Reduced fecundity is the cost of cheating in RNA virus phi6

Co-infection by multiple viruses affords opportunities for the evolution of cheating strategies to use intracellular resources. Cheating may be costly, however, when viruses infect cells alone. We previously allowed the RNA bacteriophage phi6 to evolve for 250 generations in replicated environments allowing co-infection of Pseudomonas phaseolicola bacteria. Derived genotypes showed great capacity to compete during co-infection, but suffered reduced performance in solo infections. Thus, the evolved viruses appear to be cheaters that sacrifice between-host fitness for within-host fitness. It is unknown, however, which stage of the lytic growth cycle is linked to the cost of cheating. Here, we examine the cost through burst assays, where lytic infection can be separated into three discrete phases (analogous to phage life history): dispersal stage, latent period (juvenile stage), and burst (adult stage). We compared growth of a representative cheater and its ancestor in environments where the cost occurs. The cost of cheating was shown to be reduced fecundity, because cheaters feature a significantly smaller burst size (progeny produced per infected cell) when infecting on their own. Interestingly, latent period (average burst time) of the evolved virus was much longer than that of the ancestor, indicating the cost does not follow a life history trade-off between timing of reproduction and lifetime fecundity. Our data suggest that interference competition allows high fitness of derived cheaters in mixed infections, and we discuss preferential encapsidation as one possible mechanism.     

Concurrent coevolution of intra‐organismal cheaters and resisters

The evolution of multicellularity is a major transition that is not yet fully understood. Specifically, we do not know whether there are any mechanisms by which multicellularity can be maintained without a single‐cell bottleneck or other relatedness‐enhancing mechanisms. Under low relatedness, cheaters can evolve that benefit from the altruistic behaviour of others without themselves sacrificing. If these are obligate cheaters, incapable of cooperating, their spread can lead to the demise of multicellularity. One possibility, however, is that cooperators can evolve resistance to cheaters. We tested this idea in a facultatively multicellular social amoeba, Dictyostelium discoideum. This amoeba usually exists as a single cell but, when stressed, thousands of cells aggregate to form a multicellular organism in which some of the cells sacrifice for the good of others. We used lineages that had undergone experimental evolution at very low relatedness, during which time obligate cheaters evolved. Unlike earlier experiments, which found resistance to cheaters that were prevented from evolving, we competed cheaters and noncheaters that evolved together, and cheaters with their ancestors. We found that noncheaters can evolve resistance to cheating before cheating sweeps through the population and multicellularity is lost. Our results provide insight into cheater–resister coevolutionary dynamics, in turn providing experimental evidence for the maintenance of at least a simple form of multicellularity by means other than high relatedness.     

Competitive fates of bacterial social parasites: persistence and self-induced extinction of Myxococcus xanthus cheaters

Cooperative biological systems are susceptible to disruption by cheating. Using the social bacterium Myxococcus xanthus, we have tested the short-term competitive fates of mixed cheater and wild-type strains over multiple cycles of cooperative development. Cheater/wild-type mixes underwent several cycles of starvation-induced multicellular development followed by spore germination and vegetative population growth. The population sizes of cheater and wild-type strains in each pairwise mixture were measured at the end of each developmental phase and each growth phase. Cheater genotypes showed several distinct competitive fates, including cheater persistence at high frequencies with little effect on total population dynamics, cheater persistence after major disruption of total population dynamics, self-extinction of cheaters with wild-type survival, and total population extinction. Our results empirically demonstrate that social exploitation can destabilize a cooperative biological system and increase the risk of local extinction events.     

Cheating does not explain selective differences at high and low relatedness in a social amoeba

Background  

Altruism can be favored by high relatedness among interactants. We tested the effect of relatedness in experimental populations of the social amoeba Dictyostelium discoideum, where altruism occurs in a starvation-induced social stage when some amoebae die to form a stalk that lifts the fertile spores above the soil facilitating dispersal. The single cells that aggregate during the social stage can be genetically diverse, which can lead to conflict over spore and stalk allocation. We mixed eight genetically distinct wild isolates and maintained twelve replicated populations at a high and a low relatedness treatment. After one and ten social generations we assessed the strain composition of the populations. We expected that some strains would be out-competed in both treatments. In addition, we expected that low relatedness might allow the persistence of social cheaters as it provides opportunity to exploit other strains.     

Kin Discrimination Increases with Genetic Distance in a Social Amoeba

In the social amoeba Dictyostelium discoideum, thousands of cells aggregate upon starvation to form a multicellular fruiting body, and approximately 20% of them die to form a stalk that benefits the others. The aggregative nature of multicellular development makes the cells vulnerable to exploitation by cheaters, and the potential for cheating is indeed high. Cells might avoid being victimized if they can discriminate among individuals and avoid those that are genetically different. We tested how widely social amoebae cooperate by mixing isolates from different localities that cover most of their natural range. We show here that different isolates partially exclude one another during aggregation, and there is a positive relationship between the extent of this exclusion and the genetic distance between strains. Our findings demonstrate that D. discoideum cells co-aggregate more with genetically similar than dissimilar individuals, suggesting the existence of a mechanism that discerns the degree of genetic similarity between individuals in this social microorganism.     

Cheater genotypes in the parthenogenetic ant Pristomyrmex punctatus

Cooperation is subject to cheating strategies that exploit the benefits of cooperation without paying the fair costs, and it has been a major goal of evolutionary biology to explain the origin and maintenance of cooperation against such cheaters. Here, we report that cheater genotypes indeed coexist in field colonies of a social insect, the parthenogenetic ant Pristomyrmex punctatus. The life history of this species is exceptional, in that there is no reproductive division of labour: all females fulfil both reproduction and cooperative tasks. Previous studies reported sporadic occurrence of larger individuals when compared with their nest-mates. These larger ants lay more eggs and hardly take part in cooperative tasks, resulting in lower fitness of the whole colony. Population genetic analysis showed that at least some of these large-bodied individuals form a genetically distinct lineage, isolated from cooperators by parthenogenesis. A phylogenetic study confirmed that this cheater lineage originated intraspecifically. Coexistence of cheaters and cooperators in this species provides a good model system to investigate the evolution of cooperation in nature.     

Innate Adjustment of Visitation Behavior to Rewarding and Reward‐Minimized Petunia axillaris (Solanacea) Plants by Hawkmoth Manduca sexta (Sphingidae)

Plant–pollinator interactions offer an excellent system to study the stability of mutualistic interactions. While nectar production requires resources and a reduction could in principle benefit plant fitness, only few angiosperms lack nectar, and thus cheat from a pollinator's perspective. Cheating behavior may be scarce because of pollinator foraging behaviors that select for nectariferous plants. Shorter inspection duration, interaction with fewer flowers, or even complete avoidance of plants with low/no nectar may reduce the fitness of cheating plants. The effectiveness of pollinator strategies may depend on how they are implemented. Innate strategies would invariably decrease the fitness of a cheating plant, while learned responses allow cheaters to exploit naïve pollinators. Here, we studied the foraging strategies of the hawkmoth Manduca sexta during interactions with nectariferous and reward‐minimized Petunia axillaris. We found that neither naïve nor experienced hawkmoths discriminated a priori between rewarding and nectar‐less plants. However, naïve hawkmoths displayed reduced probing time per flower and number of flowers visited on reward‐minimized plants during the first trial, without showing further improved discrimination with experience. In conclusion, the foraging decision rules of hawkmoths that may reduce the fitness of reward‐minimized plants appear to be innate, with little scope for additional learning.     

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.     

RAPID EVOLUTION OF CHEATING MITOCHONDRIAL GENOMES IN SMALL YEAST POPULATIONS

Outcrossed sex exposes genes to competition with their homologues, allowing alleles that transmit more often than their competitors to spread despite organismal fitness costs. Mitochondrial populations in species with biparental inheritance are thought to be especially susceptible to such cheaters because they lack strict transmission rules like meiosis or maternal inheritance. Yet the interaction between mutation and natural selection in the evolution of cheating mitochondrial genomes has not been tested experimentally. Using yeast experimental populations, we show that although cheaters were rare in a large sample of spontaneous respiratory‐deficient mitochondrial mutations (petites), cheaters evolve under experimentally enforced outcrossing even when mutation supply and selection are restricted by repeatedly bottlenecking populations.     

Kin selection-mutation balance: a model for the origin, maintenance, and consequences of social cheating

Social conflict, in the form of intraspecific selfish "cheating," has been observed in a number of natural systems. However, a formal, evolutionary genetic theory of social cheating that provides an explanatory, predictive framework for these observations is lacking. Here we derive the kin selection-mutation balance, which provides an evolutionary null hypothesis for the statics and dynamics of cheating. When social interactions have linear fitness effects and Hamilton's rule is satisfied, selection is never strong enough to eliminate recurrent cheater mutants from a population, but cheater lineages are transient and do not invade. Instead, cheating lineages are eliminated by kin selection but are constantly reintroduced by mutation, maintaining a stable equilibrium frequency of cheaters. The presence of cheaters at equilibrium creates a "cheater load" that selects for mechanisms of cheater control, such as policing. We find that increasing relatedness reduces the cheater load more efficiently than does policing the costs and benefits of cooperation. Our results provide new insight into the effects of genetic systems, mating systems, ecology, and patterns of sex-limited expression on social evolution. We offer an explanation for the widespread cheater/altruist polymorphism found in nature and suggest that the common fear of conflict-induced social collapse is unwarranted.     

Variable responses of hawkmoths to nectar-depleted plants in two native Petunia axillaris (Solanaceae) populations

Pollination success of deceptive orchids is affected by the density and distribution of nectar providing plant species and overall plant density. Here we extended the framework of how plant density can affect pollination to examine how it may promote the success of plant intraspecific cheaters. We compared hawkmoth behaviour in two native populations of Petunia axillaris, where we simultaneously offered rewarding and manually depleted P. axillaris. We asked whether pollinator foraging strategies change as a function of plant density and whether such changes may differentially affect nectarless plants. We observed the first choice and number of flowers visited by pollinators and found that in the dense population, pollinators visited more flowers on rewarding plants than on nectar-depleted plants. In the sparse population, such discrimination was absent. As we found no differences in nectar volume between plants of the two populations, the observed differences in plant density may be temporal. We reason that if differences were more permanent, an equivalent of the remote habitat hypothesis prevails: in a sparse population, cheating plants benefit from the absence of inter- and intraspecific competitors because pollinators tend to visit all potential resources. In a denser population, a pollinator’s optimal foraging strategy involves more selectivity. This would cause between-plant competition for pollinators in a pollinator-limited context, which applies to most hawkmoth-pollinated systems. We propose that nectar-provisioning of plants can be density-dependant, with cheaters able to persist in low density areas.     

Variation,Sex, and Social Cooperation: Molecular Population Genetics of the Social Amoeba Dictyostelium discoideum

Dictyostelium discoideum is a eukaryotic microbial model system for multicellular development, cell–cell signaling, and social behavior. Key models of social evolution require an understanding of genetic relationships between individuals across the genome or possibly at specific genes, but the nature of variation within D. discoideum is largely unknown. We re-sequenced 137 gene fragments in wild North American strains of D. discoideum and examined the levels and patterns of nucleotide variation in this social microbial species. We observe surprisingly low levels of nucleotide variation in D. discoideum across these strains, with a mean nucleotide diversity (π) of 0.08%, and no strong population stratification among North American strains. We also do not find any clear relationship between nucleotide divergence between strains and levels of social dominance and kin discrimination. Kin discrimination experiments, however, show that strains collected from the same location show greater ability to distinguish self from non-self than do strains from different geographic areas. This suggests that a greater ability to recognize self versus non-self may arise among strains that are more likely to encounter each other in nature, which would lead to preferential formation of fruiting bodies with clonemates and may prevent the evolution of cheating behaviors within D. discoideum populations. Finally, despite the fact that sex has rarely been observed in this species, we document a rapid decay of linkage disequilibrium between SNPs, the presence of recombinant genotypes among natural strains, and high estimates of the population recombination parameter ρ. The SNP data indicate that recombination is widespread within D. discoideum and that sex as a form of social interaction is likely to be an important aspect of the life cycle.     

Persistence of the single lineage of transmissible 'social cancer' in an asexual ant

How cooperation can arise and persist, given the threat of cheating phenotypes, is a central problem in evolutionary biology, but the actual significance of cheating in natural populations is still poorly understood. Theories of social evolution predict that cheater lineages are evolutionarily short-lived. However, an exception comes from obligate socially parasitic species, some of which thought to have arisen as cheaters within cooperator colonies and then diverged through sympatric speciation. This process requires the cheater lineage to persist by avoiding rapid extinction that would result from the fact that the cheaters inflict fitness cost on their host. We examined whether this prerequisite is fulfilled, by estimating the persistence time of cheaters in a field population of the parthenogenetic ant Pristomyrmex punctatus. Population genetic analysis found that the cheaters belong to one monophyletic lineage which we infer has persisted for 200-9200 generations. We show that the cheaters migrate and are thus horizontally transmitted between colonies, a trait allowing the lineage to avoid rapid extinction with its host colony. Although horizontal transmission of disruptive cheaters has the potential to induce extinction of the entire population, such collapse is likely averted when there is spatially restricted migration in a structured population, a scenario that matches the observed isolation by distance pattern that we found. We compare our result with other examples of disruptive and horizontally transmissible cheater lineages in nature.     

Complex genotype interactions influence social fitness during the developmental phase of the social amoeba Dictyostelium discoideum

When individuals interact, phenotypic variation can be partitioned into direct genetic effects (DGEs) of the individuals’ own genotypes, indirect genetic effects (IGEs) of their social partners’ genotypes and epistatic interactions between the genotypes of interacting individuals (‘genotype‐by‐genotype (G×G) epistasis’). These components can all play important roles in evolutionary processes, but few empirical studies have examined their importance. The social amoeba Dictyostelium discoideum provides an ideal system to measure these effects during social interactions and development. When starved, free‐living amoebae aggregate and differentiate into a multicellular fruiting body with a dead stalk that holds aloft viable spores. By measuring interactions among a set of natural strains, we quantify DGEs, IGEs and G×G epistasis affecting spore formation. We find that DGEs explain most of the phenotypic variance (57.6%) whereas IGEs explain a smaller (13.3%) but highly significant component. Interestingly, G×G epistasis explains nearly a quarter of the variance (23.0%), highlighting the complex nature of genotype interactions. These results demonstrate the large impact that social interactions can have on development and suggest that social effects should play an important role in developmental evolution in this system.     

The excuse principle can maintain cooperation through forgivable defection in the Prisoner's Dilemma game

Reciprocal altruism describes a situation in which an organism acts in a manner that temporarily reduces its fitness while increasing another organism''s fitness, but there is an ultimate fitness benefit based on an expectation that the other organism will act in a similar manner at a later time. It creates the obvious dilemma in which there is always a short-term benefit to cheating, therefore cooperating individuals must avoid being exploited by non-cooperating cheaters. This is achieved by following various decision rules, usually variants of the tit-for-tat (TFT) strategy. The strength of TFT, however, is also its weakness—mistakes in implementation or interpretation of moves, or the inability to cooperate, lead to a permanent breakdown in cooperation. We show that pied flycatchers (Ficedula hypoleuca) use a TFT with an embedded ‘excuse principle’ to forgive the neighbours that were perceived as unable to cooperate during mobbing of predators. The excuse principle dramatically increases the stability of TFT-like behavioural strategies within the Prisoner''s Dilemma game.     

Loss and resiliency of social amoeba symbiosis under simulated warming

Anthropogenic global change is increasingly raising concerns about collapses of symbiotic interactions worldwide. Therefore, understanding how climate change affects symbioses remains a challenge and demands more study. Here, we look at how simulated warming affects the social ameba Dictyostelium discoideum and its relationship with its facultative bacterial symbionts, Paraburkholderia hayleyella and Paraburkholderia agricolaris. We cured and cross‐infected ameba hosts with different symbionts. We found that warming significantly decreased D. discoideum''s fitness, and we found no sign of local adaptation in two wild populations. Experimental warming had complex effects on these symbioses with responses determined by both symbiont and host. Neither of these facultative symbionts increases its hosts’ thermal tolerance. The nearly obligate symbiont with a reduced genome, P. hayleyella, actually decreases D. discoideum''s thermal tolerance and even causes symbiosis breakdown. Our study shows how facultative symbioses may have complex responses to global change.     

NONADAPTIVE PROCESSES CAN CREATE THE APPEARANCE OF FACULTATIVE CHEATING IN MICROBES

Adaptations to social life may take the form of facultative cheating, in which organisms cooperate with genetically similar individuals but exploit others. Consistent with this possibility, many strains of social microbes like Myxococcus bacteria and Dictyostelium amoebae have equal fitness in single‐genotype social groups but outcompete other strains in mixed‐genotype groups. Here we show that these observations are also consistent with an alternative, nonadaptive scenario: kin selection‐mutation balance under local competition. Using simple mathematical models, we show that deleterious mutations that reduce competitiveness within social groups (growth rate, e.g.) without affecting group productivity can create fitness effects that are only expressed in the presence of other strains. In Myxococcus, mutations that delay sporulation may strongly reduce developmental competitiveness. Deleterious mutations are expected to accumulate when high levels of kin selection relatedness relax selection within groups. Interestingly, local resource competition can create nonzero “cost” and “benefit” terms in Hamilton's rule even in the absence of any cooperative trait. Our results show how deleterious mutations can play a significant role even in organisms with large populations and highlight the need to test evolutionary causes of social competition among microbes.     

Within-host competitive interactions as a mechanism for the maintenance of parasite diversity

Variation among parasite strains can affect the progression of disease or the effectiveness of treatment. What maintains parasite diversity? Here I argue that competition among parasites within the host is a major cause of variation among parasites. The competitive environment within the host can vary depending on the parasite genotypes present. For example, parasite strategies that target specific competitors, such as bacteriocins, are dependent on the presence and susceptibility of those competitors for success. Accordingly, which parasite traits are favoured by within-host selection can vary from host to host. Given the fluctuating fitness landscape across hosts, genotype by genotype (G×G) interactions among parasites should be prevalent. Moreover, selection should vary in a frequency-dependent manner, as attacking genotypes select for resistance and genotypes producing public goods select for cheaters. I review competitive coexistence theory with regard to parasites and highlight a few key examples where within-host competition promotes diversity. Finally, I discuss how within-host competition affects host health and our ability to successfully treat infectious diseases.     

Stalk size and altruism investment within and among populations of the social amoeba

Reproductive division of labour is common in many societies, including those of eusocial insects, cooperatively breeding vertebrates, and most forms of multicellularity. However, conflict over what is best for the individual vs. the group can prevent an optimal division of labour from being achieved. In the social amoeba Dictyostelium discoideum, cells aggregate to become multicellular and a fraction behaves altruistically, forming a dead stalk that supports the rest. Theory suggests that intra‐organismal conflict over spore–stalk cell fate can drive rapid evolutionary change in allocation traits, leading to polymorphisms within populations or rapid divergence between them. Here, we assess several proxies for stalk size and spore–stalk allocation as metrics of altruism investment among strains and across geographic regions. We observe geographic divergence in stalk height that can be partly explained by differences in multicellular size, as well as variation among strains in clonal spore–stalk allocation, suggesting within‐population variation in altruism investment. Analyses of chimeras comprised of strains from the same vs. different populations indicated genotype‐by‐genotype epistasis, where the morphology of the chimeras deviated significantly from the average morphology of the strains developed clonally. The significantly negative epistasis observed for allopatric pairings suggests that populations are diverging in their spore–stalk allocation behaviours, generating incompatibilities when they encounter one another. Our results demonstrate divergence in microbial social traits across geographically separated populations and demonstrate how quantification of genotype‐by‐genotype interactions can elucidate the trajectory of social trait evolution in nature.