Mutualism at the leading edge: insights into the eco-evolutionary dynamics of host-symbiont communities during range expansion

The evolution of mutualism between host and symbiont communities plays an essential role in maintaining ecosystem function and should therefore have a profound effect on their range expansion dynamics. In particular, the presence of mutualistic symbionts at the leading edge of a host-symbiont community should enhance its propagation in space. We develop a theoretical framework that captures the eco-evolutionary dynamics of host-symbiont communities, to investigate how the evolution of resource exchange may shape community structure during range expansion. We consider a community with symbionts that are mutualistic or parasitic to various degrees, where parasitic symbionts receive the same amount of resource from the host as mutualistic symbionts, but at a lower cost. The selective advantage of parasitic symbionts over mutualistic ones is increased with resource availability (i.e. with host density), promoting mutualism at the range edges, where host density is low, and parasitism at the population core, where host density is higher. This spatial selection also influences the speed of spread. We find that the host growth rate (which depends on the average benefit provided by the symbionts) is maximal at the range edges, where symbionts are more mutualistic, and that host-symbiont communities with high symbiont density at their core (e.g. resulting from more mutualistic hosts) spread faster into new territories. These results indicate that the expansion of host-symbiont communities is pulled by the hosts but pushed by the symbionts, in a unique push-pull dynamic where both the host and symbionts are active and tightly-linked players.

A hotspot of Toxoplasma gondii Africa 1 lineage in Benin: How new genotypes from West Africa contribute to understand the parasite genetic diversity worldwide

Through international trades, Europe, Africa and South America share a long history of exchanges, potentially of pathogens. We used the worldwide parasite Toxoplasma gondii to test the hypothesis of a historical influence on pathogen genetic diversity in Benin, a West African country with a longstanding sea trade history. In Africa, T. gondii spatial structure is still non-uniformly studied and very few articles have reported strain genetic diversity in fauna and clinical forms of human toxoplasmosis so far, even in African diaspora. Sera from 758 domestic animals (mainly poultry) in two coastal areas (Cotonou and Ouidah) and two inland areas (Parakou and Natitingou) were tested for T. gondii antibodies using a Modified Agglutination Test (MAT). The hearts and brains of 69 seropositive animals were collected for parasite isolation in a mouse bioassay. Forty-five strains were obtained and 39 genotypes could be described via 15-microsatellite genotyping, with a predominance of the autochthonous African lineage Africa 1 (36/39). The remaining genotypes were Africa 4 variant TUB2 (1/39) and two identical isolates (clone) of Type III (2/39). No difference in terms of genotype distribution between inland and coastal sampling sites was found. In particular, contrarily to what has been described in Senegal, no type II (mostly present in Europe) was isolated in poultry from coastal cities. This result seems to refute a possible role of European maritime trade in Benin despite it was one of the most important hubs during the slave trade period. However, the presence of the Africa 1 genotype in Brazil, predominant in Benin, and genetic analyses suggest that the triangular trade was a route for the intercontinental dissemination of genetic strains from Africa to South America. This supports the possibility of contamination in humans and animals with potentially imported virulent strains.

The parasite Toxoplasma gondii is a worldwide-distributed pathogen, able to infect all warm-blooded animals. There are important differences in the clinical expression of the infection in direct relation with the parasite genetic profile. In some regions, the geographical structuration of its genetic diversity points towards a crucial role of human activities in some lineages introduction or sorting. Benin is a West African country with a history of extensive transcontinental exchanges. Our genetic study of Toxoplasma in Benin shows a surprisingly homogeneous and autochthonous diversity, which contrasts with previous studies from other West and Central African countries. In Benin, the absence of European Toxoplasma lineages may be explained by the extreme rarity of the house mouse (Mus musculus), a host species that was previously described as highly susceptible to the mouse-virulent African strains. Might Benin be the origin region for the Africa 1 lineage, our results suggest that Guinean Gulf coasts may be a starting point of this lineage towards South America, especially Brazil, during the slave trade. As a whole, the present study provides further insights into the recent evolutionary history of Toxoplasma gondii and its consequences on human and animal health.

When the exception becomes the rule: An integrative approach to symbiosis

Symbiosis, mainly due to the advances in -omics technology and to the microbiome revolution, is being increasingly acknowledged as fundamental to explain any aspect of life existence. Previously considered an exception, a peculiar characteristic of few systems like lichens, corals and mycorrhizas, symbiosis is nowadays recognized as the rule, with the microbiome being part of all living entities and systems. However, our knowledge of the ecological meaning and functioning of many symbiotic systems is still limited. Here, we discuss a new, integrative approach based on current findings that looks at commonalities among symbiotic systems to produce theoretical models and conceptual knowledge that would allow a more efficient exploitation of symbiosis-based biotechnologies. The microbiome recruitment and assemblage processes are indicated as one of the potential targets where a holistic approach could bring advantages. Finally, we reflect on the potential socio-economic and environmental consequences of a symbiotic view of the world, where co-dependence is the matrix of life.

Aseptic Rearing and Infection with Gut Bacteria Improve the Fitness of Transgenic Diamondback Moth, Plutella xylostella

Mass insect rearing can have a range of applications, for example in biological control of pests. The competitive fitness of released insects is extremely important in a number of applications. Here, we investigated how to improve the fitness of a transgenic diamondback moth, which has shown variation in mating ability when reared in different insectaries. Specifically we tested whether infection with a gut bacteria, Enterobacter cloacae, and aseptic rearing of larvae could improve insect growth and male performance. All larvae were readily infected with E. cloacae. Under aseptic rearing, pupal weights were reduced and there was a marginal reduction in larval survival. However, aseptic rearing substantially improved the fitness of transgenic males. In addition, under aseptic rearing, inoculation with E. cloacae increased pupal weights and male fitness, increasing the proportion of transgenic progeny from 20% to 30% relative to uninfected insects. Aseptic conditions may improve the fitness of transgenic males by excluding microbial contaminants, while symbiont inoculation could further improve fitness by providing additional protection against infection, or by normalizing insect physiology. The simple innovation of incorporating antibiotic into diet, and inoculating insects with symbiotic bacteria that are resistant to that antibiotic, could provide a readily transferable tool for other insect rearing systems.

Reciprocal immune benefit based on complementary production of antibiotics by the leech Hirudo verbana and its gut symbiont Aeromonas veronii

The medicinal leech has established a long-term mutualistic association with Aeromonas veronii, a versatile bacterium which can also display free-living waterborne and fish- or human-pathogenic lifestyles. Here, we investigated the role of antibiotics in the dynamics of interaction between the leech and its gut symbiont Aeromonas. By combining biochemical and molecular approaches, we isolated and identified for the first time the antimicrobial peptides (AMPs) produced by the leech digestive tract and by its symbiont Aeromonas. Immunohistochemistry data and PCR analyses evidenced that leech AMP genes are induced in the gut epithelial cells when Aeromonas load is low (starved animals), while repressed when Aeromonas abundance is the highest (post blood feeding). The asynchronous production of AMPs by both partners suggests that these antibiotic substances (i) provide them with reciprocal protection against invasive bacteria and (ii) contribute to the unusual simplicity of the gut microflora of the leech. This immune benefit substantially reinforces the evidence of an evolutionarily stable association between H. verbana and A. veronii. Altogether these data may provide insights into the processes making the association with an Aeromonas species in the digestive tract either deleterious or beneficial.

Cheaters must prosper: reconciling theoretical and empirical perspectives on cheating in mutualism

Cheating is a focal concept in the study of mutualism, with the majority of researchers considering cheating to be both prevalent and highly damaging. However, current definitions of cheating do not reliably capture the evolutionary threat that has been a central motivation for the study of cheating. We describe the development of the cheating concept and distill a relative-fitness-based definition of cheating that encapsulates the evolutionary threat posed by cheating, i.e. that cheaters will spread and erode the benefits of mutualism. We then describe experiments required to conclude that cheating is occurring and to quantify fitness conflict more generally. Next, we discuss how our definition and methods can generate comparability and integration of theory and experiments, which are currently divided by their respective prioritisations of fitness consequences and traits. To evaluate the current empirical evidence for cheating, we review the literature on several of the best-studied mutualisms. We find that although there are numerous observations of low-quality partners, there is currently very little support from fitness data that any of these meet our criteria to be considered cheaters. Finally, we highlight future directions for research on conflict in mutualisms, including novel research avenues opened by a relative-fitness-based definition of cheating.

Evolutionary signals of symbiotic persistence in the legume-rhizobia mutualism

Understanding the origins and evolutionary trajectories of symbiotic partnerships remains a major challenge. Why are some symbioses lost over evolutionary time whereas others become crucial for survival? Here, we use a quantitative trait reconstruction method to characterize different evolutionary stages in the ancient symbiosis between legumes (Fabaceae) and nitrogen-fixing bacteria, asking how labile is symbiosis across different host clades. We find that more than half of the 1,195 extant nodulating legumes analyzed have a high likelihood (>95%) of being in a state of high symbiotic persistence, meaning that they show a continued capacity to form the symbiosis over evolutionary time, even though the partnership has remained facultative and is not obligate. To explore patterns associated with the likelihood of loss and retention of the N2-fixing symbiosis, we tested for correlations between symbiotic persistence and legume distribution, climate, soil and trait data. We found a strong latitudinal effect and demonstrated that low mean annual temperatures are associated with high symbiotic persistence in legumes. Although no significant correlations between soil variables and symbiotic persistence were found, nitrogen and phosphorus leaf contents were positively correlated with legumes in a state of high symbiotic persistence. This pattern suggests that highly demanding nutrient lifestyles are associated with more stable partnerships, potentially because they "lock" the hosts into symbiotic dependency. Quantitative reconstruction methods are emerging as a powerful comparative tool to study broad patterns of symbiont loss and retention across diverse partnerships.

Dispersal and spatial heterogeneity allow coexistence between enemies and protective mutualists

Protective mutualisms, where a symbiont reduces the negative effects of another species on a shared host, represent a common type of species interaction in natural communities, yet it is still unclear what ecological conditions might favor their emergence. Studies suggest that the initial evolution of protective mutualists might involve closely related pathogenic variants with similar life histories, but different competitive abilities and impacts on host fitness. We derive a model to evaluate this hypothesis and show that, in general, a protective variant cannot spread from rarity or exclude a more pathogenic strain. While the conditions allowing mutualist invasion are more likely with increased environmental productivity, they still depend on initial densities in the invaded patch exceeding a threshold, highlighting the likely importance of spatial structure and demographic stochasticity. Using a numerical simulation approach, we show that regional coexistence is in fact possible in an explicitly spatial system and that, under some circumstances, the mutualist population can exclude the enemy. More broadly, the establishment of protective mutualists may be favored when there are other life-history differences from more pathogenic symbionts, such as vertical transmission or additional direct benefits to hosts.

Selection for cheating across disparate environments in the legume-rhizobium mutualism

The primary dilemma in evolutionarily stable mutualisms is that natural selection for cheating could overwhelm selection for cooperation. Cheating need not entail parasitism; selection favours cheating as a quantitative trait whenever less-cooperative partners are more fit than more-cooperative partners. Mutualisms might be stabilised by mechanisms that direct benefits to more-cooperative individuals, which counter selection for cheating; however, empirical evidence that natural selection favours cheating in mutualisms is sparse. We measured selection on cheating in single-partner pairings of wild legume and rhizobium lineages, which prevented legume choice. Across contrasting environments, selection consistently favoured cheating by rhizobia, but did not favour legumes that provided less benefit to rhizobium partners. This is the first simultaneous measurement of selection on cheating across both host and symbiont lineages from a natural population. We empirically confirm selection for cheating as a source of antagonistic coevolutionary pressure in mutualism and a biological dilemma for models of cooperation.

Simulating the evolution of the human family: cooperative breeding increases in harsh environments

Verbal and mathematical models that consider the costs and benefits of behavioral strategies have been useful in explaining animal behavior and are often used as the basis of evolutionary explanations of human behavior. In most cases, however, these models do not account for the effects that group structure and cultural traditions within a human population have on the costs and benefits of its members' decisions. Nor do they consider the likelihood that cultural as well as genetic traits will be subject to natural selection. In this paper, we present an agent-based model that incorporates some key aspects of human social structure and life history. We investigate the evolution of a population under conditions of different environmental harshness and in which selection can occur at the level of the group as well as the level of the individual. We focus on the evolution of a socially learned characteristic related to individuals' willingness to contribute to raising the offspring of others within their family group. We find that environmental harshness increases the frequency of individuals who make such contributions. However, under the conditions we stipulate, we also find that environmental variability can allow groups to survive with lower frequencies of helpers. The model presented here is inevitably a simplified representation of a human population, but it provides a basis for future modeling work toward evolutionary explanations of human behavior that consider the influence of both genetic and cultural transmission of behavior.

Dispersal network structure and infection mechanism shape diversity in a coevolutionary bacteria-phage system

Resource availability, dispersal and infection genetics all have the potential to fundamentally alter the coevolutionary dynamics of bacteria-bacteriophage interactions. However, it remains unclear how these factors synergise to shape diversity within bacterial populations. We used a combination of laboratory experiments and mathematical modeling to test how the structure of a dispersal network affects host phenotypic diversity in a coevolving bacteria-phage system in communities of differential resource input. Unidirectional dispersal of bacteria and phage from high to low resources consistently increased host diversity compared with a no dispersal regime. Bidirectional dispersal, on the other hand, led to a marked decrease in host diversity. Our mathematical model predicted these opposing outcomes when we incorporated modified gene-for-gene infection genetics. To further test how host diversity depended on the genetic underpinnings of the bacteria-phage interaction, we expanded our mathematical model to include different infection mechanisms. We found that the direction of dispersal had very little impact on bacterial diversity when the bacteria-phage interaction was mediated by matching alleles, gene-for-gene or related infection mechanisms. Our experimental and theoretical results demonstrate that the effects of dispersal on diversity in coevolving host-parasite systems depend on an intricate interplay of the structure of the underlying dispersal network and the specifics of the host-parasite interaction.

Coexistence of mutualists and non-mutualists in a dual-lattice model

Evolution and maintenance of mutualism have been one of the major questions in evolutionary ecology, because it is often susceptible of invasion of non-mutualistic strategy. Some previous studies using dual-lattice model suggest that spatial structures of habitat can prevent non-mutualism from prevailing over mutualism, while the detail of the dynamics is not fully revealed. Here we explore population dynamics of the two strategies (mutualism and non-mutualism) in two species engaged in Prisoner's Dilemma game on a dual-lattice space, especially focusing on whether mutualists and non-mutualists can coexist in long-term dynamics. The habitat consists of two layers, each of which a population of species inhabits, and interspecific interaction is restricted between two corresponding sites of the layers. Each individual of the both species is either a mutualist or a non-mutualist and only the former pay cost c for benefit of the partner b. The payoff of the game affects the individuals' fecundity, while the mortality is constant. Reproduction is restricted to neighboring vacant sites of the focal individuals. Our computer simulations of the model show that even if b/c ratio remains constant, mutualists become dominant in both species over wider ranges of basic reproduction rate (reproduction rate without interspecific interaction) as b and c increase. If basic reproduction rates are asymmetric between the species or basic reproduction rates were sufficiently large, mutualists and non-mutualists can coexist in one or both species, while their population sizes often fluctuate. Transition of the final state between mutualism and non-mutualism happens rather discontinuously, then total population sizes change drastically at the transition. Moreover, we also find paradoxical cases of unilateral exploitation, i.e. one species consists of mutualists and other species non-mutualists. Additional simulations reveal that accidental extinction of the non-mutualists of one species can result in extinction of mutualist of the other species.

From metabolism to ecology: cross-feeding interactions shape the balance between polymicrobial conflict and mutualism

Polymicrobial interactions are widespread in nature and play a major role in maintaining human health and ecosystems. Whenever one organism uses metabolites produced by another organism as energy or nutrient sources, it is called cross-feeding. The ecological outcomes of cross-feeding interactions are poorly understood and potentially diverse: mutualism, competition, exploitation, or commensalism. A major reason for this uncertainty is the lack of theoretical approaches linking microbial metabolism to microbial ecology. To address this issue, we explore the dynamics of a one-way interspecific cross-feeding interaction in which food can be traded for a service (detoxification). Our results show that diverse ecological interactions (competition, mutualism, exploitation) can emerge from this simple cross-feeding interaction and can be predicted by the metabolic, demographic, and environmental parameters that govern the balance of the costs and benefits of association. In particular, our model predicts stronger mutualism for intermediate by-product toxicity because the resource-service exchange is constrained to the service being neither too vital (high toxicity impairs resource provision) nor dispensable (low toxicity reduces need for service). These results support the idea that bridging microbial ecology and metabolism is a critical step toward a better understanding of the factors governing the emergence and dynamics of polymicrobial interactions.

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.

Evolutionary ecology of mycorrhizal functional diversity in agricultural systems

The root systems of most agronomic crops are colonized by diverse assemblages of arbuscular mycorrhizal fungi (AMF), varying in the functional benefits (e.g. nutrient transfer, pathogen protection, water uptake) provided to hosts. Little is known about the evolutionary processes that shape the composition of these fungal assemblages, nor is it known whether more diverse assemblages are beneficial to crop productivity. In this review we aim to identify the evolutionary selection pressures that shape AMF diversity in agricultural systems and explore whether promotion of AMF diversity can convincingly be linked to increases in agricultural productivity and/or sustainability. We then ask whether farmers can (and should) actively modify evolutionary selection pressures to increase AMF functioning. We focus on three agriculturally imposed selection regimes: tillage, fertilization, and continuous monoculture. We find that the uniform nature of these practices strongly selects for dominance of few AMF species. These species exhibit predictable, generally non-beneficial traits, namely heavy investment in reproduction at the expense of nutrient scavenging and transfer processes that are beneficial for hosts. A number of focus-points are given based on empirical and theoretical evidence that could be utilized to slow down negative selection pressures on AMF functioning, therein increasing crop benefit.

A unified model for the coevolution of resistance, tolerance, and virulence

We present a general host-parasite model that unifies previous theory by investigating the coevolution of virulence, resistance, and tolerance, with respect to multiple physiological, epidemiological, and environmental parameters. Four sets of new predictions emerge. First, compared to virulence coevolving with resistance or tolerance, three-trait coevolution promotes more virulence and less tolerance, and broadens conditions under which pure defenses evolve. Second, the cost and efficiency of virulence and the epidemiological rates are the key factors of virulence coevolving with resistance and tolerance. Maximum virulence evolves for intermediate infection rate, at which coevolved levels of resistance and tolerance are both high. The influence of host and parasite background mortalities is strong on the evolution of defenses and weak on the coevolution of virulence. Third, evolutionary correlations between defenses can switch sign along single-parameter gradients. The evolutionary trade-off between resistance and tolerance may coevolve with virulence that either increases or decreases monotonically, depending on the underlying parameter gradient. Fourth, despite global attractiveness and stability of coevolutionary equilibria, not-so-rare and not-so-small mutations can beget large variation in virulence and defenses around equilibrium, in the form of transient "evolutionary spikes." Implications for evolutionary management of infections are discussed and directions for future research are outlined.

Genetic covariation of the marine fungal symbiont Haloguignardia irritans (Ascomycota, Pezizomycotina) with its algal hosts Cystoseira and Halidrys (Phaeophyceae, Fucales) along the west coast of North America

The fungal endophyte Haloguignardia irritans induces gall formation on the brown algal genera Cystoseira and Halidrys occurring from Oregon to Baja California, Mexico. Here we examine genetic covariation and compare rDNA phylogenies to investigate the coevolutionary histories of H. irritans and its algal hosts. Despite recognition of H. irritans as a single morphological species, internal transcribed spacer rDNA sequences representative of its geographic range are characterized by sequence variation at the intraspecific to intrageneric levels. An assessment of parallel cladogenesis between endophyte and host phylogenies provides evidence for a combination of independent fungal divergence and host jumping, similar to that observed in terrestrial lichens. Our results suggest that reduced gene flow due to geographic isolation is a major contributing factor to more concerted covariation observed at one island site, rather than to differences among algal host species alone. Because geography and its effects on gene flow can create heterogeneous mosaics of coevolution for symbioses in terrestrial environments, our results support the notion that conservation efforts toward the maintenance of genetic diversity in marine environments should likewise consider geographic complexity and its effects on coevolving marine species.

Disentangling nature, strength and stability issues in the characterization of population interactions

Many recent reviews discuss the adequacy of definitions and metrics for the strength of population interactions. However, the discussion on the beneficial or detrimental nature of interactions is clearly absent, or at the most, inadvertently merged into the strength debate. This deficiency is emerging with the increasing interest in theoretical studies of interactions that shift in their nature; e.g. associations that present a mixture of mutualistic and antagonistic aspects, such as pollination; or species with changes in role, such as mutualistic ants that predate on aphid partners. By exploring these models, major controversies are revealed underlying some traditional perspectives: the original Levins'community matrix reformulated into interaction and jacobian matrices, that is, interaction coefficients reinterpreted as partial derivatives, fail to recognize the ecological context of interactions. The 'effect of one species on the other' is not necessarily quantified by 'the effect of varying species densities'; and shifts in the signs of jacobian elements do not correspond to shifts in types of interaction but to stability properties. Thus, the generalised use of these approaches must be revised. On the other hand, the comparison of ultimate performances of populations when growing alone or in association, here referred to as the relative performance approach, conceptually represents the original meaning of the community matrix. This conception, although measured at population levels, is a reflection of properties at the individual level. This article inspects and discusses the formalities and ecological contexts of these approaches to characterization by means of known population interaction models: linear and non-linear, variable and non-variable; aiming to disentangle crucial conceptions that are usually mingled in the literature: the strength (magnitude) and the nature (detrimental or beneficial) of the interaction, which are sometimes used interchangeably, and the stability properties of the system, which have been misleadingly associated with the latter.

Evolutionary biology, community ecology and avian influenza research

The epidemiology of H5N1 HPAI is still unclear despite the efforts of the research community. Studies bringing new insights add more variability in the host-pathogen system and uncertainty in the prediction of local risks. Global analyses of the pathways of wild birds in parallel with virus outbreaks have brought limited conclusions once the raw information was extracted from relevant maps. In this article, we propose an integration of epidemiology, evolutionary biology and community ecology on a local level in a research framework. This multidisciplinary approach aims at understanding the pathogen transmission processes at the interface between different bird groups whether wild or domesticated. We believe that this ecological data brought together with the epidemiological and molecular data is a key element to explore the mechanism of the AIV ecology in their hosts.

Evolution of pathogens in a man-made world

Human activities have resulted in substantial, large-scale environmental modifications, especially in the past century. Ecologists and evolutionary biologists are increasingly coming to realize that parasites and pathogens, like free-living organisms, evolve as the consequence of these anthropogenic changes. Although this area now commands the attention of a variety of researchers, a broad predictive framework is lacking, mainly because the links between human activities, the environment and parasite evolution are complex. From empirical and theoretical examples chosen in the literature, we give an overview of the ways in which humans can directly or indirectly influence the evolution of different traits in parasites (e.g. specificity, virulence, polymorphism). We discuss the role of direct and indirect factors as diverse as habitat fragmentation, pollution, biodiversity loss, climate change, introduction of species, use of vaccines and antibiotics, ageing of the population, etc. We also present challenging questions for further research. Understanding the links between anthropogenic changes and parasite evolution needs to become a cornerstone of public health planning, economic development and conservation biology.

Gene flow reverses an adaptive cline in a coevolving host-parasitoid interaction

Many natural populations are characterized by clinal patterns of adaptation, but it is unclear how gene flow and environmental gradients interact to drive such clines. We addressed this question by directly manipulating dispersal and productivity in an experimental landscape containing a microbial parasitoid, the bacteriophage T7, and its host, the bacterium Escherichia coli. We observed that the adaptation of parasitoids increased on hosts originating from lower-productivity communities in the absence of gene flow. However, adaptation decreased along the same productivity gradient with experimentally imposed gene flow of the host and parasitoid. This occurred despite relatively low rates of gene flow.

Coevolution of symbiotic mutualists and parasites in a community context

Recent advances in our knowledge of parasitic and mutualistic associations have confirmed the central role of coevolutionary interactions in population and community ecology. Here, we discuss the potential coevolutionary interdependence of the strength and specificity of symbiotic interactions with the complexity and productivity of their environment. We predict that interactions become less beneficial with increasing environmental quality and that the association of productivity with symbiont specificity depends on the relative strengths of tradeoffs between host range and other life-history parameters. However, as biotic complexity increases, pathogen specificity is predicted to decline, whereas mutualist specificity will increase. Testing these predictions on a geographical scale would contribute significantly to the predictive science of coevolution, and to our ability to manage biological interactions embedded in increasingly fragmented landscapes.

Limiting cheaters in mutualism: evidence from hybridization between mutualist and cheater yucca moths

Mutualisms are balanced antagonistic interactions where both species gain a net benefit. Because mutualisms generate resources, they can be exploited by individuals that reap the benefits of the interaction without paying any cost. The presence of such 'cheaters' may have important consequences, yet we are only beginning to understand how cheaters evolve from mutualists and how their evolution may be curtailed within mutualistic lineages. The yucca-yucca moth pollination mutualism is an excellent model in this context as there have been two origins of cheating from within the yucca moth lineage. We used nuclear and mitochondrial DNA markers to examine genetic structure in a moth population where a cheater species is parapatric with a resident pollinator. The results revealed extensive hybridization between pollinators and cheaters. Hybrids were genetically intermediate to parental populations, even though all individuals in this population had a pollinator phenotype. The results suggest that mutualisms can be stable in the face of introgression of cheater genes and that the ability of cheaters to invade a given mutualism may be more limited than previously appreciated.

The evolution of plant-insect mutualisms

Mutualisms (cooperative interactions between species) have had a central role in the generation and maintenance of life on earth. Insects and plants are involved in diverse forms of mutualism. Here we review evolutionary features of three prominent insect-plant mutualisms: pollination, protection and seed dispersal. We focus on addressing five central phenomena: evolutionary origins and maintenance of mutualism; the evolution of mutualistic traits; the evolution of specialization and generalization; coevolutionary processes; and the existence of cheating. Several features uniting very diverse insect-plant mutualisms are identified and their evolutionary implications are discussed: the involvement of one mobile and one sedentary partner; natural selection on plant rewards; the existence of a continuum from specialization to generalization; and the ubiquity of cheating, particularly on the part of insects. Plant-insect mutualisms have apparently both arisen and been lost repeatedly. Many adaptive hypotheses have been proposed to explain these transitions, and it is unlikely that any one of them dominates across interactions differing so widely in natural history. Evolutionary theory has a potentially important, but as yet largely unfilled, role to play in explaining the origins, maintenance, breakdown and evolution of insect-plant mutualisms.

Diet-dependent effects of gut bacteria on their insect host: the symbiosis of Erwinia sp. and western flower thrips

Studies on bacteria in the gut of insect species are numerous, but their focus is hardly ever on the impact on host performance. We showed earlier that Erwinia bacteria occur in the gut of western flower thrips, most probably acquired during feeding. Here, we investigate whether thrips gain a net benefit or pay a net cost because of these gut bacteria. On a diet of cucumber leaves, the time to maturity is shorter and the oviposition rate is higher in thrips with bacteria than in thrips without (aposymbionts). When fed on cucumber leaves and pollen, aposymbionts develop faster and lay more eggs. So Erwinia bacteria benefit or parasitize their thrips hosts depending on the diet, which is in accordance with theoretical predictions for fitness of organisms engaged in symbiotic interactions. Possibly, the transmission of gut bacteria has not become strictly vertical because of this diet-dependent fitness variability.

HAPLODIPLOIDY AS AN OUTCOME OF COEVOLUTION BETWEEN MALE-KILLING CYTOPLASMIC ELEMENTS AND THEIR HOSTS

Haplodiploidy (encompassing both arrhenotoky and paternal genome elimination) could have originated from coevolution between male-killing endosymbiotic bacteria and their hosts. In insects, haplodiploidy tends to arise in lineages that rely on maternally transmitted bacteria for nutrition and that have gregarious broods in which competition between siblings may occur. When siblings compete, there is strong selection on maternally transmitted elements to kill males. I consider a hypothetical bacterial phenotype that renders male zygotes effectively haploid by preventing chromosome decondensation in male-determining sperm nuclei. By causing high male mortality, such a phenotype can be advantageous to the bacterial lineage. By eliminating paternal genes, it can also be advantageous to the host female. A simple model shows that the host female will benefit under a wide range of values for the efficiency of resource re-allocation, the efficiency of transmission, and the viability of haploid males. This hypothesis helps to explain the ecological correlates of the origins of haplodiploidy, as well as such otherwise puzzling phenomena as obligate cannibalism by male Micromalthus beetles, reversion to diploidy by aposymbiotic male stictococcid scale insects, and the bizarre genomic constitution of scale insect bacteriomes.

Uniform vertical transmission and selection in a host–symbiont system. Non-random symbiont distribution generates apparent differential selection

We investigate the survival, prevalence, and distribution of a symbiont in its host population when the forces of vertical transmission (beta) and symbiont-induced selection (s) are uniform (invariant across host genotypes). We use host-symbiont disequilibria to quantify the role played by non-random associations between host genotypes and the symbiont in altering host genetic structure. Results show a larger part of the parameter space permits symbiont survival under mutualism (beta>/=0.25) than parasitism (beta>0.5). The nonlinear interaction between beta and s determines symbiont survival and prevalence at equilibrium; initial symbiont prevalence is a factor only in a small number of parameter combinations. The symbiont's non-random distribution generates apparent differential selection, when selective differences across host genotypes and alleles exist under uniform selection. The direction of change in host allele frequencies is determined exclusively by the signs of s and the allelic disequilibrium. Disequilibria cannot be created or maintained, and heterozygote disequilibrium changes sign in a greater number of runs and at higher magnitudes than homozygote disequilibria. This investigation increases our understanding of the interactions between vertical transmission and selection, and their effect on the coevolutionary dynamics and final states of interacting species under different selection regimes.

Coevolution between hosts and parasites with partially overlapping geographic ranges

Many host species interact with a specific parasite within only a fraction of their geographical range. Where host and parasite overlap geographically, selection may be reciprocal constituting a coevolutionary hot spot. Host evolution, however, may be driven primarily by selection imposed by alternative biotic or abiotic factors that occur outside such hot spots. To evaluate the importance of coevolutionary hot spots for host and parasite evolution, we analyse a spatially explicit genetic model for a host that overlaps with a parasite in only part of its geographical range. Our results show that there is a critical amount of overlap beyond which reciprocal selection leads to a coevolutionary response in the host. This critical amount of overlap depends upon the explicit spatial configuration of hot spots. When the amount of overlap exceeds this first critical level, host-parasite coevolution commonly generates stable allele frequency clines rather than oscillations. It is within this region that one of the primary predictions of the geographic mosaic theory is realized, and local maladaptation is prevalent in both species. Past a further threshold of overlap between the species oscillations do evolve, but allele frequencies in both species are spatially synchronous and local maladaptation is absent in both species. A consequence of such transitions between coevolutionary dynamics is that parasite adaptation is inversely proportional to the fraction of its host's range that it occupies. Hence, as the geographical range of a parasite increases, it becomes increasingly maladapted to the host. This suggests a novel mechanism through which the geographical range of parasites may be limited.

Interacting Guilds: Moving beyond the Pairwise Perspective on Mutualisms

Most textbook treatments imply, and almost all theoretical analyses assume, that mutualistic interactions take place between a single pair of interacting partner species. A major goal of this symposium is to broaden and shift this pairwise perspective and make it concordant with the emerging view that locally exclusive mutualisms between just two species are the exception and that many communities include guilds of mutualistic species on one or both sides of the interaction. Many pollination and seed-dispersal mutualisms have long been recognized as diffuse, but recent molecular analyses are revealing unrecognized partner diversity in mutualistic interactions previously thought to be locally species specific. Co-occurring species within a mutualist guild are unlikely to be ecologically equivalent in the way they locate, compete for, and/or reward partners, and so intraguild interactions have the potential to influence population dynamics and patterns of selection in species on both sides of the mutualistic interaction. To illustrate some of these potential complexities for population dynamics, I use simple path analytic models to show that positive pairwise interactions between mutualists do not necessarily translate into positive net interactions within a mutualism involving more than two species. For example, when there is intraguild competition for partners, or even for resources external to the mutualism, the presence of a lower-quality mutualist can negatively affect the partner population by reducing associations it can form with better mutualists. Variation in quality among potential partners is likely to place a premium on traits or behaviors that enhance association with better mutualists. More investigations are needed to determine how variation among interacting mutualists, with respect to characteristics such as longevity, dispersal capability, and competitive ability, influence population dynamics and selection in multispecies mutualisms.

Shared control of epidemiological traits in a coevolutionary model of host-parasite interactions

Most models concerning the evolution of a parasite's virulence and its host's resistance assume that each component of the relationship (transmission, virulence, recovery, etc.) is controlled by either the host or the parasite but not by both. We present a model that describes the coevolution of host and parasite, assuming that the rate of transmission or the virulence depends on both genotypes. The evolution of these traits is constrained by trade-offs that account for costs of defense and attack strategies, in line with previous studies on the separate evolution of the host and the parasite. Considering shared control by the host and the parasite in determining the traits of the relationship leads to several novel predictions. First, the host should evolve maximal investment in defense against parasites with an intermediate replication rate. Second, the evolution of the parasite strongly depends on the way the host's defense is described. Third, the coevolutionary process may lead to decreasing the parasite's virulence as a response to a rise in the host's background mortality, contrary to classical predictions.

A model for the coevolution of immunity and immune evasion in vector-borne diseases with implications for the epidemiology of malaria

We describe a model of host-parasite coevolution, where the interaction depends on the investments by the host in its immune response and by the parasite in its ability to suppress (or evade) its host's immune response. We base our model on the interaction between malaria parasites and their mosquito hosts and thus describe the epidemiological dynamics with the Macdonald-Ross equation of malaria epidemiology. The qualitative predictions of the model are most sensitive to the cost of the immune response and to the intensity of transmission. If transmission is weak or the cost of immunity is low, the system evolves to a coevolutionarily stable equilibrium at intermediate levels of investment (and, generally, at a low frequency of resistance). At a higher cost of immunity and as transmission intensifies, the system is not evolutionarily stable but rather cycles around intermediate levels of investment. At more intense transmission, neither host nor parasite invests any resources in dominating its partner so that no resistance is observed in the population. These results may help to explain the lack of encapsulated malaria parasites generally observed in natural populations of mosquito vectors, despite strong selection pressure for resistance in areas of very intense transmission.

Phenotype matching in wild parsnip and parsnip webworms: causes and consequences

According to the geographic mosaic theory of coevolution, selection intensity in interactions varies across a landscape, forming a selection mosaic; interaction traits match at coevolutionary hotspots where selection is reciprocal and mismatch at coldspots where reciprocity is not a factor. Chemical traits play an important role in the interaction between wild parsnip (Pastinaca sativa) and the parsnip webworm (Depressaria pastinacella). Furanocoumarins, produced as plant defenses, are detoxified by the webworms by cytochrome P450 monooxygenases; significant additive genetic variation exists for both furanocoumarin production in the plant and detoxification in the insect, making these traits available for selection. To test the hypothesis that differences in selection intensity affect the distribution of coevolutionary hotspots and coldspots in this interaction, we examined 20 populations of webworms and wild parsnips in Illinois and Wisconsin that varied in size, extent of infestation, proximity to woods (and potential vertebrate predators), and proximity to a chemically distinct alternate host plant, Heracleum lanatum (cow parsnip). Twelve of 20 populations displayed phenotype matching between plant defense and insect detoxification profiles. Of the eight mismatched populations, a logistic regression model related matching probability to two predictors: the presence of the alternate host and average content of xanthotoxin (one of the five furanocoumarins produced by P. sativa). The odds of mismatching were significantly increased by the presence of the alternate host (odds ratio = 15.4) and by increased xanthotoxin content (odds ratio = 6.053). Parsnips growing near cow parsnip displayed chemical phenotypes that were chemically intermediate between cow parsnip and parsnips growing in isolation. Rapid phenotype matching in this system is likely due in part to differential mortality every season; larvae transferred to a plant 30 m or more from the plant on which they developed tended to experience increased mortality over larvae transferred to another umbel on the same plant on which they had developed, and plant populations that mismatched in 2001 displayed a change in chemical phenotype distribution from the previous year. Trait mixing through gene flow is also a likely factor in determining mismatch frequency. Populations from which webworms were eradicated the previous year were all recolonized; in three of seven of these populations, infestation rates exceeded 90%. Our findings, consistent with the geographic mosaic theory, suggest that the presence of a chemically distinct alternate host plant can affect selection intensity in such a way as to reduce the likelihood of reciprocity in the coevolutionary interaction between wild parsnip and the parsnip webworm.

Mutualistic asexual endophytes in a native grass are usually parasitic

Asexual systemic fungi that live symbiotically within grasses are viewed as strong mutualists on the basis of theory and empirical studies of introduced agronomic grasses. Evolutionary theory predicts that microbial symbionts that lose sexuality and rely on propagules of their hosts for transmission should evolve to benefit their hosts. Fungal endophytes of some cultivated turf and pasture grasses are well known for increasing plant performance and competitive abilities, especially under stress, and increasing resistance to herbivores, pathogens, and root-feeders by virtue of fungal alkaloids. The assumption of mutualism, however, has rarely been tested in native grasses, which often harbor high but variable frequencies of systemic asexual endophytes. We tested the effect of Neotyphodium infections for the native grass Arizona fescue in a 3-yr field experiment. We strictly controlled host genotype and manipulated soil moisture and nutrients. Infection generally decreased host growth in terms of plant volume, number of tillers, and dry mass of shoots and roots. Infected plants also showed decreased reproduction in terms of number and mass of seeds, and the seeds produced by infected plants had lower germination success than plants without their endophytes, suggesting that the negative effects of the symbiont are transferred to the next generation. Plant genotype strongly influenced host's growth and reproduction and interacted with the presence of the endophyte, but the interaction was usually in the direction of negative effects. Our results challenge the notion that systemic asexual endophytes must be plant mutualists for infections to persist in nature. We propose other hypotheses to explain the variable but usually high endophyte frequencies in natural populations of grasses.

Geographic structure and dynamics of coevolutionary selection

Coevolution of species is one of the major processes organizing the Earth's biodiversity. Recent coevolutionary theory has indicated that the geographic structure of species has the potential to impose powerful and continuing effects on coevolutionary dynamics, if that structure creates selection mosaics and coevolutionary hotspots across landscapes. Here we confirm that current coevolutionary selection in interspecific interactions can be highly divergent across both narrow and broad geographic scales, thereby fueling continuing coevolution of taxa. Study of a widespread plant insect interaction across a broad range of habitats for several years showed that an insect functioning both as a pollinator and a floral parasite can be strongly mutualistic in some habitats but commensal or antagonistic in neighbouring habitats. The results for one of the habitats span seven years, demonstrating that the local structure of coevolutionary selection can remain stable across multiple generations. Conservation of the evolutionary processes maintaining long-term biological diversity may require preservation of the conditions that allow a long-term shifting geographic mosaic of coevolutionary hotspots and coldspots.