

The Role of Indirect Effects in Coevolution along the Mutualism-Antagonism Continuum

AbstractThe web of interactions in a community drives the coevolution of species. Yet it is unclear how the outcome of species interactions influences the coevolutionary dynamics of communities. This is a pressing matter, as changes to the outcome of interactions may become more common with human-induced global change. Here, we combine network and evolutionary theory to explore coevolutionary outcomes in communities harboring mutualistic and antagonistic interactions. We show that as the ratio of mutualistic to antagonistic interactions decreases, selection imposed by direct partners outweighs that imposed by indirect partners. This weakening of indirect effects results in communities composed of species with dissimilar traits and fast rates of adaptation. These changes are more pronounced when specialist consumers are the first species to engage in antagonistic interactions. Hence, a shift in the outcome of species interactions may reverberate across communities and alter the direction and speed of coevolution.

Evoregions of fleas and their small mammalian hosts: Do they coincide?

Combining the biogeography and phylogenetic patterns of parasite-host associations allows a better understanding of the history of parasite–host interactions, which can be achieved via biogeographic regionalization incorporating phylogenetic information. Recently, the concepts of evoregions (regions where a majority of species evolved from one or several ancestors inhabiting these regions) and evolutionary transition zones (regions of high phylogenetic turnover) have been proposed, coupled with a classification approach for these concepts. We applied this approach to 206 flea species and 265 host species of the Palearctic and aimed to identify evoregions and evolutionary transition zones for both fleas and hosts and to understand whether these evoregions and transition zones match each other. We identified 5 evoregions with 3 transition zones for either fleas or hosts, but neither the positions and boundaries of the flea and host evoregions nor the transition zones coincided. Indications of multiple geographic centres of diversification of the same flea lineages suggested that (a) the common evolutionary history of fleas and hosts was characterized by multiple events other than codiversification and that (b) dispersal played an important role in flea and host assemblies. Barriers to dispersal could be represented by landscape features (deserts and mountain ranges) and/or climate differences.

Mutualistic coevolution and community diversity favour persistence in metacommunities under environmental changes

Linking local to regional ecological and evolutionary processes is key to understand the response of Earth's biodiversity to environmental changes. Here we integrate evolution and mutualistic coevolution in a model of metacommunity dynamics and use numerical simulations to understand how coevolution can shape species distribution and persistence in landscapes varying in space and time. Our simulations show that coevolution and species richness can synergistically shape distribution patterns by increasing colonization and reducing extinction of populations in metacommunities. Although conflicting selective pressures emerging from mutualisms may increase mismatches with the local environment and the rate of local extinctions, coevolution increases trait matching among mutualists at the landscape scale, counteracting local maladaptation and favouring colonization and range expansions. Our results show that by facilitating colonization, coevolution can also buffer the effects of environmental changes, preventing species extinctions and the collapse of metacommunities. Our findings reveal the mechanisms whereby coevolution can favour persistence under environmental changes and highlight that these positive effects are greater in more diverse systems that retain landscape connectivity.

Ehrlich and Raven escape and radiate coevolution hypothesis at different levels of organization: Past and future perspectives

The classic paper by Ehrlich and Raven on coevolution will soon be 60 years old. Although they were not the first to develop the idea of coevolution, their thought-provoking paper certainly popularized this idea and inspired several generations of scientists interested in coevolution. Here, we describe some of their main contributions, quantitatively measure the impact of their seminal paper on different fields of research, and discuss how ideas related to their original paper might push the study of coevolution forward. To guide our discussion, we explore their original hypothesis into three research fields that are associated with distinct scales/levels of organization: (1) the genetic mechanisms underlying coevolutionary interactions; (2) the potential association between coevolutionary diversification and the organization of ecological networks; and (3) the micro- and macroevolutionary mechanisms and expected patterns under their hypothesis. By doing so, we discuss potentially overlooked aspects and future directions for the study of coevolutionary dynamics and diversification.

Endoparasite community structure of an anuran assemblage in the Caatinga, Northeastern Neotropical Region

Amphibians are a widespread Chordata taxon and are important for maintaining the balance of both terrestrial and aquatic ecosystems. Brazil has a rich amphibian fauna; however, little is known about the role of their ecology and phylogenetic relationships during the assembly processes of associated endoparasite communities. Herein, we describe an endoparasite community in an anuran assemblage in the Caatinga, a unique biome of dry forests in north-eastern Brazil. We studied endoparasite diversity, as well as the effects of body length, body mass, body volume and sex on parasite abundance. We also investigated the influence of ecological and historical factors and anuran microhabitat use on endoparasite composition. We analysed individuals from 13 anuran species distributed across five families: Odontophrynidae (Proceratophrys cristiceps); Leptodactylidae (Leptodactylus fuscus, Leptodactylus vastus, Leptodactylus macrosternum, Leptodactylus troglodytes and Physalaemus cuvieri); Hylidae (Pithecopus gonzagai, Scinax x-signatus, Boana raniceps and Dendropsophus nanus); Bufonidae (Rhinella diptycha and Rhinella granulosa); and Microhylidae (Dermatonotus muelleri). We found nine species of endoparasites, including seven nematodes (Aplectana membranosa, Cosmocerca sp., Oswaldocruzia mazzai, Raillietnema spectans, Rhabdias fuelleborni, Schrankiana sp. and Physaloptera sp.), one species of Trematoda (Glypthelmins pseudium) and one non-identified cestode. There was no significant relationship between endoparasite abundance and host body length, body mass, body volume and sex. A phylogenetic principal component analysis showed that ecological factors had a greater influence on endoparasite assemblage than historical factors. Similarly, our results showed that ecological factors had a greater influence on anuran microhabitat use compared to historical factors, which contributed to the generalist characteristics presented by most of the sampled endoparasite species.

Lipid droplets and the transcriptome of Mycobacterium tuberculosis from direct sputa: a literature review

Mycobacterium tuberculosis (Mtb), the main etiology of tuberculosis (TB), is predominantly an intracellular pathogen that has caused infection, disease and death in humans for centuries. Lipid droplets (LDs) are dynamic intracellular organelles that are found across the evolutionary tree of life. This review is an evaluation of the current state of knowledge regarding Mtb-LD formation and associated Mtb transcriptome directly from sputa.Based on the LD content, Mtb in sputum may be classified into three groups: LD positive, LD negative and LD borderline. However, the clinical and evolutionary importance of each state is not well elaborated. Mounting evidence supports the view that the presence of LD positive Mtb bacilli in sputum is a biomarker of slow growth, low energy state, towards lipid degradation, and drug tolerance. In Mtb, LD may serve as a source of chemical energy, scavenger of toxic compounds, prevent destruction of Mtb through autophagy, delay trafficking of lysosomes towards the phagosome, and contribute to Mtb persistence. It is suggest that LD is a key player in the induction of a spectrum of phenotypic and metabolic states of Mtb in the macrophage, granuloma and extracellular sputum microenvironment. Tuberculosis patients with high proportion of LD positive Mtb in pretreatment sputum was associated with higher rate of poor treatment outcome, indicating that LD may have a clinical application in predicting treatment outcome.The propensity for LD formation among Mtb lineages is largely unknown. The role of LD on Mtb transmission and disease phenotype (pulmonary TB vs extra-pulmonary TB) is not well understood. Thus, further studies are needed to understand the relationships between LD positivity and Mtb lineage, Mtb transmission and clinical types.

Phenotypic and functional variation in venom and venom resistance of two sympatric rattlesnakes and their prey

Predator-prey interactions often lead to the coevolution of adaptations associated with avoiding predation and, for predators, overcoming those defences. Antagonistic coevolutionary relationships are often not simple interactions between a single predator and prey but rather a complex web of interactions between multiple coexisting species. Coevolution between venomous rattlesnakes and small mammals has led to physiological venom resistance in several mammalian taxa. In general, viperid venoms contain large quantities of snake venom metalloproteinase toxins (SVMPs), which are inactivated by SVMP inhibitors expressed in resistant mammals. We explored variation in venom chemistry, SVMP expression, and SVMP resistance across four co-distributed species (California Ground Squirrels, Bryant's Woodrats, Southern Pacific Rattlesnakes, and Red Diamond Rattlesnakes) collected from four different populations in Southern California. Our aim was to understand phenotypic and functional variation in venom and venom resistance in order to compare coevolutionary dynamics of a system involving two sympatric predator-prey pairs to past studies that have focused on single pairs. Proteomic analysis of venoms indicated that these rattlesnakes express different phenotypes when in sympatry, with Red Diamonds expressing more typical viperid venom (with a diversity of SVMPs) and Southern Pacifics expressing a more atypical venom with a broader range of non-enzymatic toxins. We also found that although blood sera from both mammals were generally able to inhibit SVMPs from both rattlesnake species, inhibition depended strongly on the snake population, with snakes from one geographic site expressing SVMPs to which few mammals were resistant. Additionally, we found that Red Diamond venom, rather than woodrat resistance, was locally adapted. Our findings highlight the complexity of coevolutionary relationships between multiple predators and prey that exhibit similar offensive and defensive strategies in sympatry.

Colonization of a novel host by fleas: changes in egg production and egg size

We studied the success of fleas, Synosternus cleopatrae and Xenopsylla ramesis, in switching to a novel host by establishing experimental lines maintained on different hosts for 18 generations. Fleas fed on principal (P-line) or novel hosts, either sympatric with (S-line) or allopatric to (A-line) a flea and its principal host, then we assessed their reproductive performance via the number and size of eggs. We compared reproductive performance between hosts within a line and between lines within a host asking: (a) whether fleas adapt to a novel host species after multiple generations; (b) if yes, whether the pattern of adaptation differs between novel host species sympatric with or allopatric to a flea and its principal host; and (c) adaptation to a novel host is accompanied with a loss of success in exploitation of an original host. Fleas from the S- and A-lines increased their egg production on a novel host (except X. ramesis from the S-line). S. cleopatrae from the S-line but not the A-line increased egg size on a novel host, whereas X. ramesis from the A-line but not the S-line produced larger eggs from a novel host. We found no indication of a loss of reproductive performance on the original host while adapting to a novel host. We conclude that fleas are able to switch rapidly to a new host with the pattern of a switch to either sympatric or an allopatric host depending on the identities of both flea and host species.

Coevolutionary patterns caused by prey selection

Many theoretical models have been formulated to better understand the coevolutionary patterns that emerge from antagonistic interactions. These models usually assume that the attacks by the exploiters are random, so the effect of victim selection by exploiters on coevolutionary patterns remains unexplored. Here we analytically studied the payoff for predators and prey under coevolution assuming that every individual predator can attack only a small number of prey any given time, considering two scenarios: (i) predation occurs at random; (ii) predators select prey according to phenotype matching. We also develop an individual based model to verify the robustness of our analytical prediction. We show that both scenarios result in well known similar coevolutionary patterns if population sizes are sufficiently high: symmetrical coevolutionary branching and symmetrical coevolutionary cycling (Red Queen dynamics). However, for small population sizes, prey selection can cause unexpected coevolutionary patterns. One is the breaking of symmetry of the coevolutionary pattern, where the phenotypes evolve towards one of two evolutionarily stable patterns. As population size increases, the phenotypes oscillate between these two values in a novel form of Red Queen dynamics, the episodic reversal between the two stable patterns. Thus, prey selection causes prey phenotypes to evolve towards more extreme values, which reduces the fitness of both predators and prey, increasing the likelihood of extinction.

Distinct responses of antagonistic and mutualistic networks to agricultural intensification

Species interaction networks, which govern the maintenance of biodiversity and ecosystem processes within ecological communities, are being rapidly altered by anthropogenic activities worldwide. Studies on the response of species interaction networks to anthropogenic disturbance have almost exclusively focused on one interaction type at a time, such as mutualistic or antagonistic interactions, making it challenging to decipher how networks of different interaction types respond to the same anthropogenic disturbance. Moreover, few studies have simultaneously focused on the two main components of network structure: network topology (i.e., architecture) and network ecology (i.e., species identities and interaction turnover), thereby limiting our understanding of the ecological drivers underlying changes in network topology in response to anthropogenic disturbance. Here, we used 16,400 plant-pollinator and plant-herbivore interaction observations from 16 sites along an agricultural intensification gradient to compare changes in network topology and ecology between mutualistic and antagonistic networks. We measured two aspects of network topology-nestedness and modularity-and found that although the mutualistic networks were consistently more nested than antagonistic networks and antagonistic networks were consistently more modular, the rate of change in nestedness and modularity along the gradient was comparable between the two network types. Change in network ecology, however, was distinct between mutualistic and antagonistic networks, with partner switching making a significantly larger contribution to interaction turnover in the mutualistic networks than in the antagonistic networks, and species turnover being a strong contributor to interaction turnover in the antagonistic networks. The ecological and topological changes we observed in the antagonistic and mutualistic networks have different implications for pollinator and herbivore communities in agricultural landscapes, and support the idea that pollinators are more labile in their interaction partner choice, whereas herbivores form more reciprocally specialized, and therefore more vulnerable, interactions. Our results also demonstrate that studying both topological and ecological network structure can help to elucidate the effects of anthropogenic disturbance on ecological communities, with applications for conservation and restoration of species interactions and the ecosystem processes they maintain.

Resistance of South American opossums to vWF-binding venom C-type lectins

Opossums in the clade Didelphini are well known to be resistant to snake venom due to endogenous circulating inhibitors which target metalloproteinases and phospholipases. However, the mechanisms through which these opossums cope with a variety of other damaging venom proteins are unknown. A protein involved in blood clotting (von Willebrand Factor) has been found to have undergone rapid adaptive evolution in venom-resistant opossums. This protein is a known target for a subset of snake venom C-type lectins (CTLs), which bind it and then induce it to bind platelets, causing hemostatic disruption. Several amino acid changes in vWF unique to these opossums could explain their resistance; however, experimental evidence that these changes disrupt venom CTL binding was lacking. We used platelet aggregation assays to quantify resistance to a venom-induced platelet response in two species of venom-resistant opossums (Didelphis virginiana, Didelphis aurita), and one venom-sensitive opossum (Monodelphis domestica). We found that all three species have lost nearly all their aggregation response to the venom CTLs tested. Using washed platelet assays we showed that this loss of aggregation response is not due to inhibitors in the plasma, but rather to the failure of either vWF or platelets (or both) to respond to venom CTLs. These results demonstrate the potential adaptive function of a trait previously shown to be evolving under positive selection. Surprisingly, these findings also expand the list of potentially venom tolerant species to include Monodelphis domestica and suggest that an ecological relationship between opossums and vipers may be a broader driver of adaptive evolution across South American marsupials than previously thought.

The Role of Evolution in Shaping Ecological Networks

The structure of ecological networks reflects the evolutionary history of their biotic components, and their dynamics are strongly driven by ecoevolutionary processes. Here, we present an appraisal of recent relevant research, in which the pervasive role of evolution within ecological networks is manifest. Although evolutionary processes are most evident at macroevolutionary scales, they are also important drivers of local network structure and dynamics. We propose components of a blueprint for further research, emphasising process-based models, experimental evolution, and phenotypic variation, across a range of distinct spatial and temporal scales. Evolutionary dimensions are required to advance our understanding of foundational properties of community assembly and to enhance our capability of predicting how networks will respond to impending changes.

Traits of litter-dwelling forest arthropod predators and detritivores covary spatially with traits of their resources

The functional trait approach proposes that relating traits of organisms within a community to variation in abiotic and biotic characteristics of their environment will provide insight on the mechanisms of community assembly. As traits at a given trophic level might act as filters for the selection of traits at another trophic level, we hypothesized that traits of consumers and of their resources covary in space. We evaluated complementary predictions about top‐down (negative) and bottom‐up (positive) trait covariation in a detrital food web. Additionally, we tested whether positive trait covariation was better explained by the Resource Concentration Hypothesis (i.e., most commonly represented trait values attract abundant consumers) or the Resource Specialization Hypothesis (i.e., resource diversity increases niche availability for the consumers). Macroarthopods were collected with pitfall traps over two summers in three forested sites of southern Quebec in 110 plots that varied in tree species composition. Six feeding traits of consumers (detritivores and predators) and six palatability traits of their resources (leaf litter and prey) were matched to assess spatial covariation. Trait matches included consumer biting force/resource toughness, detritivore mandibular gape/leaf thickness, predator/prey body size ratio, etc. Our results demonstrate for the first time a covariation between feeding traits of detritivores and palatability traits of leaf litter (31–34%), and between feeding traits of litter‐dwelling predators and palatability traits of potential prey (38–44%). The observed positive covariation supports both the Resource Concentration Hypothesis and Resource Specialization Hypothesis. Spatial covariation of consumer and resource traits provides a new tool to partially predict the structure of the detrital food web. Nonetheless, top‐down regulation remains difficult to confirm. Further research on top‐down processes will be undoubtedly necessary to refine our capacity to interpret the effect of biotic interactions on co‐distribution.

Patterns of host use by brood parasitic Maculinea butterflies across Europe

The range of hosts exploited by a parasite is determined by several factors, including host availability, infectivity and exploitability. Each of these can be the target of natural selection on both host and parasite, which will determine the local outcome of interactions, and potentially lead to coevolution. However, geographical variation in host use and specificity has rarely been investigated. Maculinea (= Phengaris) butterflies are brood parasites of Myrmica ants that are patchily distributed across the Palæarctic and have been studied extensively in Europe. Here, we review the published records of ant host use by the European Maculinea species, as well as providing new host ant records for more than 100 sites across Europe. This comprehensive survey demonstrates that while all but one of the Myrmica species found on Maculinea sites have been recorded as hosts, the most common is often disproportionately highly exploited. Host sharing and host switching are both relatively common, but there is evidence of specialization at many sites, which varies among Maculinea species. We show that most Maculinea display the features expected for coevolution to occur in a geographic mosaic, which has probably allowed these rare butterflies to persist in Europe. This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.

Can we predict the success of a parasite to colonise an invasive host?

To understand whether a parasite can exploit a novel invasive host species, we measured reproductive performance (number of eggs per female per day, egg size, development rate and size of new imagoes) of fleas from the Negev desert in Israel (two host generalists, Synosternus cleopatrae and Xenopsylla ramesis, and a host specialist, Parapulex chephrenis) when they exploited either a local murid host (Gerbillus andersoni, Meriones crassus and Acomys cahirinus) or two alien hosts (North American heteromyids, Chaetodipus penicillatus and Dipodomys merriami). We asked whether (1) reproductive performance of a flea differs between an alien and a characteristic hosts and (2) this difference is greater in a host specialist than in host generalists. The three fleas performed poorly on alien hosts as compared to local hosts, but the pattern of performance differed both among fleas and within fleas between alien hosts. The response to alien hosts did not depend on the degree of host specificity of a flea. We conclude that successful parasite colonisation of an invasive host is determined by some physiological, immunological and/or behavioural compatibility between a host and a parasite. This compatibility is unique for each host-parasite association, so that the success of a parasite to colonise an invasive host is unpredictable.

Comparative Genomics of Bacillus thuringiensis Reveals a Path to Specialized Exploitation of Multiple Invertebrate Hosts

Understanding the genetic basis of host shifts is a key genomic question for pathogen and parasite biology. The Bacillus cereus group, which encompasses Bacillus thuringiensis and Bacillus anthracis, contains pathogens that can infect insects, nematodes, and vertebrates. Since the target range of the essential virulence factors (Cry toxins) and many isolates is well known, this group presents a powerful system for investigating how pathogens can diversify and adapt to phylogenetically distant hosts. Specialization to exploit insects occurs at the level of the major clade and is associated with substantial changes in the core genome, and host switching between insect orders has occurred repeatedly within subclades. The transfer of plasmids with linked cry genes may account for much of the adaptation to particular insect orders, and network analysis implies that host specialization has produced strong associations between key toxin genes with similar targets. Analysis of the distribution of plasmid minireplicons shows that plasmids with orf156 and orf157, which carry genes encoding toxins against Lepidoptera or Diptera, were contained only by B. thuringiensis in the specialized insect clade (clade 2), indicating that tight genome/plasmid associations have been important in adaptation to invertebrate hosts. Moreover, the accumulation of multiple virulence factors on transposable elements suggests that cotransfer of diverse virulence factors is advantageous in terms of expanding the insecticidal spectrum, overcoming insect resistance, or through gains in pathogenicity via synergistic interactions between toxins.IMPORTANCE Population genomics have provided many new insights into the formation, evolution, and dynamics of bacterial pathogens of humans and other higher animals, but these pathogens usually have very narrow host ranges. As a pathogen of insects and nematodes, Bacillus thuringiensis, which produces toxins showing toxicity to many orders of insects and other invertebrates, can be used as a model to study the evolution of pathogens with wide host ranges. Phylogenomic analysis revealed that host specialization and switching occur at the level of the major clade and subclade, respectively. A toxin gene co-occurrence network indicates that multiple toxins with similar targets were accumulated by the same cell in the whole species. This accumulation may be one of the strategies that B. thuringiensis has used to fight against host resistance. This kind of formation and evolution of pathogens represents a different path used against multiple invertebrate hosts from that used against higher animals.

Parasite performance and host alternation: is there a negative effect in host-specific and host-opportunistic parasites?

Environmental fluctuations are expected to require special adaptations only if they are associated with a decrease in fitness. We compared reproductive performance between fleas fed on alternating (preferred and non-preferred) hosts and fleas fed solely on either a preferred or a non-preferred host to determine whether (1) host alternation incurs an immediate negative effect, and, if yes, then (2) whether this effect is greater in a host specialist (Parapulex chephrenis) than in host generalists (Xenopsylla conformis and Synosternus cleopatrae). We also compared flea performance under alternating host regimes with different host order (initial feeding on either a preferred or a non-preferred host). An immediate negative effect of alternating hosts on reproductive performance was found in P. chephrenis only. These fleas produced 44·3% less eggs that were 3·6% smaller when they fed on alternating hosts as compared with a preferred host. In contrast, X. conformis and S. cleopatrae appeared to be able to adapt their reproductive strategy to host alternation by producing higher quality offspring (on average, 3·1% faster development and 2·1% larger size) without compromising offspring number. However, the former produced eggs that were slightly, albeit significantly, smaller when it fed on alternating hosts as compared with a preferred host. Moreover, host order affected reproductive performance in host generalists (e.g. 2·8% larger eggs when the first feeding was performed on a non-preferred host), but not in a host specialist. We conclude that immediate effects of environmental fluctuation on parasite fitness depend on the degree of host specialization.

Network structure and local adaptation in co-evolving bacteria-phage interactions

Numerous theoretical and experimental studies have investigated antagonistic co-evolution between parasites and their hosts. Although experimental tests of theory from a range of biological systems are largely concordant regarding the influence of several driving processes, we know little as to how mechanisms acting at the smallest scales (individual molecular and phenotypic changes) may result in the emergence of structures at larger scales, such as co-evolutionary dynamics and local adaptation. We capitalized on methods commonly employed in community ecology to quantify how the structure of community interaction matrices, so-called bipartite networks, reflected observed co-evolutionary dynamics, and how phages from these communities may or may not have adapted locally to their bacterial hosts. We found a consistent nested network structure for two phage types, one previously demonstrated to exhibit arms race co-evolutionary dynamics and the other fluctuating co-evolutionary dynamics. Both phages increased their host ranges through evolutionary time, but we found no evidence for a trade-off with impact on bacteria. Finally, only bacteria from the arms race phage showed local adaptation, and we provide preliminary evidence that these bacteria underwent (sometimes different) molecular changes in the wzy gene associated with the LPS receptor, while bacteria co-evolving with the fluctuating selection phage did not show local adaptation and had partial deletions of the pilF gene associated with type IV pili. We conclude that the structure of phage-bacteria interaction networks is not necessarily specific to co-evolutionary dynamics, and discuss hypotheses for why only one of the two phages was, nevertheless, locally adapted.

Relative importance of evolutionary dynamics depends on the composition of microbial predator-prey community

Community dynamics are often studied in subsets of pairwise interactions. Scaling pairwise interactions back to the community level is, however, problematic because one given interaction might not reflect ecological and evolutionary outcomes of other functionally similar species interactions or capture the emergent eco-evolutionary dynamics arising only in more complex communities. Here we studied this experimentally by exposing Pseudomonas fluorescens SBW25 prey bacterium to four different protist predators (Tetrahymena pyriformis, Tetrahymena vorax, Chilomonas paramecium and Acanthamoeba polyphaga) in all possible single-predator, two-predator and four-predator communities for hundreds of prey generations covering both ecological and evolutionary timescales. We found that only T. pyriformis selected for prey defence in single-predator communities. Although T. pyriformis selection was constrained in the presence of the intraguild predator, T. vorax, T. pyriformis selection led to evolution of specialised prey defence strategies in the presence of C. paramecium or A. polyphaga. At the ecological level, adapted prey populations were phenotypically more diverse, less stable and less productive compared with non-adapted prey populations. These results suggest that predator community composition affects the relative importance of ecological and evolutionary processes and can crucially determine when rapid evolution has the potential to change ecological properties of microbial communities.

Long-term coevolution between avian brood parasites and their hosts

Coevolutionary theory predicts that the most common long-term outcome of the relationships between brood parasites and their hosts should be coevolutionary cycles based on a dynamic change selecting the currently least-defended host species, given that when well-defended hosts are abandoned, hosts will be selected to decrease their defences as these are usually assumed to be costly. This is assumed to be the case also in brood parasite-host systems. Here I examine the frequency of the three potential long-term outcomes of brood parasite-host coevolution (coevolutionary cycles, lack of rejection, and successful resistance) in 182 host species. The results of simple exploratory comparisons show that coevolutionary cycles are very scarce while the lack of rejection and successful resistance, which are considered evolutionary enigmas, are much more frequent. I discuss these results considering (i) the importance of different host defences at all stages of the breeding cycle, (ii) the role of phenotypic plasticity in long-term coevolution, and (iii) the evolutionary history of host selection. I suggest that in purely antagonistic coevolutionary interactions, such as those involving brood parasites and their hosts, that although cycles will exist during an intermediate phase of the interactions, the arms race will end with the extinction of the host or with the host acquiring successful resistance. As evolutionary time passes, this resistance will force brood parasites to use previously less suitable host species. Furthermore, I present a model that represents the long-term trajectories and outcomes of coevolutionary interactions between brood parasites and their hosts with respect to the evolution of egg-rejection defence. This model suggests that as an increasing number of species acquire successful resistance, other unparasitized host species become more profitable and their parasitism rate and the costs imposed by brood parasitism at the population level will increase, selecting for the evolution of host defences. This means that although acceptance is adaptive when the parasitism rate and the costs of parasitism are very low, this cannot be considered to represent an evolutionary equilibrium, as conventional theory has done to date, because it is not stable.

Sympatric inhibition and niche differentiation suggest alternative coevolutionary trajectories among Streptomycetes

Soil bacteria produce a diverse array of antibiotics, yet our understanding of the specific roles of antibiotics in the ecological and evolutionary dynamics of microbial interactions in natural habitats remains limited. Here, we show a significant role for antibiotics in mediating antagonistic interactions and nutrient competition among locally coexisting Streptomycete populations from soil. We found that antibiotic inhibition is significantly more intense among sympatric than allopatric Streptomycete populations, indicating local selection for inhibitory phenotypes. For sympatric but not allopatric populations, antibiotic inhibition is significantly positively correlated with niche overlap, indicating that inhibition is targeted toward bacteria that pose the greatest competitive threat. Our results support the hypothesis that antibiotics serve as weapons in mediating local microbial interactions in soil and suggest that coevolutionary niche displacement may reduce the likelihood of an antibiotic arms race. Further insight into the diverse roles of antibiotics in microbial ecology and evolution has significant implications for understanding the persistence of antibiotic inhibitory and resistance phenotypes in environmental microbes, optimizing antibiotic drug discovery and developing strategies for managing microbial coevolutionary dynamics to enhance inhibitory phenotypes.

Energy expenditure for egg production in arthropod ectoparasites: the effect of host species

We studied the energy cost of egg production in two flea species (Parapulex chephrenis and Xenopsylla ramesis) feeding on principal (Acomys cahirinus and Meriones crassus, respectively) and auxiliary (M. crassus and A. cahirinus, respectively) rodent hosts. We predicted that fleas feeding on principal as compared with auxiliary hosts will (a) expend less energy for egg production; (b) produce larger eggs and (c) live longer after oviposition. Both fleas produced more eggs and spent less energy per egg when exploiting principal hosts. Parapulex chephrenis produced larger eggs after exploiting auxiliary hosts, while the opposite was true for X. ramesis. After oviposition, P. chephrenis fed on the auxiliary hosts survived for a shorter time than those fed on the principal hosts, while in X. ramesis the survival time did not differ among hosts. Our results suggested that one of the proximate causes for lower reproductive performance and subsequent lower abundance of fleas on auxiliary hosts is the higher energy cost of egg production. However, in some species, lower offspring number may be compensated to some extent by their size, although this compensation may also compromise their future reproduction via decreased survival. In addition, the reproductive strategy of exploitation of low profitable (i.e. auxiliary) hosts may differ between flea species.

When directional selection reduces geographic variation in traits mediating species interactions

Although we often focus on the causes of geographic variation, understanding processes that act to reduce geographic variation is also important. Here, we consider a process whereby adaptive foraging across the landscape and directional selection exerted by a conifer seed predator, the common crossbill (Loxia curvirostra), potentially act to homogenize geographic variation in the defensive traits of its prey. We measured seed predation and phenotypic selection exerted by crossbills on black pine (Pinus nigra) at two sites in the Pindos Mountains, Greece. Seed predation by crossbills was over an order of magnitude higher at the site where cone scale thickness was significantly thinner, which was also the cone trait that was the target of selection at the high predation site. Additional comparisons of selection differentials demonstrate that crossbills exert selection on black pine that is consistent in form across space and time, and increases in strength with increasing seed predation. If predators distribute themselves in relation to the defensive traits of their prey and the strength of selection predators exert is proportional to the amount of predation, then predators may act to homogenize trait variation among populations of their prey in a process analogous to coevolutionary alternation with escalation.

Effects of predation on real-time host-parasite coevolutionary dynamics

The impact of community complexity on pairwise coevolutionary dynamics is theoretically dependent on the extent to which species evolve generalised or specialised adaptations to the multiple species they interact with. Here, we show that the bacteria Pseudomonas fluorescens diversifies into defence specialists, when coevolved simultaneously with a virus and a predatory protist, as a result of fitness trade-offs between defences against the two enemies. Strong bacteria-virus pairwise coevolution persisted, despite strong protist-imposed selection. However, the arms race dynamic (escalation of host resistance and parasite infectivity ranges) associated with bacteria-virus coevolution broke down to a greater extent in the presence of the protist, presumably through the elevated genetic and demographic costs of increased bacteria resistance ranges. These findings suggest that strong pairwise coevolution can persist even in complex communities, when conflicting selection leads to evolutionary diversification of different defence strategies.

Modelling the maintenance of egg polymorphism in avian brood parasites and their hosts

In avian brood parasitism, egg phenotype plays a key role for both host and parasite reproduction. Several parrotbill species of the genus Paradoxornis are parasitized by the common cuckoo Cuculus canorus, and clear polymorphism in egg phenotype is observed. In this article, we develop a population genetics model in order to identify the key parameters that control the maintenance of egg polymorphism. The model analyses show that egg polymorphism can be maintained either statically as an equilibrium or dynamically with frequency oscillations depending on the sensitivity of the host against unlike eggs and how the parasite targets host nests with specific egg phenotypes. On the basis of the model, we discuss egg polymorphism observed in parrotbills and other host species parasitized by the cuckoo. We suggest the possibility that frequencies of egg phenotypes oscillate and we appeal for monitoring of cuckoo-host interactions over a large spatiotemporal scale.

Compositional and phylogenetic dissimilarity of host communities drives dissimilarity of ectoparasite assemblages: geographical variation and scale-dependence

We tested the hypothesis that compositional and/or phylogenetic dissimilarity of host assemblages affect compositional and/or phylogenetic dissimilarity of parasite assemblages, to different extents depending on scale, using regional surveys of fleas parasitic on small mammals from 4 biogeographical realms. Using phylogenetic community dissimilarity metric, we calculated the compositional and phylogenetic dissimilarity components between all pairs of host and parasite communities within realms and hemispheres. We then quantified the effect of compositional or phylogenetic dissimilarity in host regional assemblages, and geographical distance between assemblages, on the compositional or phylogenetic dissimilarity of flea regional assemblages within a realm, respectively. The compositional dissimilarity in host assemblages strongly affected compositional dissimilarity in flea assemblages within all realms and within both hemispheres. However, the effect of phylogenetic dissimilarity of host assemblages on that of flea assemblages was mostly confined to the Neotropics and Nearctic, but was detected in both the Old and New World at the higher scale, possibly because of phylogenetic heterogeneity in flea and host faunas between realms. The clearer effect of the compositional rather than the phylogenetic component of host community dissimilarity on flea community dissimilarity suggests important roles for host switching and ecological fitting during the assembly history of flea communities.

Bacteria-virus coevolution

Phages, viruses of bacteria, are ubiquitous. Many phages require host cell death to successfully complete their life cycle, resulting in reciprocal evolution of bacterial resistance and phage infectivity (antagonistic coevolution). Such coevolution can have profound consequences at all levels of biological organisation. Here, we review genetic and ecological factors that contribute to determining coevolutionary dynamics between bacteria and phages. We also consider some of the consequences of bacteria-phage coevolution, such as determining rates of molecular evolution and structuring communities, and how these in turn feedback into driving coevolutionary dynamics.

The ecological and evolutionary implications of merging different types of networks

Interactions among species drive the ecological and evolutionary processes in ecological communities. These interactions are effectively key components of biodiversity. Studies that use a network approach to study the structure and dynamics of communities of interacting species have revealed many patterns and associated processes. Historically these studies were restricted to trophic interactions, although network approaches are now used to study a wide range of interactions, including for example the reproductive mutualisms. However, each interaction type remains studied largely in isolation from others. Merging the various interaction types within a single integrative framework is necessary if we want to further our understanding of the ecological and evolutionary dynamics of communities. Dividing the networks up is a methodological convenience as in the field the networks occur together in space and time and will be linked by shared species. Herein, we outline a conceptual framework for studying networks composed of more than one type of interaction, highlighting key questions and research areas that would benefit from their study.

Climatic gradients of arms race coevolution

In nature, spatiotemporally dynamic coevolutionary processes play major roles in the foundation and maintenance of biodiversity. Here, we examined the arms race coevolution involving a seed-eating weevil with a long snout and its camellia plant host with a thick fruit coat (pericarp) throughout the marked climatic gradient observed across the Japanese islands. Results demonstrated that female weevils, which bored holes through camellia pericarps to lay eggs into seeds, had evolved much longer snouts than males, especially in areas in which Japanese camellia pericarps were very thick. The thickness of the plant pericarp was heritable, and the camellia plant evolved a significantly thicker pericarp on islands with the weevil than on islands without it. Across populations with weevils, resource allocation to plant defense increased with increasing annual mean temperature or annual precipitation, thereby geographically differentiating the evolutionary and ecological interactions between the two species. Given that the coevolutionary relationship exhibited appreciable variation across a relatively small range of annual mean temperatures, ongoing global climatic change can dramatically alter the coevolutionary process, thereby changing the ecological interaction between these species.

Spatial geographic mosaic in an aquatic predator-prey network

The geographic mosaic theory of coevolution predicts 1) spatial variation in predatory structures as well as prey defensive traits, and 2) trait matching in some areas and trait mismatching in others mediated by gene flow. We examined gene flow and documented spatial variation in crushing resistance in the freshwater snails Mexipyrgus churinceanus, Mexithauma quadripaludium, Nymphophilus minckleyi, and its relationship to the relative frequency of the crushing morphotype in the trophically polymorphic fish Herichthys minckleyi. Crushing resistance and the frequency of the crushing morphotype did show spatial variation among 11 naturally replicated communities in the Cuatro Ciénegas valley in Mexico where these species are all endemic. The variation in crushing resistance among populations was not explained by geographic proximity or by genetic similarity in any species. We detected clear phylogeographic patterns and limited gene flow for the snails but not for the fish. Gene flow among snail populations in Cuatro Ciénegas could explain the mosaic of local divergence in shell strength and be preventing the fixation of the crushing morphotype in Herichthys minckleyi. Finally, consistent with trait matching across the mosaic, the frequency of the fish morphotype was negatively correlated with shell crushing resistance likely reflecting the relative disadvantage of the crushing morphotype in communities where the snails exhibit relatively high crushing resistance.

The coevolving web of life

Coevolution--reciprocal evolutionary change in interacting species--is one of the central biological processes organizing the web of life, and most species are involved in one or more coevolved interactions. We have learned in recent years that coevolution is a highly dynamic process that continually reshapes interactions among species across ecosystems, creating geographic mosaics over timescales sometimes as short as thousands or even hundreds of years. If we take that as our starting point, what should we now be asking about the coevolutionary process? Here I suggest five major questions that we need to answer if we are to understand how coevolution shapes the web of life. How evolutionarily dynamic is specialization to other species, and what is the role of coevolutionary alternation in driving those dynamics? Does the geographic mosaic of coevolution shape adaptation in fundamentally different ways in different forms of interaction? How does the geographic mosaic of coevolution shape speciation? How does the structure of reciprocal selection change during the assembly of large webs of interacting species? How important are genomic events such as whole-genome duplication (i.e., polyploidy) and whole-genome capture (i.e., hybridization) in generating novel webs of interacting species? I end by suggesting four points about coevolution that we should tell every new student or researcher in biology.

Retention of mutualism in a geographically diverging interaction

A current challenge in coevolutionary biology is to understand how interactions between pairs of species change as they diversify into multispecific interactions. We tested whether the previously demonstrated pairwise mutualism between the widespread pollinating floral parasite Greya politella and its Lithophragma hostplants is ecologically enhanced or diminished in a region in which another Greya species, Greya obscura, uses the same host, Lithophragma cymbalaria. Field surveys and experimental trials showed that pollination efficacy by G. politella was more than an order of magnitude higher than by G. obscura, but G. politella abundance varied greatly between years. Greya obscura had a strongly positive effect on seed set in a year when G. politella densities were exceptionally low. Our results suggest that the coevolving mutualism between Greya and Lithophragma has potentially been enhanced rather than diminished as this interaction has diversified in the number of pollinating Greya species.