Evolution and persistence of obligate mutualists and exploiters: competition for partners and evolutionary immunization

Mutualisms are ubiquitous in nature, as is their exploitation by both conspecific and heterospecific cheaters. Yet, evolutionary theory predicts that cheating should be favoured by natural selection. Here, we show theoretically that asymmetrical competition for partners generally determines the evolutionary fate of obligate mutualisms facing exploitation by third-species invaders. When asymmetry in partner competition is relatively weak, mutualists may either exclude exploiters or coexist with them, in which case their co-evolutionary response to exploitation is usually benign. When asymmetry is strong, the mutualists evolve towards evolutionary attractors where they become extremely vulnerable to exploiter invasion. However, exploiter invasion at an early stage of the mutualism's history can deflect mutualists' co-evolutionary trajectories towards slightly different attractors that confer long-term stability against further exploitation. Thus, coexistence of mutualists and exploiters may often involve an historical effect whereby exploiters are co-opted early in mutualism history and provide lasting 'evolutionary immunization' against further invasion.     

Global stability of obligate mutualism in community modules with facultative mutualists

Mutualism is a fundamental building block of ecological communities and an important driver of biotic evolution. Classic theory suggests that a pairwise two‐species obligate mutualism is fragile, with a large perturbation potentially driving both mutualist populations into extinction. In nature, however, there are many cases of pairwise obligate mutualism. Such pairwise obligate mutualisms are occasionally associated with additional interactions with facultative mutualists. Here, we use a mathematical model to show that when a two‐species obligate mutualism has a single additional link to a third facultative mutualist, the obligate mutualism can become permanently persistent. In the model, a facultative mutualist interacts with one of two inter‐dependent obligate mutualists, and the facultative mutualist enhances the persistence not only of its directly interacting obligate mutualist, but also that of the other obligate mutualist indirectly, enabling the permanent coexistence of the three mutualist species. The effect of the facultative mutualist is strong; it can allow a three‐species permanent coexistence even when two obligate mutualists by themselves are not sustainable (i.e. not locally stable). These results suggest that facultative mutualists can play a pivotal role for the persistence of obligate mutualisms, and contribute to a better understanding on the mechanisms maintaining more complex mutualistic networks of multiple species.     

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 cryptic role of biodiversity in the emergence of host–microbial mutualisms

The persistence of mutualisms in host‐microbial – or holobiont – systems is difficult to explain because microbial mutualists, who bear the costs of providing benefits to their host, are always prone to being competitively displaced by non‐mutualist ‘cheater’ species. This disruptive effect of competition is expected to be particularly strong when the benefits provided by the mutualists entail costs such as reduced competitive ability. Using a metacommunity model, we show that competition between multiple cheaters within the host's microbiome, when combined with the spatial structure of host–microbial interactions, can have a constructive rather than a disruptive effect by allowing the emergence and maintenance of mutualistic microorganisms within the host. These results indicate that many of the microorganisms inhabiting a host's microbiome, including those that would otherwise be considered opportunistic or even potential pathogens, play a cryptic yet critical role in promoting the health and persistence of the holobiont across spatial scales.     

Three-way coexistence in obligate mutualist-exploiter interactions: the potential role of competition

Many mutualisms host "exploiter" species that consume the benefits provided by one or both mutualists without reciprocating. Exploiters have been widely assumed to destabilize mutualisms, yet they are common. We develop models to explore conditions for local coexistence of obligate plant/pollinating seed parasite mutualisms and nonpollinating exploiters. As the larvae of both pollinators and (at a later time) exploiters consume seeds, we examine the importance of intraspecific and (asymmetric) interspecific competition among and between pollinators and exploiters for achieving three-way coexistence. With weak intra- and interspecific competition, exploiters can invade the stable mutualism and coexist with the mutualists (either stably or with oscillations), provided the exploiters' intrinsic birthrate (b(E)) slightly exceeds that of the pollinators. At higher b(E), all three species go locally extinct. When facing strong interspecific competition, exploiters cannot invade and coexist with the mutualists if intraspecific competition in pollinators and exploiters is weak. However, strong intraspecific competition in pollinators and exploiters facilitates exploiter invasion and coexistence and greatly expands the range of b(E) over which stable coexistence occurs. Our results suggest that mutualist/exploiter coexistence may be more easily achieved than previously thought, thus highlighting the need for a better understanding of competition among and between mutualists and exploiters.     

Persistence of mutualisms with bidirectional interactions in a two-species system

In Lotka–Volterra equations (LVEs) of mutualisms, population densities of mutualists will increase infinitely if the mutualisms between them are strong, which is called the divergence problem. In order to avoid the problem, a mutualism system of two species is analyzed in this work. The model is derived from reactions on lattice and has a form similar to that of LVEs. Population densities of species will not increase infinitely because of spatial limitation on the lattice. Stability analysis of the model demonstrates basic mechanisms by which the mutualisms lead to coexistence/extinction of the species. When in coexistence, intermediate mutualistic effect is shown to lead to the maximal density in certain parameter ranges, while a strong or weak mutualistic effect is not so good. Furthermore, the stability analysis exhibits that extremely strong/weak mutualisms will result in extinction of one/both species.     

Phytophagous insect-microbe mutualisms and adaptive evolutionary diversification.

Adaptive diversification is a process intrinsically tied to species interactions. Yet, the influence of most types of interspecific interactions on adaptive evolutionary diversification remains poorly understood. In particular, the role of mutualistic interactions in shaping adaptive radiations has been largely unexplored, despite the ubiquity of mutualisms and increasing evidence of their ecological and evolutionary importance. Our aim here is to encourage empirical inquiry into the relationship between mutualism and evolutionary diversification, using herbivorous insects and their microbial mutualists as exemplars. Phytophagous insects have long been used to test theories of evolutionary diversification; moreover, the diversification of a number of phytophagous insect lineages has been linked to mutualisms with microbes. In this perspective, we examine microbial mutualist mediation of ecological opportunity and ecologically based divergent natural selection for their insect hosts. We also explore the conditions and mechanisms by which microbial mutualists may either facilitate or impede adaptive evolutionary diversification. These include effects on the availability of novel host plants or adaptive zones, modifying host-associated fitness trade-offs during host shifts, creating or reducing enemy-free space, and, overall, shaping the evolution of ecological (host plant) specialization. Although the conceptual framework presented here is built on phytophagous insect-microbe mutualisms, many of the processes and predictions are broadly applicable to other mutualisms in which host ecology is altered by mutualistic interactions.     

PHYTOPHAGOUS INSECT–MICROBE MUTUALISMS AND ADAPTIVE EVOLUTIONARY DIVERSIFICATION

Adaptive diversification is a process intrinsically tied to species interactions. Yet, the influence of most types of interspecific interactions on adaptive evolutionary diversification remains poorly understood. In particular, the role of mutualistic interactions in shaping adaptive radiations has been largely unexplored, despite the ubiquity of mutualisms and increasing evidence of their ecological and evolutionary importance. Our aim here is to encourage empirical inquiry into the relationship between mutualism and evolutionary diversification, using herbivorous insects and their microbial mutualists as exemplars. Phytophagous insects have long been used to test theories of evolutionary diversification; moreover, the diversification of a number of phytophagous insect lineages has been linked to mutualisms with microbes. In this perspective, we examine microbial mutualist mediation of ecological opportunity and ecologically based divergent natural selection for their insect hosts. We also explore the conditions and mechanisms by which microbial mutualists may either facilitate or impede adaptive evolutionary diversification. These include effects on the availability of novel host plants or adaptive zones, modifying host-associated fitness trade-offs during host shifts, creating or reducing enemy-free space, and, overall, shaping the evolution of ecological (host plant) specialization. Although the conceptual framework presented here is built on phytophagous insect–microbe mutualisms, many of the processes and predictions are broadly applicable to other mutualisms in which host ecology is altered by mutualistic interactions.     

HOW TO PREVENT CHEATING: A DIGESTIVE SPECIALIZATION TIES MUTUALISTIC PLANT-ANTS TO THEIR ANT-PLANT PARTNERS

Mutualisms often involve reciprocal adaptations of both partners. Acacia ant-plants defended by symbiotic Pseudomyrmex ant mutualists secrete sucrose-free extrafloral nectar, which is unattractive to generalists. We aimed to investigate whether this extrafloral nectar can also exclude exploiters, that is nondefending ant species. Mutualist workers discriminated against sucrose whereas exploiters and generalists with no affinity toward Acacia myrmecophytes preferred sucrose, because mutualist workers lacked the sucrose-cleaving enzyme invertase, which is present in workers of the other two groups. Sucrose uptake induced invertase activity in workers of parasites and generalists, but not mutualists, and in larvae of all species: the mutualists loose invertase during their ontogeny. This reduced metabolic capacity ties the mutualists to their plant hosts, but it does not completely prevent the mutualism from exploitation. We therefore investigated whether the exploiters studied here are cheaters (i.e., have evolved from former mutualists) or parasites (exploiters with no mutualistic ancestor). A molecular phylogeny demonstrates that the exploiter species did not evolve from former mutualists, and no evidence for cheaters was found. We conclude that being specialized to their partner can prevent mutualists from becoming cheaters, whereas other mechanisms are required to stabilize a mutualism against the exploitation by parasites.     

Invasive mutualists erode native pollination webs

Plant–animal mutualisms are characterized by weak or asymmetric mutual dependences between interacting species, a feature that could increase community stability. If invasive species integrate into mutualistic webs, they may alter web structure, with consequences for species persistence. However, the effect of alien mutualists on the architecture of plant–pollinator webs remains largely unexplored. We analyzed the extent of mutual dependency between interacting species, as a measure of mutualism strength, and the connectivity of 10 paired plant–pollinator webs, eight from forests of the southern Andes and two from oceanic islands, with different incidences of alien species. Highly invaded webs exhibited weaker mutualism than less-invaded webs. This potential increase in network stability was the result of a disproportionate increase in the importance and participation of alien species in the most asymmetric interactions. The integration of alien mutualists did not alter overall network connectivity, but links were transferred from generalist native species to super-generalist alien species during invasion. Therefore, connectivity among native species declined in highly invaded webs. These modifications in the structure of pollination webs, due to dominance of alien mutualists, can leave many native species subject to novel ecological and evolutionary dynamics.     

Parasites may help stabilize cooperative relationships

Background  

The persistence of cooperative relationships is an evolutionary paradox; selection should favor those individuals that exploit their partners (cheating), resulting in the breakdown of cooperation over evolutionary time. Our current understanding of the evolutionary stability of mutualisms (cooperation between species) is strongly shaped by the view that they are often maintained by partners having mechanisms to avoid or retaliate against exploitation by cheaters. In contrast, we empirically and theoretically examine how additional symbionts, specifically specialized parasites, potentially influence the stability of bipartite mutualistic associations. In our empirical work we focus on the obligate mutualism between fungus-growing ants and the fungi they cultivate for food. This mutualism is exploited by specialized microfungal parasites (genus Escovopsis) that infect the ant's fungal gardens. Using sub-colonies of fungus-growing ants, we investigate the interactions between the fungus garden parasite and cooperative and experimentally-enforced uncooperative ("cheating") pairs of ants and fungi. To further examine if parasites have the potential to help stabilize some mutualisms we conduct Iterative Prisoner's Dilemma (IPD) simulations, a common framework for predicting the outcomes of cooperative/non-cooperative interactions, which incorporate parasitism as an additional factor.     

Ecological rescue of host-microbial systems under environmental change

Beneficial mutations can promote persistence via evolutionary rescue in species experiencing environmental change. However, in long-lived organisms, the pace of evolution is often too slow relative to that of environmental change for evolutionary rescue to occur. Using a spatially implicit metacommunity model, we demonstrate how interactions between slow-growing hosts and their fast-growing microbiomes can promote persistence under rapid environmental change. We show that microbial mutualists can rescue their hosts by allowing them to persist under deteriorating environmental conditions. This form of mutualist-mediated ecological rescue can be jeopardized by competitively dominant microbial cheaters, which can destabilize host population dynamics and promote the risk of stochastic extinction. However, when microbial diversity is high, (meta)community-level interactions among multiple microbial species can buffer the disruptive effect of cheaters and give rise to a more potent form of ecological rescue mediated by the entire microbiome that promotes the abundance, stability, and persistence of the host in the face of environmental change. Our results address two critical problems associated with the viability of rescue in macroorganisms: the temporal mismatch between rapid environmental change and slow organismal response and the potential disruption of rescue by microbial cheaters.     

Parasites of mutualisms

Cooperation invites cheating, and nowhere is this more apparent than when different species cooperate, known as mutualism. In almost all mutualisms studied, specialist parasites have been identified that purloin the benefits that one mutualist provides another. Explaining how parasites are kept from driving mutualisms extinct remains an unsolved problem because existing theories explaining the maintenance of cooperation do not apply to parasites of mutualisms. Nonetheless, these theories can be summarized in such a way as to suggest how mutualisms can persist in the face of parasites. (1) For cooperation to occur, the recipient of a benefit must reciprocate, and the recriprocated benefit must be captured by the initial giver or its offspring. (2) For cooperation to persist, the mutualism must be re-assembled each generation. Because most mutualisms are of the "by-product' type, broadly defined, the first condition is normally always fulfilled. Thus, the maintenance of mutualism usually requires enforcement of the second condition: reliable re-assembly. Hence, I argue that the persistence of mutualism is best understood by using theories of species coexistence, because each mutualist can be considered a resource for the other, and species coexistence theory explains how multiple taxa (e.g. parasites and mutualists) can stably partition a resource over multiple generations. This approach connects the study of mutualism to theories of population regulation and helps to identify key factors that have promoted the evolution, maintenance and breakdown of mutualism. I discuss how these ideas might apply to and be tested in ant-plant, fig-wasp and yucca-moth mutualisms.     

Proteases hold the key to an exclusive mutualism

Mutualisms, cooperative interactions between species, generally involve an economic exchange: species exchange commodities that are cheap for them to provide, for ones that cannot be obtained affordably or at all. But these associations can only succeed if effective partners can be enticed to interact. In some mutualisms, partners can actively seek one another out. However, plants, which use mutualists for a wide array of essential life history functions, do not have this option. Instead, natural selection has repeatedly favoured the evolution of rewards – nutritional substances (such as sugar‐rich nectar and fleshy fruit) with which plants attract certain organisms whose feeding activities can then be co‐opted for their own benefit. The trouble with rewards, however, is that they are usually also attractive to organisms that confer no benefits at all. Losing rewards to ‘exploiters’ makes a plant immediately less attractive to the mutualists it requires; if the reward cannot be renewed quickly (or at all), then mutualistic service is precluded entirely. Thus, it is in plants' interests to either restrict rewards to only the most beneficial partners or somehow punish or deter exploiters. Yet, at least in cases where the rewards are highly nutritious, we can expect counter‐selection for exploiter traits that permit them to skirt such control. How, then, can mutualisms persist? In this issue, Orona‐Tamayo et al. ( 2013 ) describe a remarkable adaptation that safeguards one particularly costly reward from nonmutualists. Their study helps to explain the evolutionary success of an iconic interaction and illuminates one way in which mutualism as a whole can persist in the face of exploitation.     

Invasive belowground mutualists of woody plants

Most plants require mutualistic associations to survive, which can be an important limitation on their ability to become invasive. There are four strategies that permit plants to become invasive without being limited by a lack of mutualists. One is to not be dependent on mutualists. The other three strategies are to form novel mutualisms, form associations with cosmopolitan species, or co-invade with mutualists from their native range. Historically there has been a bias to study mutualisms from a plant perspective, with little consideration of soil biota as invasive species in their own right. Here we address this by reviewing the literature on belowground invasive mutualists of woody plants. We focus on woody invaders as ecosystem-transforming plants that frequently have a high dependence on belowground mutualists. We found that co-invasions are common, with many ectomycorrhizal plant species and N-fixing species co-invading with their mutualists. Other groups, such as arbuscular mycorrhizal plants, tend to associate with cosmopolitan fungal species or to form novel associations in their exotic range. Only limited evidence exists of direct negative effects of co-invading mutualists on native mutualist communities, and effects on native plants appear to be largely driven by altered environmental conditions rather than direct interactions. Mutualists that introduce novel ecosystem functions have effects greater than would be predicted based solely on their biomass. Focusing on the belowground aspects of plant invasions provides novel insights into the impacts, processes and management of invasions of both soil organisms and woody plant species.     

Interactions among mutualism,competition, and predation foster species coexistence in diverse communities

In natural systems, organisms are simultaneously engaged in mutualistic, competitive, and predatory interactions. Theory predicts that species persistence and community stability are feasible when the beneficial effects of mutualisms are balanced by density-dependent negative feedbacks. Enemy-mediated negative feedbacks can foster plant species coexistence in diverse communities, but empirical evidence remains mixed. Disparity between theoretical expectations and empirical results may arise from the effects of mutualistic mycorrhizal fungi. Here, we build a multiprey species/predator model combined with a bidirectional resource exchange system, which simulates mutualistic interactions between plants and fungi. To reach population persistence, (1) the per capita rate of increase of all plant population must exceed the sum of the negative per capita effects of predation, interspecific competition, and costs of mycorrhizal association, and (2) the per capita numerical response of enemies to mycorrhizal plants must exceed the magnitude of the per capita enemy rate of mortality. These conditions reflect the balance between regulation and facilitation in the system. Interactions between plant natural enemies and mycorrhizal fungi lead to shifts in the strength and direction of net mycorrhizal effects on plants over time, with common plant species deriving greater benefits from mycorrhizal associations than rare plant species.     

Evolutionary origins and diversification of proteobacterial mutualists

Mutualistic bacteria infect most eukaryotic species in nearly every biome. Nonetheless, two dilemmas remain unresolved about bacterial–eukaryote mutualisms: how do mutualist phenotypes originate in bacterial lineages and to what degree do mutualists traits drive or hinder bacterial diversification? Here, we reconstructed the phylogeny of the hyperdiverse phylum Proteobacteria to investigate the origins and evolutionary diversification of mutualistic bacterial phenotypes. Our ancestral state reconstructions (ASRs) inferred a range of 34–39 independent origins of mutualist phenotypes in Proteobacteria, revealing the surprising frequency with which host-beneficial traits have evolved in this phylum. We found proteobacterial mutualists to be more often derived from parasitic than from free-living ancestors, consistent with the untested paradigm that bacterial mutualists most often evolve from pathogens. Strikingly, we inferred that mutualists exhibit a negative net diversification rate (speciation minus extinction), which suggests that mutualism evolves primarily via transitions from other states rather than diversification within mutualist taxa. Moreover, our ASRs infer that proteobacterial mutualist lineages exhibit a paucity of reversals to parasitism or to free-living status. This evolutionary conservatism of mutualism is contrary to long-standing theory, which predicts that selection should often favour mutants in microbial mutualist populations that exploit or abandon more slowly evolving eukaryotic hosts.     

Partner choice in nitrogen-fixation mutualisms of legumes and rhizobia

Mutualistic interactions are widespread and obligatory for manyorganisms, yet their evolutionary persistence in the face ofcheating is theoretically puzzling. Nutrient-acquisition symbiosesbetween plants and soil microbes are critically important toplant evolution and ecosystem function, yet we know almost nothingabout the evolutionary dynamics and mechanisms of persistenceof these ancient mutualisms. Partner-choice and partner-fidelityare mechanisms for dealing with cheaters, and can theoreticallyallow mutualisms to persist despite cheaters. Many models of cooperative behavior assume pairwise interactions,while most plant-microbe nutrient-acquisition symbioses involvea single plant interacting with numerous microbes. Market models,in contrast, are well suited to mutualisms in which single plantsattempt to conduct mutually beneficial resource exchange withmultiple individuals. Market models assume that one partnerchooses to trade with a subset of individuals selected froma market of potential partners. Hence, determining whether partner-choiceoccurs in plant-microbe mutualisms is critical to understandingthe evolutionary persistence and dynamics of these symbioses.The nitrogen-fixation/carbon-fixation mutualism between leguminousplants and rhizobial bacteria is widespread, ancient, and importantfor ecosystem function and human nutrition. It also involvessingle plants interacting simultaneously with several to manybacterial partners, including ineffective ("cheating") strains.We review the existing literature and find that this mutualismdisplays several elements of partner-choice, and may match therequirements of the market paradigm. We conclude by identifyingprofitable questions for future research.     

Competition hierarchies among ants and predation by birds jointly determine the strength of multi‐species ant–aphid mutualisms

Protection mutualisms often involve multiple species of protector that vary in quality as mutualists. Because protectors may compete for access to mutualists, concordance between competitive ability and degree of benefit will determine the overall strength of multi‐species mutualisms. We compared the abilities of two similarly sized congener ants as competitors for, and mutualists of pine‐feeding aphids, and how insectivorous birds affected each ant–aphid mutualism. Formica planipilis and F. podzolica were indistinguishable in forager abundance, but the former was 13‐fold more abundant at competition baits and provided 11‐fold more benefits to aphids. These results highlight how, in a single environment, a great ecological distance can exist between two congener ants of similar size. Insectivorous birds disrupted the two mutualisms to a similar extent, reducing aphid and ant abundance by 91% and 39% respectively. Nevertheless, birds had an important influence on the relative benefits of the two ants to aphids: where F. planipilis consistently benefited aphids, F. podzolica only did so in the absence of birds. Consequently, the presence of insectivorous birds and ant species identity jointly determined whether ant–aphid mutualisms occurred in pine canopies and the strength of such interactions. Our study highlights the inter‐relatedness of competition, predation and mutualism, and how competition can serve to strengthen mutualism by filtering inferior mutualists.     

On the evolution of non-specific mutualism

It has been argued that mutualisms are non-specific when mutualistic interactions are weak and transient, and become more specific as interactions increase in strength. However, this runs counter to the observation that there exist tightly linked mutualisms of great antiquity that are highly nonspecific. Here we argue that mutualism generates positive, interspecific, frequency-dependent selection, which acts as a cohesive evolutionary force, discouraging evolution of specificity. A simple mathematical model is constructed to analyse the evolution of a community consisting of two guilds of species with mutualistic between-guild interactions, two competing species in each guild and two genetically distinct phenotypes within each species. With some simplifying assumptions, the trajectories in the neighbourhood of the only interior equilibrium point are determined analytically in terms of interactions between individuals. These show that the equilibrium is locally stable (no evolution) when there is little differentiation between phenotypes in mutualistic and interspecific, competitive interactions. On the other hand, when there is strong differentiation between phenotypes in their mutualistic interactions, the equilibrium is unstable and the community starts to evolve towards non-specificity. There are, however, two forces counteracting this tendency which, if sufficiently potent, cause evolution towards specificity. The first is generated by strong differentiation between phenotypes in interspecific competition; the second is caused by specificity which already exists between species in their mutualistic interactions. Thus, the tendency for non-specificity or specificity to evolve depends on the interplay between antagonistic and mutualistic interactions in the community. We illustrate these results with some numerical examples and, finally, survey some data on specificity of mutualisms in the light of the analysis.