Cheating and the evolutionary stability of mutualisms

A general model for the evolution of mutualisms

The evolution of mutualisms presents a puzzle. Why does selection favour cooperation among species rather than cheaters that accept benefits but provide nothing in return? Here we present a general model that predicts three key factors will be important in mutualism evolution: (i) high benefit to cost ratio, (ii) high within-species relatedness and (iii) high between-species fidelity. These factors operate by moderating three types of feedback benefit from mutualism: cooperator association, partner-fidelity feedback and partner choice. In defining the relationship between these processes, our model also allows an assessment of their relative importance. Importantly, the model suggests that phenotypic feedbacks (partner-fidelity feedback, partner choice) are a more important explanation for between-species cooperation than the development of genetic correlations among species (cooperator association). We explain the relationship of our model to existing theories and discuss the empirical evidence for our predictions.

Unifying the Theories of Inclusive Fitness and Reciprocal Altruism

Inclusive fitness and reciprocal altruism are widely thought to be distinct explanations for how altruism evolves. Here we show that they rely on the same underlying mechanism. We demonstrate this commonality by applying Hamilton's rule, normally associated with inclusive fitness, to two simple models of reciprocal altruism: one, an iterated prisoner's dilemma model with conditional behavior; the other, a mutualistic symbiosis model where two interacting species differ in conditional behaviors, fitness benefits, and costs. We employ Queller's generalization of Hamilton's rule because the traditional version of this rule does not apply when genotype and phenotype frequencies differ or when fitness effects are nonadditive, both of which are true in classic models of reciprocal altruism. Queller's equation is more general in that it applies to all situations covered by earlier versions of Hamilton's rule but also handles nonadditivity, conditional behavior, and lack of genetic similarity between altruists and recipients. Our results suggest changes to standard interpretations of Hamilton's rule that focus on kinship and indirect fitness. Despite being more than 20 years old, Queller's generalization of Hamilton's rule is not sufficiently appreciated, especially its implications for the unification of the theories of inclusive fitness and reciprocal altruism.

Coevolution of slow-fast populations: evolutionary sliding, evolutionary pseudo-equilibria and complex Red Queen dynamics

We study the interplay of ecological and evolutionary dynamics in communities composed of populations with contrasting time-scales. In such communities, genetic variation of individual traits can cause population transitions between stationary and cyclic ecological regimes, hence abrupt variations in fitness. Such abrupt variations raise ridges in the adaptive landscape, where the populations are poised between equilibrium and cyclic coexistence and along which evolutionary trajectories can remain sliding for long times or halt at special points called evolutionary pseudo-equilibria. These novel phenomena should be generic to all systems in which ecological interactions cause fitness to vary discontinuously. They are demonstrated by the analysis of a predator-prey community, with one adaptive trait for each population. The eco-evolutionary dynamics of the system show a number of other distinctive features, including evolutionary extinction and two forms of Red Queen dynamics. One of them is characterized by intermittent bouts of cyclic oscillations of the two populations.

THE ECONOMICS OF MUTUALISMS: OPTIMAL UTILIZATION OF MYCORRHIZAL MUTUALISTIC PARTNERS BY PLANTS

Can choice of mutualistic partners and the degree of their utilization determine (1) mutualistic partner coexistence, (2) relative abundance of mutualistic partners, and (3) environment-dependent changes in relative abundance? We investigate these questions in the context of the plant-mycorrhizal fungal mutualism by building a biological market model potentially applicable to other mutualisms as well. We examine the situation where a single plant selectively utilizes member(s) of a group of ectomycorrhizal potential trading partners. Under biologically realistic circumstances, the plant may simultaneously utilize multiple partners, its degree of utilization determining the community structure of the fungi. If utilization of multiple partners is optimal, the marginal cost of acquiring additional nitrogen from every trading partner must be equal while the marginal cost of acquiring it from any unutilized partner must be larger. Because the plant's nitrogen demand is light dependent, the composition of the fungal species among its trading partners changes along light-availability gradients. We discuss the design of an experiment to test the key prediction of our model, the equalization of marginal cost.

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.

Ecological and evolutionary forces shaping microbial diversity in the human intestine

The human gut is populated with as many as 100 trillion cells, whose collective genome, the microbiome, is a reflection of evolutionary selection pressures acting at the level of the host and at the level of the microbial cell. The ecological rules that govern the shape of microbial diversity in the gut apply to mutualists and pathogens alike.

Surprising evolutionary predictions from enhanced ecological realism

A focus on the eco-evolutionary feedback continually operating between a population's evolution and its environment helps to appreciate the generality of ESS theory. Here we illustrate, through a sequence of four examples, how respecting such feedback in the evolutionary dynamics of quantitative traits may result in qualitatively unexpected outcomes. Reviewing existing insights and complementing these with new results, we show (1) that evolutionary matrix games are fundamentally degenerate and allow a natural unfolding, (2) that selection-driven extinction may not be rare in nature, (3) that evolutionary epidemiology should not rely on R0 maximization, and (4) why the occurrence of Hardy-Weinberg proportions generically requires an evolutionary explanation.

Unifying evolutionary dynamics: from individual stochastic processes to macroscopic models

A distinctive signature of living systems is Darwinian evolution, that is, a propensity to generate as well as self-select individual diversity. To capture this essential feature of life while describing the dynamics of populations, mathematical models must be rooted in the microscopic, stochastic description of discrete individuals characterized by one or several adaptive traits and interacting with each other. The simplest models assume asexual reproduction and haploid genetics: an offspring usually inherits the trait values of her progenitor, except when a mutation causes the offspring to take a mutation step to new trait values; selection follows from ecological interactions among individuals. Here we present a rigorous construction of the microscopic population process that captures the probabilistic dynamics over continuous time of birth, mutation, and death, as influenced by the trait values of each individual, and interactions between individuals. A by-product of this formal construction is a general algorithm for efficient numerical simulation of the individual-level model. Once the microscopic process is in place, we derive different macroscopic models of adaptive evolution. These models differ in the renormalization they assume, i.e. in the limits taken, in specific orders, on population size, mutation rate, mutation step, while rescaling time accordingly. The macroscopic models also differ in their mathematical nature: deterministic, in the form of ordinary, integro-, or partial differential equations, or probabilistic, like stochastic partial differential equations or superprocesses. These models include extensions of Kimura's equation (and of its approximation for small mutation effects) to frequency- and density-dependent selection. A novel class of macroscopic models obtains when assuming that individual birth and death occur on a short timescale compared with the timescale of typical population growth. On a timescale of very rare mutations, we establish rigorously the models of "trait substitution sequences" and their approximation known as the "canonical equation of adaptive dynamics". We extend these models to account for mutation bias and random drift between multiple evolutionary attractors. The renormalization approach used in this study also opens promising avenues to study and predict patterns of life-history allometries, thereby bridging individual physiology, genetic variation, and ecological interactions in a common evolutionary framework.

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.

Diversification of transmission modes and the evolution of mutualism

In order for mutualism to evolve, some force must align the interests of the two interacting partners. Vertical transmission can fill this role, but it is still unknown whether mutualism can be stable when vertically transmitted symbionts can evolve toward horizontal transmission. In this article, we investigate how symbionts' transmission mode and virulence should evolve, depending on the relationship between these two traits. We show that pathogens that reduce their host's fecundity can have more complex evolutionary dynamics than those that increase mortality. In some cases, runaway evolution of virulence can drive the host population extinct. In most cases, evolutionary branching results in the differentiation of avirulent, vertically transmitted symbionts from virulent, contagious pathogens. The population of symbionts then becomes polymorphic, and because the least virulent symbionts are the most frequent, the average virulence of symbionts is much lower than it would be in a monomorphic population. When the link between transmission and virulence results from correlated mutational changes and not from fixed constraints, vertically transmitted symbionts do not simply lose virulence; they evolve toward mutualism. We show that the force that stabilizes mutualism in such situations is the competition for transmission between symbionts.

Testing the out-of-Florida hypothesis on the origin of cheating in the yucca-yucca moth mutualism

Mutualistic interactions can be exploited by cheaters that take the rewards offered by mutualists without providing services in return. The evolution of cheater species from mutualist ancestors is thought to be possible under particular ecological conditions. Here we provide a test of the first explicit model of the transition from mutualism to antagonism. We used the obligate pollination mutualism between yuccas and yucca moths to examine the origins of a nonpollinating cheater moth, Tegeticula intermedia, and its pollinating sister species, T. cassandra. Based on geographic distribution and ecological factors affecting the pollinators, previous research had indicated that the cheaters evolved in Florida as a result of sympatry of T. cassandra and another pollinator species. We used mitochondrial DNA (mtDNA) sequences and amplified fragment length polymorphism (AFLP) data to investigate the phylogeographic history of the pollinator-cheater sister pair and to test whether the cheaters arose in Florida. Contrary to predictions, phylogenetic and population genetic analyses suggested that the cheaters evolved in the western United States and subsequently spread eastward. Western populations of cheaters had the most ancestral haplotypes and the highest genetic diversity, and there was also significant genetic structure associated with a geographic split between eastern and western populations. In comparison, there was evidence for weak genetic structure between northern and southern pollinator populations, suggesting a long history in Florida. The western origin of the cheaters indicated that the pollinators have more recently become restricted to the southeastern United States. This was supported by AFLP analyses that indicated that the pollinators were more closely related to the western cheaters than they were to geographically proximate cheaters in the east. Shared mtDNA between pollinators and eastern cheaters suggested hybridization, possibly in a secondary contact zone. The results negate the out-of-Florida hypothesis and reveal instead a long, complex, and disparate history for the pollinator-cheater sister pair.

Evolutionary stability of mutualism: interspecific population regulation as an evolutionarily stable strategy

Interspecific mutualisms are often vulnerable to instability because low benefit : cost ratios can rapidly lead to extinction or to the conversion of mutualism to parasite-host or predator-prey interactions. We hypothesize that the evolutionary stability of mutualism can depend on how benefits and costs to one mutualist vary with the population density of its partner, and that stability can be maintained if a mutualist can influence demographic rates and regulate the population density of its partner. We test this hypothesis in a model of mutualism with key features of senita cactus (Pachycereus schottii)-senita moth (Upiga virescens) interactions, in which benefits of pollination and costs of larval seed consumption to plant fitness depend on pollinator density. We show that plants can maximize their fitness by allocating resources to the production of excess flowers at the expense of fruit. Fruit abortion resulting from excess flower production reduces pre-adult survival of the pollinating seed-consumer, and maintains its density beneath a threshold that would destabilize the mutualism. Such a strategy of excess flower production and fruit abortion is convergent and evolutionarily stable against invasion by cheater plants that produce few flowers and abort few to no fruit. This novel mechanism of achieving evolutionarily stable mutualism, namely interspecific population regulation, is qualitatively different from other mechanisms invoking partner choice or selective rewards, and may be a general process that helps to preserve mutualistic interactions in nature.

Ecological Dynamics of Mutualist/Antagonist Communities

One approach to understanding how mutualisms function in community settings is to model well-studied pairwise interactions in the presence of the few species with which they interact most strongly. In nature, such species are often specialized antagonists of one or both mutualists. Hence, these models can also shed light on the problem of when and how mutualisms are able to persist in the face of exploitation. We used spatial stochastic simulations to model the ecological dynamics of obligate, species-specific mutualisms between plants and pollinating seed parasite insects (e.g., yuccas and yucca moths) in the presence of one of two obligate antagonist species: flower-feeding insects (florivores) or insects that parasitize seeds but fail to pollinate (exploiters). Our results suggest that mutualisms can persist surprisingly well in the presence of highly specialized antagonists but that they exhibit distinctly different temporal and spatial dynamics when antagonists are present. In our models, antagonists tend to induce oscillations in the mutualist populations. As the number of per capita visits by antagonists increase, the system's oscillatory dynamics become more extreme, finally leading to the extinction of one or more of the three species. When the antagonists exhibit high per capita visitation frequencies and long dispersal distances, significant spatial patchiness emerges within these tripartite interactions. We found surprisingly little difference between the ecological effects of florivores and exploiters, although in general florivores tended to drive themselves (and sometimes the mutualists) to extinction at parameter values at which the exploiters were able to persist. These theoretical results suggest several testable hypotheses regarding the ecological and evolutionary persistence of mutualisms. More broadly, they point to the critical importance of studying the dynamics of pairwise interactions in community contexts.

Host sanctions and the legume-rhizobium mutualism

Explaining mutualistic cooperation between species remains one of the greatest problems for evolutionary biology. Why do symbionts provide costly services to a host, indirectly benefiting competitors sharing the same individual host? Host monitoring of symbiont performance and the imposition of sanctions on 'cheats' could stabilize mutualism. Here we show that soybeans penalize rhizobia that fail to fix N(2) inside their root nodules. We prevented a normally mutualistic rhizobium strain from cooperating (fixing N(2)) by replacing air with an N(2)-free atmosphere (Ar:O(2)). A series of experiments at three spatial scales (whole plants, half root systems and individual nodules) demonstrated that forcing non-cooperation (analogous to cheating) decreased the reproductive success of rhizobia by about 50%. Non-invasive monitoring implicated decreased O(2) supply as a possible mechanism for sanctions against cheating rhizobia. More generally, such sanctions by one or both partners may be important in stabilizing a wide range of mutualistic symbioses.

Honesty and cheating in cleaning symbioses: evolutionarily stable strategies defined by variable pay-offs

Game-theory models have indicated that the evolution of mixed strategies of cheating and honesty in many mutualisms is unlikely. Moreover, the mutualistic nature of interspecific interactions has often been difficult to demonstrate empirically. We present a game-theory analysis that addresses these issues using cleaning symbioses among fishes as a model system. We show that the assumption of constant pay-offs in existing models prevents the evolution of evolutionarily stable mixed strategies of cheating and honesty. However, when interaction pay-offs are assumed to be density dependent, mixed strategies of cheating and honesty become possible. In nature, cheating by clients often takes the form of retaliation by clients against cheating cleaners, and we show that mixed strategies of cheating and honesty evolve within the cleaner population when clients retaliate. The dynamics of strategies include both negative and positive effects of interactions, as well as density-dependent interactions. Consequently, the effects of perturbations to the model are nonlinear. In particular, we show that under certain conditions the removal of cleaners may have little impact on client populations. This indicates that the underlying mutualistic nature of some interspecific interactions may be difficult to demonstrate using simple manipulation experiments.

Genomes at the interface between bacteria and organelles

The topic of the transition of the genome of a free-living bacterial organism to that of an organelle is addressed by considering three cases. Two of these are relatively clear-cut as involving respectively organisms (cyanobacteria) and organelles (plastids). Cyanobacteria are usually free-living but some are involved in symbioses with a range of eukaryotes in which the cyanobacterial partner contributes photosynthesis, nitrogen fixation, or both of these. In several of these symbioses the cyanobacterium is vertically transmitted, and in a few instances, sufficient unsuccessful attempts have been made to culture the cyanobiont independently for the association to be considered obligate for the cyanobacterium. Plastids clearly had a cyanobacterial ancestor but cannot grow independently of the host eukaryote. Plastid genomes have at most 15% of the number of genes encoded by the cyanobacterium with the smallest number of genes; more genes than are retained in the plastid genome have been transferred to the eukaryote nuclear genome, while the rest of the cyanobacterial genes have been lost. Even the most cyanobacteria-like plastids, for example the "cyanelles" of glaucocystophyte algae, are functionally and genetically very similar to other plastids and give little help in indicating intermediates in the evolution of plastids. The third case considered is the vertically transmitted intracellular bacterial symbionts of insects where the symbiosis is usually obligate for both partners. The number of genes encoded by the genomes of these obligate symbionts is intermediate between that of organelles and that of free-living bacteria, and the genomes of the insect symbionts also show rapid rates of sequence evolution and AT (adenine, thymine) bias. Genetically and functionally, these insect symbionts show considerable similarity to organelles.

The Red King effect: when the slowest runner wins the coevolutionary race

Mutualisms provide benefits to those who participate in them. As a mutualism evolves, how will these benefits come to be allocated among the participants? We approach this question by using evolutionary game theory and explore the ways in which the coevolutionary process determines the allocation of benefits in mutualistic interactions. Motivated by the Red Queen theory, which states that coevolutionary processes favor rapid rates of evolution, we pay particular attention to the role of evolutionary rates in the establishment of mutualism and the partitioning of benefits among mutualist partners. We find that, contrary to the Red Queen, in mutualism evolution the slowly evolving species is likely to gain a disproportionate share of the benefits. Moreover, population structure serves to magnify the advantage to the slower species.