Macroevolutionary patterns in the origin of mutualisms involving ants

Ants are a diverse and abundant insect group that form mutualistic associations with a number of different organisms from fungi to insects and plants. Here, we use a phylogenetic approach to identify ecological factors that explain macroevolutionary trends in the mutualism between ants and honeydew-producing Homoptera. We also consider association between ant-Homoptera, ant-fungi and ant-plant mutualisms. Homoptera-tending ants are more likely to be forest dwelling, polygynous, ecologically dominant and arboreal nesting with large colonies of 10(4)-10(5) individuals. Mutualistic ants (including those that garden fungi and inhabit ant-plants) are found in under half of the formicid subfamilies. At the genus level, however, we find a negative association between ant-Homoptera and ant-fungi mutualisms, whereas there is a positive association between ant-Homoptera and ant-plant mutualisms. We suggest that species can only specialize in multiple mutualisms simultaneously when there is no trade-off in requirements from the different partners and no redundancy of rewards.

Specificity, stability and comparative physiology of coral-Symbiodinium mutualisms: Evaluating the potential for acclimation and/or adaptation in reef corals

The relationship between reef corals and endosymbiotic dinoflagellates is fundamental to the existence of coral reefs. To evaluate the fidelity of coral-Symbiodinium mutualisms, corals maintained in aquaria for years were analyzed by denaturant gradient gel electrophoresis (DGGE). Comparing Symbiodinium populations of captive aquarium colonies with known associations in nature is a practical way of examining partner flexibility. The finding of "normal" symbiont populations in corals existing under highly variable conditions supports the premise that most coral colonies possess stable associations. High sensitivity real-time PCR (rtPCR) was used to evaluate background populations of the putatively stress-tolerant Symbiodinium D in reef corals of the Caribbean. Analyses of samples collected during periods of environmental stability indicate the ability of Symbiodinium D to associate with a wider diversity of host taxa than previously recognized. To gain a broader perspective with regard to the ecology of Symbiodinium D1a, rtPCR and DGGE were used to evaluate the symbiont populations of reef corals from Barbados before and after the 2005 mass coral bleaching. Background populations were observed in 56% of the host genera prior to observations of bleaching. These findings indicate that 'stress', not 'bleaching', caused the displacement of 'natural' symbiont population and the opportunistic proliferation of D1a in many host taxa. Of the 12 host taxa monitored before and after the bleaching event, there was a 40% increase in colonies hosting Symbiodinium D1a. Together, these studies elucidate the mechanism responsible for recent observations reporting the emergence of Symbiodinium D following thermal disturbances. These observations are now most easily explained as the disproportionate growth of existing in hospite symbiont populations, rather than novel symbiont acquisition subsequent to bleaching. To evaluate the comparative "fitness" of corals able to host multiple symbiont types, rates of calcification were measured in P. verrucosa hosting either Symbiodinium C1b-c or D1 at elevated temperature. Rates of calcification decreased significantly for both host-symbiont combinations, but differences attributable to symbiont composition were not detected. This research improves our knowledge of the symbiosis biology and ecology of reef corals and contributes information necessary to most accurately predict the response of these ecosystems to global climate changes.

Colonization sequence of non-pollinating fig wasps associated with Ficus citrifolia in Brazil

Mutualisms, such as the fig-agaonid wasp association, are susceptible to colonization by parasitic species, which exploit the resources involved therein. In most cases, they oviposit into the figs from outside without providing any pollination service. In this study, we used several different methods (adhesive traps and direct standardized field observations) to assess the colonization sequence of a diverse fig wasp fauna associated with Ficus citrifolia, section Americana, in Brazil. They consistently showed a temporal partitioning in colonization among non-pollinating fig wasp species. Idarnes species belonging to the flavicollis and incerta groups colonized figs just before or during the fig receptive phase. In contrast, Idarnes females belonging to the carme group oviposited one to three weeks later, mainly in the middle of the inter-floral phase. Eurytoma, Heterandrium, Physothorax and Torymus were later colonizers, and laid eggs either in the middle or during the late inter-floral phase. The results suggest that these Neotropical fig wasps have different strategies of resource exploitation, even among species belonging to the same genus.

Cleaning wrasse species vary with respect to dependency on the mutualism and behavioural adaptations in interactions

Interspecific mutualisms are an essential feature of life on earth, yet we know little about their evolution and stability. In many mutualisms several species are available as partners, raising questions about the similarity in function and behavioural repertoire depending on the partner species. Furthermore, variation between species in the quantity and quality of interactions resulting in variation in payoffs may allow us to infer the potential evolutionary origin of a multispecies mutualism complex. We addressed these issues in the marine cleaning mutualism, in which so-called 'cleaners' remove ectoparasites from so-called 'client' reef fish. We measured several parameters concerning the quantity and quality of cleaning interactions in six sympatric cleaner wrasse species. We found significant variation between cleaner species with respect to client diversity, the number of interactions with predatory clients, the duration of interactions, the frequency of client jolts as a correlate of 'cheating' by cleaners, and behaviours used for manipulation of client decisions. Exploratory correlations between cleaner species' dependency and our variables of interest suggest that cleaning originated as a conflict-free by-product mutualism and evolved towards more sophisticated behaviours, including strategic behaviours for interactions with predators, cheating and manipulation specifically adapted to the client type.

Coupled reproductive ecologies in a plant-pollinator-seed predator system

Résumé de la thèseBien que le mutualisme puisse être considéré comme une relation harmonieuse entre différentes espèces, son étude révèle plutôt une exploitation réciproque où chaque partenaire tente de maximiser ses bénéfices tout en réduisant ses coûts. Dans ce contexte, l'identification des facteurs qui favorisent ou contrarient, au cours de l'évolution, une issue mutualiste est une étape majeure pour pouvoir reconstruire les étapes clés menant à l'apparition et au maintien des interactions mutualistes. Le but de ce doctorat était l'identification des traits phénotypiques qui permettent à la plante Silene latofolia (Caryophyllacée)et à son pollinisateur - prédateur de graines, la phalène Hadena bicruris (Noctuidé), d'augmenter les bénéfices nets que chacun retire de l'interaction. Ce système d'étude est particulièrement bien approprié à l'étude de ces traits, car on peut assez facilement estimer la qualité et la quantité des descendants (fitness) des deux partenaires. En effet, la femelle papillon pond un oeuf dans la fleur qu'elle pollinise et sa larve se développe dans le fruit, consommant les graines de la plante. Ainsi, sur une même plante, il est possible d'estimer les succès respectifs de la plante et du papillon à obtenir une descendance. De plus, le conflit d'intérêt autour des graines qui sont indispensables, à la fois à la plante et au papillon, peut stimuler l'évolution de traits qui limitent la surexploitation réciproque des partenaires. Dans une première étude, j'ai montré que le papillon mâle était un pollinisateur efficace de S. latifolia et qu'ainsi, il permettait à la plante d'augmenter le nombre de graines produites (i.e.bénéfice) sans pour autant augmenter la quantité de larves sur la plante. Dans ce système, les papillons pondent un seul oeuf par fleur, déposé soit à l'intérieur de la fleur, dans le tube de corolle, soit sur le pétale. Ma seconde étude montre que les plantes répondent différemment à la présence des oeufs suivant leur position. Aussi, quand l'oeuf est placé dans la fleur, la plante a davantage tendance à ne pas développer le fruit de la fleur infesté ou bien à produire des fruits plus petits que lorsque l'oeuf est placé sur le pétale. Enfin, j'ai montré que la femelle du papillon pond plus souvent sur le pétale lorsque elle visite des fleurs dotées d'un long tube de corolle, et que les larves issues de ces oeufs ont moins de chances de réussir à pénétrer dans le fruit que les larves issues des oeufs placés à l'intérieur de la fleur. Aussi, la variation observée du site de ponte pourrait être causé par la morphologie de la fleur qui contraint le papillon à pondre sur le pétale. Vu dans leur ensemble, les résultats obtenus pendant ce doctorat suggèrent que la participation des mâles à la pollination, l'absence de développement des fruits et la profondeur du tube de corolle pourraient réduire les coûts que S. latifolia subit dans son interaction avec H. bicruris. Par ailleurs, je n'ai pas détecté de mécanismes qui permettraient au papillon de réduire les coûts que la plante pourrait lui imposer. La prochaine étape serait de déterminer l'effet des traits identifiés dans ce doctorat sur la fitness globale de la plante et du papillon pour estimer pleinement leur efficacité à réduire les coûts et à favoriser une issue mutualiste. De même, il faudrait évaluer l'effet de ces traits en populations naturelles pour identifier le rôle des facteurs environnementaux sur leur efficacité.AbstractAlthough mutualisms can be regarded as harmonious relationships between the interacting partners, they are best conceptualized as reciprocal exploitations in which each partner attempts to increase its own benefits and decrease its costs. To date, identifying the factors which promote or discourage mutualistic outcomes remains a major goal to reconstruct the ecological conditions leading to mutualisms. The aim of this PhD thesis was to identify phenotypic traits that may increase the net benefits of each partner in the interaction between the plant Silene latifolia (Caryophyllaceae) and its pollinator / seed predator, the moth Hadena bicruris (Noctuidae). This study system is particularly well suited because the fitness of both interacting species can be assessed. The female moth lays its egg in the flower it pollinated, and its offspring grows in the fruit, feeding on the seeds of the plant, which allows for the follow-up of both larva and fruit fates. Furthermore, the inherent conflict of interest over the seeds as plant progeny vs. larval resource may stimulate the evolution of traits that reduce overexploitation in both the moth and plant. In a first study, I show that male moths are efficient pollinators, hence increasing seed production without increasing oviposition. The contribution of male moths to pollination might thus improve the net benefits of the interaction for the host plant. Females of the H. bicruris moth lay a single egg per flower, and place it either inside the corolla tube or on the petal. My second study shows that plants are more likely to abort the infested flower or to produce a smaller fruit when the egg was experimentally placed inside the flower compared to plants that received an egg on the petal. Finally, female moths were found to lay their eggs more frequently on the petal when visiting a flower with a deep corolla tube, and larvae hatching from these eggs less likely to successfully attack the fruit. Variation in egg position on the flower may thus be the result of a constraint imposed by floral morphology. Overall, this PhD work suggests that the pollination by male moths, flower abortion, and deep corolla tube may efficiently reduce the costs experienced by S. latifolia in its interaction with H. bicruris. Interestingly, no apparent mechanism of costs reduction was detected for the moth. Further studies should focus on the effects of these traits (i) in the long term fitness of both the plant and the insect and (ii) their interactions with environmental factors (biotic and abiotic) that may affect their efficiency in natural populations.

Cost limitation through constrained oviposition site in a plant-pollinator/seed predator mutualism

Mutualism often involves reciprocal exploitation due to individual selection for increased benefits even at the expense of the partner. Therefore, stability and outcomes of such interactions crucially depend on cost limitation mechanisms. In the plant, pollinator /seed predator interaction between Silene latifolia (Caryophyllaceae) and Hadena bicruris (Lepidoptera: Noctuidae), moths generate pollination benefits as adults but impose seed predation costs as larvae. We examined whether floral morphology limits over-exploitation by constraining oviposition site. Oviposition site varies naturally inside vs. outside the corolla tube, but neither its determinants nor its effect on the interaction have been investigated. In a common garden with plants originating from eight populations, corolla tube length predicted oviposition site, but not egg presence or pollination efficiency, suggesting that long corolla tubes constrain the moth to lay eggs on petals. Egg position was also predicted by the combined effect of corolla tube and moth ovipositor lengths, with shorter ovipositor than corolla tube resulting in higher probability for eggs outside. Egg position on a given plant was repeatable over different exposure nights. When egg position was experimentally manipulated, eggs placed on the petal resulted in significantly fewer successful fruit attacks compared with eggs placed inside the corolla tube, suggesting differences in egg/larval mortality. Egg position also differently affected larval mass, fruit mass and fruit development. Our results indicate that constraining oviposition site through a long corolla tube reduces seed predation costs suffered by the plant without negatively affecting pollination efficiency and, hence may act to limit over-exploitation. However, the net effects of corolla tube depth variation on this interaction may fluctuate with extrinsic factors affecting egg mortality, and with patterns of gene flow affecting trait matching between the interacting species. The intermediate fitness costs incurred by both plant and insect associated with the different egg positions may reduce selective pressures for this interaction to evolve towards antagonism, favouring instead a mutualistic outcome. While a role for oviposition site variation in cost limitation is a novel finding in this system, it may apply more generally also to other mutualisms involving pollinating seed predators.

Tolerance traits and the stability of mutualism

Identifying factors which allow the evolution and persistence of cooperative interactions between species is a fundamental issue in evolutionary ecology. Various hypotheses have been suggested which generally focus on mechanisms that allow cooperative genotypes in different species to maintain interactions over space and time. Here, we emphasise the fact that even within mutualisms (interactions with net positive fitness effects for both partners), there may still be inherent costs, such as the occasional predation by ants upon aphids. Individuals engaged in mutualisms benefit from minimising these costs as long as it is not at the expense of breaking the interspecific interaction, which offers a net positive benefit. The most common and obvious defence traits to minimise interspecific interaction costs are resistance traits, which act to reduce encounter rate between two organisms. Tolerance traits, in contrast, minimise fitness costs to the actor, but without reducing encounter rate. Given that, by definition, it is beneficial to remain in mutualistic interactions, the only viable traits to minimise costs are tolerance-based 'defence' strategies. Thus, we propose that tolerance traits are an important factor promoting stability in mutualisms. Furthermore, because resistance traits tend to propagate coevolutionary arms races between antagonists, whilst tolerance traits do not, we also suggest that tolerance-based defence strategies may be important in facilitating the transition from antagonistic interactions into mutualisms. For example, the mutualism between ants and aphids has been suggested to have evolved from parasitism. We describe how phenotypic plasticity in honeydew production may be a tolerance trait that has prevented escalation into an antagonistic arms race and instead led to mutualistic coevolution.

A role for parasites in stabilising the fig-pollinator mutualism

Mutualisms are interspecific interactions in which both players benefit. Explaining their maintenance is problematic, because cheaters should outcompete cooperative conspecifics, leading to mutualism instability. Monoecious figs (Ficus) are pollinated by host-specific wasps (Agaonidae), whose larvae gall ovules in their "fruits'' (syconia). Female pollinating wasps oviposit directly into Ficus ovules from inside the receptive syconium. Across Ficus species, there is a widely documented segregation of pollinator galls in inner ovules and seeds in outer ovules. This pattern suggests that wasps avoid, or are prevented from ovipositing into, outer ovules, and this results in mutualism stability. However, the mechanisms preventing wasps from exploiting outer ovules remain unknown. We report that in Ficus rubiginosa, offspring in outer ovules are vulnerable to attack by parasitic wasps that oviposit from outside the syconium. Parasitism risk decreases towards the centre of the syconium, where inner ovules provide enemy-free space for pollinator offspring. We suggest that the resulting gradient in offspring viability is likely to contribute to selection on pollinators to avoid outer ovules, and by forcing wasps to focus on a subset of ovules, reduces their galling rates. This previously unidentified mechanism may therefore contribute to mutualism persistence independent of additional factors that invoke plant defences against pollinator oviposition, or physiological constraints on pollinators that prevent oviposition in all available ovules.

When are ant-attractant devices a worthwhile investment? Vicia faba extrafloral nectaries and Lasius niger ants

Most studies aiming to determine the beneficial effect of ants on plants simply consider the effects of the presence or exclusion of ants on plant yield. This approach is often inadequate, however, as ants interact with both non-tended herbivores and tended Homoptera. Moreover, the interaction with these groups of organisms is dependent on ant density, and these functional relationships are likely to be non-linear. A model is presented here that segregates plant herbivores into two categories depending on the sign of their numerical response to ants (myrmecophiles increase with ants, non-tended herbivores decline). The changes in these two components of herbivores with increasing ant density and the resulting implications for ant-plant mutualisms are considered. It emerges that a wide range of ant densities needs to be considered as the interaction sign (mutualism or parasitism) and strength is likely to change with ant density. The model is used to interpret the results of an experimental study that varied levels of Aphis fabae infestation and Lasius niger ant attendance on Vicia faba bean plants. Increasing ant density consistently reduced plant fitness and thus, in this location, the interaction between the ants and the plant can be considered parasitic. In the Vicia faba system, these costs of ants are unlikely to be offset by other beneficial agents (e.g., parasitoids), which also visit extrafloral nectaries.

The curious case of the camelthorn: competition, coexistence and nest-site limitation in a multispecies mutualism

Myrmecophyte plants house ants in domatia in exchange for protection from herbivores. Ant-myrmecophyte mutualisms exhibit two general patterns due to competition between ants for plant occupancy: i) domatia nest-sites are a limiting resource and ii) each individual plant hosts one ant species at a time. However, individual camelthorn trees (Vachellia erioloba) typically host two to four ant species simultaneously, often coexisting in adjacent domatia on the same branch. Such fine-grain spatial coexistence brings into question the conventional wisdom on ant-myrmecophyte mutualisms. Camelthorn ants appear not to be nest-site limited, despite low abundance of suitable domatia, and have random distributions of nest-sites within and across trees. These patterns suggest a lack of competition between ants for domatia and contrast strongly with other ant-myrmecophyte systems. Comparison of this unusual case with others suggests that spatial scale is crucial to coexistence or competitive exclusion involving multiple ant species. Furthermore, coexistence may be facilitated when co-occurring ant species diverge strongly on at least one niche axis. Our conclusions provide recommendations for future ant-myrmecophyte research, particularly in utilising multispecies systems to further our understanding of mutualism biology.

A trophic cascade induced by predatory ants in a fig-fig wasp mutualism

Summary 1. A trophic cascade occurs when predators directly decrease the densities, or change the behaviour, of herbivores and thus indirectly increase plant productivity. The predator–herbivore– plant context is well known, but some predators attack species beneficial to plants (e.g. pollinators) and/or enemies of herbivores (e.g. parasites), and their role in the dynamics of mutualisms remains largely unexplored. 2. We surveyed the predatory ant species and studied predation by the dominant ant species, the weaver ant Oecophylla smaragdina, associated with the fig tree Ficus racemosa in southwest China. We then tested the effects of weaver ants on the oviposition behaviour of pollinating and non-pollinating fig wasps in an ant-exclusion experiment. The effects of weaver ants on fig wasp community structure and fig seed production were then compared between trees with and without O. smaragdina. 3. Oecophylla smaragdina captured more non-pollinating wasps (Platyneura mayri) than pollinators as the insects arrived to lay eggs. When ants were excluded, more non-pollinators laid eggs into figs and fewer pollinators entered figs. Furthermore, trees with O. smaragdina produced more pollinator offspring and fewer non-pollinator offspring, shifting the community structure significantly. In addition, F. racemosa produced significantly more seeds on trees inhabited by weaver ants. 4. Oecophylla smaragdina predation reverses the dominance of the two commonest wasp species at the egg-laying stage and favours the pollinators. This behavioural pattern is mirrored by wasp offspring production, with pollinators’ offspring dominating figs produced by trees inhabited by weaver ants, and offspring of the non-pollinator P. mayri most abundant in figs on trees inhabited by other ants. 5. Overall, our results suggest that predation by weaver ants limits the success of the non-pollinating P. mayri and therefore indirectly benefits the mutualism by increasing the reproductive success of both the pollinators and the plant. Predation is thus a key functional factor that can shape the community structure of a pollinator-plant mutualistic system. Key-words: competitive release, fig wasp, mutualism, predation, predator-exclusion experiment, trophic cascade

Spider-fed bromeliads: seasonal and interspecific variation in plant performance

Background and Aims Several animals that live on bromeliads can contribute to plant nutrition through nitrogen provisioning (digestive mutualism). The bromeliad-living spider Psecas chapoda (Salticidae) inhabits and breeds on Bromelia balansae in regions of South America, but in specific regions can also appear on Ananas comosus (pineapple) plantations and Aechmea distichantha. Methods Using isotopic and physiological methods in greenhouse experiments, the role of labelled ((15)N) spider faeces and Drosophila melanogaster flies in the nutrition and growth of each host plant was evaluated, as well as seasonal variation in the importance of this digestive mutualism. Key Results Spiders contributed 0.6 +/- 0.2% (mean +/- s.e.; dry season) to 2.7 +/- 1% (wet season) to the total nitrogen in B. balansae, 2.4 +/- 0.4% (dry) to 4.1 +/- 0.3% (wet) in An. comosus and 3.8 +/- 0.4% (dry) to 5 +/- 1% (wet) in Ae. distichantha. In contrast, flies did not contribute to the nutrition of these bromeliads. Chlorophylls and carotenoid concentrations did not differ among treatments. Plants that received faeces had higher soluble protein concentrations and leaf growth (RGR) only during the wet season. Conclusions These results indicate that the mutualism between spiders and bromeliads is seasonally restricted, generating a conditional outcome. There was interspecific variation in nutrient uptake, probably related to each species` performance and photosynthetic pathways. Whereas B. balansae seems to use nitrogen for growth, Ae. distichantha apparently stores nitrogen for stressful nutritional conditions. Bromeliads absorbed more nitrogen coming from spider faeces than from flies, reinforcing the beneficial role played by predators in these digestive mutualisms.

The modularity of seed dispersal: differences in structure and robustness between bat- and bird-fruit networks

In networks of plant-animal mutualisms, different animal groups interact preferentially with different plants, thus forming distinct modules responsible for different parts of the service. However, what we currently know about seed dispersal networks is based only on birds. Therefore, we wished to fill this gap by studying bat-fruit networks and testing how they differ from bird-fruit networks. As dietary overlap of Neotropical bats and birds is low, they should form distinct mutualistic modules within local networks. Furthermore, since frugivory evolved only once among Neotropical bats, but several times independently among Neotropical birds, greater dietary overlap is expected among bats, and thus connectance and nestedness should be higher in bat-fruit networks. If bat-fruit networks have higher nestedness and connectance, they should be more robust to extinctions. We analyzed 1 mixed network of both bats and birds and 20 networks that consisted exclusively of either bats (11) or birds (9). As expected, the structure of the mixed network was both modular (M = 0.45) and nested (NODF = 0.31); one module contained only birds and two only bats. In 20 datasets with only one disperser group, bat-fruit networks (NODF = 0.53 +/- A 0.09, C = 0.30 +/- A 0.11) were more nested and had a higher connectance than bird-fruit networks (NODF = 0.42 +/- A 0.07, C = 0.22 +/- A 0.09). Unexpectedly, robustness to extinction of animal species was higher in bird-fruit networks (R = 0.60 +/- A 0.13) than in bat-fruit networks (R = 0.54 +/- A 0.09), and differences were explained mainly by species richness. These findings suggest that a modular structure also occurs in seed dispersal networks, similar to pollination networks. The higher nestedness and connectance observed in bat-fruit networks compared with bird-fruit networks may be explained by the monophyletic evolution of frugivory in Neotropical bats, among which the diets of specialists seem to have evolved from the pool of fruits consumed by generalists.

Cheaters in mutualism networks

Mutualism-network studies assume that all interacting species are mutualistic partners and consider that all links are of one kind. However, the influence of different types of links, such as cheating links, on network organization remains unexplored. We studied two flower-visitation networks (Malpighiaceae and Bignoniaceae and their flower visitors), and divide the types of link into cheaters (i.e. robbers and thieves of flower rewards) and effective pollinators. We investigated if there were topological differences among networks with and without cheaters, especially with respect to nestedness and modularity. The Malpighiaceae network was nested, but not modular, and it was dominated by pollinators and had much fewer cheater species than Bignoniaceae network (28% versus 75%). The Bignoniaceae network was mainly a plant-cheater network, being modular because of the presence of pollen robbers and showing no nestedness. In the Malpighiaceae network, removal of cheaters had no major consequences for topology. In contrast, removal of cheaters broke down the modularity of the Bignoniaceae network. As cheaters are ubiquitous in all mutualisms, the results presented here show that they have a strong impact upon network topology.