The roles of sensory traps in the origin, maintenance, and breakdown of mutualism

A sensory bias overrides learned preferences of bumblebees for honest signals in Mimulus guttatus

Insect pollinators readily learn olfactory cues, and this is expected to select for 'honest signals' that provide reliable information about floral rewards. However, plants might alternatively produce signals that exploit pollinators' sensory biases, thereby relaxing selection for signal honesty. We examined the innate and learned preferences of Bombus impatiens for Mimulus guttatus floral scent phenotypes corresponding to different levels of pollen rewards in the presence and absence of the innately attractive floral volatile compound β-trans-bergamotene. Bees learned to prefer honest signals after foraging on live M. guttatus flowers, but only exhibited this preference when presented floral scent phenotypes that did not include β-trans-bergamotene. Our results suggest that a sensory bias for β-trans-bergamotene overrides the ability of B. impatiens to use honest signals when foraging on M. guttatus. This may represent a deceptive pollination strategy that allows plants to minimize investment in costly rewards without incurring reduced rates of pollinator visitation.

Endosymbiosis before eukaryotes: mitochondrial establishment in protoeukaryotes

Endosymbiosis and organellogenesis are virtually unknown among prokaryotes. The single presumed example is the endosymbiogenetic origin of mitochondria, which is hidden behind the event horizon of the last eukaryotic common ancestor. While eukaryotes are monophyletic, it is unlikely that during billions of years, there were no other prokaryote-prokaryote endosymbioses as symbiosis is extremely common among prokaryotes, e.g., in biofilms. Therefore, it is even more precarious to draw conclusions about potentially existing (or once existing) prokaryotic endosymbioses based on a single example. It is yet unknown if the bacterial endosymbiont was captured by a prokaryote or by a (proto-)eukaryote, and if the process of internalization was parasitic infection, slow engulfment, or phagocytosis. In this review, we accordingly explore multiple mechanisms and processes that could drive the evolution of unicellular microbial symbioses with a special attention to prokaryote-prokaryote interactions and to the mitochondrion, possibly the single prokaryotic endosymbiosis that turned out to be a major evolutionary transition. We investigate the ecology and evolutionary stability of inter-species microbial interactions based on dependence, physical proximity, cost-benefit budget, and the types of benefits, investments, and controls. We identify challenges that had to be conquered for the mitochondrial host to establish a stable eukaryotic lineage. Any assumption about the initial interaction of the mitochondrial ancestor and its contemporary host based solely on their modern relationship is rather perilous. As a result, we warn against assuming an initial mutually beneficial interaction based on modern mitochondria-host cooperation. This assumption is twice fallacious: (i) endosymbioses are known to evolve from exploitative interactions and (ii) cooperativity does not necessarily lead to stable mutualism. We point out that the lack of evidence so far on the evolution of endosymbiosis from mutual syntrophy supports the idea that mitochondria emerged from an exploitative (parasitic or phagotrophic) interaction rather than from syntrophy.

Compartmentalization drives the evolution of symbiotic cooperation

Across the tree of life, hosts have evolved mechanisms to control and mediate interactions with symbiotic partners. We suggest that the evolution of physical structures that allow hosts to spatially separate symbionts, termed compartmentalization, is a common mechanism used by hosts. Such compartmentalization allows hosts to: (i) isolate symbionts and control their reproduction; (ii) reward cooperative symbionts and punish or stop interactions with non-cooperative symbionts; and (iii) reduce direct conflict among different symbionts strains in a single host. Compartmentalization has allowed hosts to increase the benefits that they obtain from symbiotic partners across a diversity of interactions, including legumes and rhizobia, plants and fungi, squid and Vibrio, insects and nutrient provisioning bacteria, plants and insects, and the human microbiome. In cases where compartmentalization has not evolved, we ask why not. We argue that when partners interact in a competitive hierarchy, or when hosts engage in partnerships which are less costly, compartmentalization is less likely to evolve. We conclude that compartmentalization is key to understanding the evolution of symbiotic cooperation. This article is part of the theme issue 'The role of the microbiome in host evolution'.

The Non-Legume Parasponia andersonii Mediates the Fitness of Nitrogen-Fixing Rhizobial Symbionts Under High Nitrogen Conditions

Organisms rely on symbiotic associations for metabolism, protection, and energy. However, these intimate partnerships can be vulnerable to exploitation. What prevents microbial mutualists from parasitizing their hosts? In legumes, there is evidence that hosts have evolved sophisticated mechanisms to manage their symbiotic rhizobia, but the generality and evolutionary origins of these control mechanisms are under debate. Here, we focused on the symbiosis between Parasponia hosts and N2-fixing rhizobium bacteria. Parasponia is the only non-legume lineage to have evolved a rhizobial symbiosis and thus provides an evolutionary replicate to test how rhizobial exploitation is controlled. A key question is whether Parasponia hosts can prevent colonization of rhizobia under high nitrogen conditions, when the contribution of the symbiont becomes nonessential. We grew Parasponia andersonii inoculated with Bradyrhizobium elkanii under four ammonium nitrate concentrations in a controlled growth chamber. We measured shoot and root dry weight, nodule number, nodule fresh weight, nodule volume. To quantify viable rhizobial populations in planta, we crushed nodules and determined colony forming units (CFU), as a rhizobia fitness proxy. We show that, like legumes and actinorhizal plants, P. andersonii is able to control nodule symbiosis in response to exogenous nitrogen. While the relative host growth benefits of inoculation decreased with nitrogen fertilization, our highest ammonium nitrate concentration (3.75 mM) was sufficient to prevent nodule formation on inoculated roots. Rhizobial populations were highest in nitrogen free medium. While we do not yet know the mechanism, our results suggest that control mechanisms over rhizobia are not exclusive to the legume clade.

Local adaptation mediates floral responses to water limitation in an annual wildflower

PREMISE

Understanding how environmental stress affects the strength of mutualisms is critically important given observed and projected environmental changes. In particular, the frequency and duration of drought have been increasing worldwide. We investigated how water availability affects plant traits that mediate a pollination mutualism.

METHODS

For butterfly-pollinated Phlox drummondii, we determined how moisture availability affects flower size, nectar volume, and nectar sugar amount. Furthermore, we explored the role that local adaptation may play in responses to moisture availability by including plants collected from regions that differ in aridity. Finally, we determined whether responses of plant populations to selection may differ under drought by calculating heritability of traits under control and dry conditions.

RESULTS

Flower size was generally smaller in dry plants than in control plants. Early in the treatment period, nectar volume and sugar were higher in dry plants than in control plants for plants from both arid and wetter regions. With prolonged dry treatment, nectar volume and sugar remained higher only in plants from the arid region. Heritability of floral traits was lower for water-limited plants than for control plants.

CONCLUSIONS

Plant investment into pollination mutualisms under environmental stress may depend on the extent to which populations are already locally adapted to such conditions, suggesting that mutualism may remain strong, at least in arid regions. However, decreases in heritability under water-limitation suggest that responses to selection imposed by pollinators may be low, even if drought-adapted plants maintain production of rewards to pollinators.

Ants Associated with Turnera subulata (Turneraceae): Elaiosome Attraction, Seed Dispersion and Germination

Symbiosis between plants and ants include examples in which the plant provides shelter and/or food for ants that, in turn, act in the defense or in the dispersion of seeds from the host plant. Although traditionally referred as mutualistic, the results of these interactions may vary with the ecological context in which patterns are involved. A range of species have facultative association with Turnera subulata (Turneraceae). Here, using behavioral bioassays, we investigated the effects of the most frequent ant species associated with T. subulata (Brachymyrmex sp.1, Camponotus blandus (Smith), Dorymyrmex sp.1, Crematogaster obscurata Emery, and Solenopsis invicta Buren) in the dispersion of plant host seeds and in the number of seedlings around the associated ant nests. We also evaluated the effects of these ant species in the germination of T. subulata seeds, in the consumption of elaiosome, and in the attractiveness to elaiosome odor. Our results showed that the ant species associated with T. subulata presented variation in the attraction by the odor and in the rate of consumption of the elaiosomes. However, none of the ant species studied contributed significantly to the increase of seed germination and seedling growth. Our results suggest that the consumption of the elaiosome by ant species is not a determinant factor to the success of germination of T. subulata. However, such species could contribute indirectly to seed germination by carrying seeds to sites more fertile to germination. In general, our results help to elucidate the results of ecological interactions involving ants and plants.

Reward regulation in plant-frugivore networks requires only weak cues

Theory assumes that fair trade among mutualists requires highly reliable communication. In plant–animal mutualisms the reliability of cues that indicate reward quality is often low. Therefore, it is controversial whether communication allows animal mutualists to regulate their reward intake. Here we show that even loose relationships between fruit brightness and nutritional rewards (r² = 0.11–0.35) allow birds to regulate their nutrient intake across distinct European plant–frugivore networks. Resident, over-wintering generalist frugivores that interact with diverse plant species select bright, lipid-rich fruits, whereas migratory birds select dark, sugar- and antioxidant-rich fruits. Both nutritional strategies are consistent with previous physiological experiments suggesting that over-wintering generalists aim to maximize their energy intake, whereas migrants aim to enhance the build-up of body fat, their immune response and oxidative status during migration. Our results suggest that animal mutualists require only weak cues to regulate their reward intake according to specific nutritional strategies.

A challenge for mutualists is that partner cue reliability is often low. Here, the authors show that though fruit brightness is weakly predictive of nutritional content, the diets of birds (e.g. migrants vs. residents) are structured by fruit brightness in alignment with expected nutritional needs.

Innate Receiver Bias: Its Role in the Ecology and Evolution of Plant–Animal Interactions

Receiver bias in plant–animal interactions is here defined as “selection mediated by behavioral responses of animals, where those responses have evolved in a context outside the interactions.” As a consequence, the responses are not necessarily linked to fitness gains in interacting animals. Thus, receiver bias can help explain seemingly maladaptive patterns of behavior in interacting animals and the evolution of plant traits that trigger such behavior. In this review, I discuss principles of receiver bias, show its overlap with mimicry and how it differs from mimicry, and outline examples in different plant–animal interactions. The most numerous and best documented examples of receiver bias occur within plant–pollinator interactions. I elaborate on the ability of some plants to heat up their flowers (i.e., floral thermogenesis) and argue that this trait likely evolved under receiver bias, especially in pollination systems with oviposition mimicry. Further examples include signals in insect-mediated seed dispersal and plant defense through repellence of aphids. These examples show that receiver bias is widespread in different plant–animal interactions. For a broader understanding of the role of receiver bias in those interactions, we need more data on how animals respond to plant signals, the context and evolutionary history of those behaviors, and the evolutionary patterns of plant signals.

Fruit colour conceals endocarp dimorphism from avian seed dispersers in a tropical beach plant, Scaevola taccada (Goodeniaceae), found in Okinawa

Theory predicts that honest signalling strategies will not always be evolutionarily stable in interspecific communication, yet to demonstrate such a transition of signalling modality between honesty and dishonesty in the wild would be difficult. An endocarp dimorphism has been found in Scaevola taccada fruits: a morph with a cork substrate that facilitates ocean current seed dispersal and a morph without the cork. Both types of fruit are covered with sugar-containing flesh, and are similar in size and colour to one another (at least from a human perspective). The apparent lack of external differences between morphotypes could potentially degrade mutualistic relations between the plant and seed-dispersing birds because the presence of a cork could lower the fruit's nutritional value. Thus, unless seed dispersers can discriminate between the different types of fruit, this system may provide an example of a transition between honest and dishonest signalling. We examined S. taccada fruit and leaf colours from an avian visual perspective. Even though the fruits and leaves were different in colour from one another to birds, there was no perceivable difference in the colours between fruit morphotypes. Therefore, fruit colour is not an honest indicator of reward to seed dispersers. Further, we propose an adoption of a statistical method in avian visual modelling studies that avoids the common statistical errors, such as violation of the congruence principle.

Parasponia: a novel system for studying mutualism stability

Understanding how mutualistic interactions are stabilized in the presence of cheaters is a major question in evolutionary biology. The legume-rhizobia mutualism has become a model system for studying how plants control cheating partners. However, the generality and evolutionary origins of these control mechanisms are intensely debated. In this Opinion article, we argue that a novel system--the Parasponia-rhizobia mutualism--will significantly advance research in mutualism stability. Parasponia is the only non-legume lineage to have evolved a rhizobial symbiosis, which provides an evolutionary replicate to test how rhizobial exploitation is controlled. Evidence also suggests that this symbiosis is young. This allows studies at an earlier evolutionary stage in mutualisms, so the origin of control mechanisms can be better understood.

Niche construction initiates the evolution of mutualistic interactions

Niche construction theory explains how organisms' niche modifications may feed back to affect their evolutionary trajectories. In theory, the evolution of other species accessing the same modified niche may also be affected. We propose that this niche construction may be a general mechanism driving the evolution of mutualisms. Drosophilid flies benefit from accessing yeast-infested fruits, but the consequences of this interaction for yeasts are unknown. We reveal high levels of variation among strains of Saccharomyces cerevisiae in their ability to modify fruits and attract Drosophila simulans. More attractive yeasts are dispersed more frequently, both in the lab and in the field, and flies associated with more attractive yeasts have higher fecundity. Although there may be multiple natural yeast and fly species interactions, our controlled assays in the lab and field provide evidence of a mutualistic interaction, facilitated by the yeast's niche modification.

Birds see the true colours of fruits to live off the fat of the land

Communication is a characteristic of life, but its reliability and basic definition are hotly debated. Theory predicts that trade among mutualists requires high reliability. Here, we show that moderate reliability already allows mutualists to optimize their rewards. The colours of Mediterranean fleshy-fruits indicate lipid rewards (but not other nutrients) to avian seed dispersers on regional and local scales. On the regional scale, fruits with high lipid content were significantly darker and less chromatic than congeners with lower lipid content. On the local scale, two warbler species (Sylvia atricapilla and Sylvia borin) selected fruit colours that were less chromatic, and thereby maximized their intake of lipids-a critical resource during migration and wintering. Crucially, birds were able to maximize lipid rewards with moderate reliability from visual fruit colours (r(2) = 0.44-0.60). We suggest that mutualisms require only that any association between the quality and sensory aspects of signallers is learned through multiple, repeated interactions. Because these conditions are often fulfilled, also in social communication systems, we contend that selection on reliability is less intense than hitherto assumed. This may contribute to explaining the extraordinary diversity of signals, including that of plant reproductive displays.

Signals can trump rewards in attracting seed-dispersing ants

Both rewards and signals are important in mutualisms. In myrmecochory, or seed dispersal by ants, the benefits to plants are relatively well studied, but less is known about why ants pick up and move seeds. We examined seed dispersal by the ant Aphaenogaster rudis of four co-occurring species of plants, and tested whether morphology, chemical signaling, or the nutritional quality of fatty seed appendages called elaiosomes influenced dispersal rates. In removal trials, ants quickly collected diaspores (seeds plus elaiosomes) of Asarum canadense, Trillium grandiflorum, and Sanguinaria canadensis, but largely neglected those of T. erectum. This discrepancy was not explained by differences in the bulk cost-benefit ratio, as assessed by the ratio of seed to elaiosome mass. We also provisioned colonies with diaspores from one of these four plant species or no diaspores as a control. Colonies performed best when fed S. canadensis diaspores, worst when fed T. grandiflorum, and intermediately when fed A. canadense, T. erectum, or no diaspores. Thus, the nutritional rewards in elaiosomes affected colony performance, but did not completely predict seed removal. Instead, high levels of oleic acid in T. grandiflorum elaiosomes may explain why ants disperse these diaspores even though they reduce ant colony performance. We show for the first time that different elaiosome-bearing plants provide rewards of different quality to ant colonies, but also that ants appear unable to accurately assess reward quality when encountering seeds. Instead, we suggest that signals can trump rewards as attractants of ants to seeds.

Pollinator-mediated evolution of floral signals

Because most plants rely on animals for pollination, insights from animal sensory ecology and behavior are essential for understanding the evolution of flowers. In this review, we compare and contrast three main types of pollinator responses to floral signals--receiver bias, 'adaptive' innate preferences, and associative learning--and discuss how they can shape selection on floral signals. We show that pollinator-mediated selection on floral signals can be strong and that the molecular bases of floral signal variation are often surprisingly simple. These new empirical and conceptual insights into pollinator-mediated evolution provide a framework for understanding patterns of both convergent (pollination syndromes) and advergent (floral mimicry) floral signal evolution.

Who resembles whom? Mimetic and coincidental look-alikes among tropical reef fishes

Studies of mimicry among tropical reef-fishes usually give little or no consideration to alternative explanations for behavioral associations between unrelated, look-alike species that benefit the supposed mimic. I propose and assess such an alternative explanation. With mimicry the mimic resembles its model, evolved to do so in response to selection by the mimicry target, and gains evolved benefits from that resemblance. In the alternative, the social-trap hypothesis, a coincidental resemblance of the model to the “mimic” inadvertently attracts the latter to it, and reinforcement of this social trapping by learned benefits leads to the “mimic” regularly associating with the model. I examine three well known cases of supposed aggressive mimicry among reef-fishes in relation to nine predictions from these hypotheses, and assess which hypothesis offers a better explanation for each. One case, involving precise and complex morphological and behavioral resemblance, is strongly consistent with mimicry, one is inconclusive, and one is more consistent with a social-trap based on coincidental, imprecise resemblance. Few cases of supposed interspecific mimicry among tropical reef fishes have been examined in depth, and many such associations may involve social traps arising from generalized, coincidental resemblance. Mimicry may be much less common among these fishes than is generally thought.

Evolution of minimal specificity and promiscuity in steroid hormone receptors

Most proteins are regulated by physical interactions with other molecules; some are highly specific, but others interact with many partners. Despite much speculation, we know little about how and why specificity/promiscuity evolves in natural proteins. It is widely assumed that specific proteins evolved from more promiscuous ancient forms and that most proteins' specificity has been tuned to an optimal state by selection. Here we use ancestral protein reconstruction to trace the evolutionary history of ligand recognition in the steroid hormone receptors (SRs), a family of hormone-regulated animal transcription factors. We resurrected the deepest ancestral proteins in the SR family and characterized the structure-activity relationships by which they distinguished among ligands. We found that that the most ancient split in SR evolution involved a discrete switch from an ancient receptor for aromatized estrogens--including xenobiotics--to a derived receptor that recognized non-aromatized progestagens and corticosteroids. The family's history, viewed in relation to the evolution of their ligands, suggests that SRs evolved according to a principle of minimal specificity: at each point in time, receptors evolved ligand recognition criteria that were just specific enough to parse the set of endogenous substances to which they were exposed. By studying the atomic structures of resurrected SR proteins, we found that their promiscuity evolved because the ancestral binding cavity was larger than the primary ligand and contained excess hydrogen bonding capacity, allowing adventitious recognition of larger molecules with additional functional groups. Our findings provide an historical explanation for the sensitivity of modern SRs to natural and synthetic ligands--including endocrine-disrupting drugs and pollutants--and show that knowledge of history can contribute to ligand prediction. They suggest that SR promiscuity may reflect the limited power of selection within real biological systems to discriminate between perfect and "good enough."

By-product information can stabilize the reliability of communication

Although communication underpins many biological processes, its function and basic definition remain contentious. In particular, researchers have debated whether information should be an integral part of a definition of communication and how it remains reliable. So far the handicap principle, assuming signal costs to stabilize reliable communication, has been the predominant paradigm in the study of animal communication. The role of by-product information produced by mechanisms other than the communicative interaction has been neglected in the debate on signal reliability. We argue that by-product information is common and that it provides the starting point for ritualization as the process of the evolution of communication. Second, by-product information remains unchanged during ritualization and enforces reliable communication by restricting the options for manipulation and cheating. Third, this perspective changes the focus of research on communication from studying signal costs to studying the costs of cheating. It can thus explain the reliability of signalling in many communication systems that do not rely on handicaps. We emphasize that communication can often be informative but that the evolution of communication does not cause the evolution of information because by-product information often predates and stimulates the evolution of communication. Communication is thus a consequence but not a cause of reliability. Communication is the interplay of inadvertent, informative traits and evolved traits that increase the stimulation and perception of perceivers. Viewing communication as a complex of inadvertent and derived traits facilitates understanding of the selective pressures shaping communication and those shaping information and its reliability. This viewpoint further contributes to resolving the current controversy on the role of information in communication.

The fundamental role of competition in the ecology and evolution of mutualisms

Mutualisms are interspecific interactions that yield reciprocal benefits. Here, by adopting a consumer-resource perspective, we show how considering competition is necessary in order to understand the evolutionary and ecological dynamics of mutualism. We first review the ways in which competition shapes the ecology of mutualisms, using a graphical framework based on resource flows rather than net effects to highlight the opportunities for competition. We then describe the known mechanisms of competition and show how it is a critical driver of the evolutionary dynamics, persistence, and diversification of mutualism. We argue that empirical and theoretical research on the ecology and evolution of mutualisms will jointly progress by addressing four key points: (i) the existence and shape of physiological trade-offs among cooperation, competition, and other life-history and functional traits; (ii) the capacity for individuals to express conditional responses to variation in their mutualistic and competitive environment; (iii) the existence of heritable variation for mutualistic and competitive traits and their potentially conditional expression; and (iv) the structure of the network of consumer-resource interactions in which individuals are embedded.

Evidence for intersexual chemical mimicry in a dioecious plant

The dioecious Mediterranean fig, Ficus carica, displays a unique phenology in which males sometimes bloom synchronously with females (in summer), and sometimes not (in spring). Ficus carica is engaged in an obligatory mutualism with a specific pollinating wasp, which reproduces only within figs, localising them by their specific scents. We show that scents emitted by male figs show seasonal variation within individual trees. Scents of summer male figs resemble those of the co-flowering females, and are different from those of the same male trees in spring, when female figs are absent. These differences hold even if only compounds electrophysiologically active for pollinators are considered. The similar scents of summer males and females may explain why the rewardless females are still pollinated. These results offer a tractable model for future studies of intersexual chemical mimicry in mutualistic pollination interactions.

Cooperation for direct fitness benefits

Studies of the evolution of helping have traditionally used the explanatory frameworks of reciprocity and altruism towards relatives, but recently there has been an increasing interest in other kinds of explanations. We review the success or otherwise of work investigating alternative processes and mechanisms, most of which fall under the heading of cooperation for direct benefits. We evaluate to what extent concepts such as by-product benefits, pseudo-reciprocity, sanctions and partner choice, markets and the build-up of cross-species spatial trait correlations have contributed to the study of the evolution of cooperation. We conclude that these alternative ideas are successful and show potential to further increase our understanding of cooperation. We also bring up the origin and role of common interest in the evolution of cooperation, including the appearance of organisms. We note that there are still unresolved questions about the main processes contributing to the evolution of common interest. Commenting on the broader significance of the recent developments, we argue that they represent a justified balancing of the importance given to different major hypotheses for the evolution of cooperation. This balancing is beneficial because it widens considerably the range of phenomena addressed and, crucially, encourages empirical testing of important theoretical alternatives.

The evolution of floral scent and insect chemical communication

Plants have evolved a range of strategies to manipulate the behaviour of their insect partners. One powerful strategy is to produce signals that already have a role in the animals' own communication systems. To investigate to what extent the evolution of floral scents is correlated with chemical communication in insects, I analyse the occurrence, commonness, and evolutionary patterns of the 71 most common 'floral' volatile organic compounds (VOCs) in 96 plant families and 87 insect families. I found an overlap of 87% in VOCs produced by plants and insects. 'Floral' monoterpenes showed strong positive correlation in commonness between plants (both gymnosperms and angiosperms) and herbivores, whereas the commonness of 'floral' aromatics was positively correlated between angiosperms and both pollinators and herbivores. According to a multivariate regression analysis the commonness of 'floral' aromatics was best explained by their commonness in pollinators, whereas monoterpenes were best explained by herbivores. Among pollinator orders, aromatics were significantly more common in Lepidoptera than in Hymenoptera, whereas monoterpenes showed no difference among the two orders. Collectively, these patterns suggest that plants and insects converge in overall patterns of volatile production, both for attraction and defence. Monoterpenes seem to have evolved primarily for defence under selection by herbivores, whereas aromatics evolved signalling functions in angiosperms, primarily for pollinator attraction.

Interference competition and high temperatures reduce the virulence of fig wasps and stabilize a fig-wasp mutualism

Fig trees are pollinated by fig wasps, which also oviposit in female flowers. The wasp larvae gall and eat developing seeds. Although fig trees benefit from allowing wasps to oviposit, because the wasp offspring disperse pollen, figs must prevent wasps from ovipositing in all flowers, or seed production would cease, and the mutualism would go extinct. In Ficus racemosa, we find that syconia ('figs') that have few foundresses (ovipositing wasps) are underexploited in the summer (few seeds, few galls, many empty ovules) and are overexploited in the winter (few seeds, many galls, few empty ovules). Conversely, syconia with many foundresses produce intermediate numbers of galls and seeds, regardless of season. We use experiments to explain these patterns, and thus, to explain how this mutualism is maintained. In the hot summer, wasps suffer short lifespans and therefore fail to oviposit in many flowers. In contrast, cooler temperatures in the winter permit longer wasp lifespans, which in turn allows most flowers to be exploited by the wasps. However, even in winter, only in syconia that happen to have few foundresses are most flowers turned into galls. In syconia with higher numbers of foundresses, interference competition reduces foundress lifespans, which reduces the proportion of flowers that are galled. We further show that syconia encourage the entry of multiple foundresses by delaying ostiole closure. Taken together, these factors allow fig trees to reduce galling in the wasp-benign winter and boost galling (and pollination) in the wasp-stressing summer. Interference competition has been shown to reduce virulence in pathogenic bacteria. Our results show that interference also maintains cooperation in a classic, cooperative symbiosis, thus linking theories of virulence and mutualism. More generally, our results reveal how frequency-dependent population regulation can occur in the fig-wasp mutualism, and how a host species can 'set the rules of the game' to ensure mutualistic behavior in its symbionts.

Reliable cues and signals of fruit quality are contingent on the habitat in black elder (Sambucus nigra)

Communication mediates interactions between organisms and can be based on signals or cues. Signals are selected for their signaling function, whereas cues evolve for reasons other than signaling. To be evolutionarily stable, communication needs to be reliable on average, but the mechanisms that enforce reliability are hotly debated in light of strong environmental influence on signals and cues. While fruit quality in black elder (Sambucus nigra) is unrelated to fruit color, it is indicated by alternative pedicel phenotypes. Information on fruit quality has thus been transferred from the fruit to the developmentally associated pedicels, which are environmentally determined cues. Within each phenotype, color variation indicates fruit quality. Communication by black elder is thus reliable, but the proximate mechanisms enforcing reliability are habitat specific. High irradiance increases both the contrasts of the visual cue and fruit quality in the anthocyanin-based red pedicel phenotype, while shaded plants of the chlorophyll-based green phenotype apparently use signals by forgoing photosynthesis. This is because lower chlorophyll content in green pedicels creates contrasting pedicels, and higher contrasts indicate higher sugar content in the fruits of green pedicels. Because anthocyanins are light-induced, plants use cues when exposed to high irradiance, whereas they apparently use costly signals in the shade by reducing chlorophyll content in the pedicels. In behavioral field and laboratory experiments we document that avian seed dispersers select among pedicel phenotypes that indicate different fruit quality. Plants can thus increase their reproductive success by sending highly informative cues. Our results indicate how reliable information transfer can be maintained both in cues and signals in spite of substantial environmental influence on visual traits.

Visualization of the spatial and spectral signals of orb-weaving spiders, Nephila pilipes, through the eyes of a honeybee

It is well known that the honeybee has good color vision. However, the spectral range in which the bee can see is different from that of the human eye. To study how bees view their world of colors, one has to see through the eyes of the bee, not the eyes of a human. A conventional way to examine the color signals that animals can detect is to measure the surface reflectance spectra and compute the quantum catches of each photoreceptor type based on its known spectral sensitivity. Color signal and color contrast are then determined from the loci of these quantum catches in the color space. While the point-by-point measurements of the reflectance spectra using a standard spectrometer have yielded a significant amount of data for analyzing color signals, the lack of spatial information and low sampling efficiency constrain their applications. Using a special filter coating technique, a set of filters with transmission spectra that were closely matched to the bee's sensitivity spectra of three photoreceptor types (UV, blue, and green) was custom made. By placing these filters in front of a UV/VIS-sensitive CCD camera and acquiring images sequentially, we could collect images of a bee's receptor with only three shots. This allowed a direct visualization of how bees view their world in a pseudo-color RGB display. With this imaging system, spatial and spectral signals of the orb-weaving spider, Nephila pilipes, were recorded,and color contrast images corresponding to the bee's spatial resolution were constructed and analyzed. The result not only confirmed that the color markings of N. pilipes are of high chromatic contrast to the eyes of a bee, but it also indicated that the spatial arrangement of these markings resemble flower patterns which may attract bees to visit them. Thus, it is likely that the orb-weaving spider (N. pilipes) deploys a similar strategy to that of the Australian crab spider (Thomisus spectabilis)to exploit the bee's pre-existing preference for flowers with color patterning.

The roles of tolerance in the evolution, maintenance and breakdown of mutualism

Tolerance strategies are cost-reduction mechanisms that enable organisms to recover some of the fitness lost to damage, but impose limited or no cost on antagonists. They are frequently invoked in studies of plant-herbivore and of host-parasite interactions, but the possible roles of tolerance in mutualism (interspecific cooperation) have yet to be thoroughly examined. This review identifies candidate roles for tolerance in the evolution, maintenance and breakdown of mutualism. Firstly, by reducing the cost of damage, tolerance provides a key pathway by which pre-mutualistic hosts can reduce the cost of association with their parasites, promoting cooperation. This holds for the evolution of 'evolved dependency' type mutualism, where a host requires an antagonist that does not direct any reward to their partner for some resource, and of 'outright mutualism', where participants directly trade benefits. Secondly, in outright mutualism, tolerance might maintain cooperation by reducing the cost of a persisting negative trait in a symbiotic partner. Finally, the evolution of tolerance might also provide a pathway out of mutualism because the host could evolve a cheaper alternative to continued cooperation with its mutualistic partner, permitting autonomy. A key consequence of tolerance is that it contrasts with partner choice mechanisms that impose large costs on cheats, and I highlight understanding any trade-off between tolerance and partner choice as an important research topic in the evolution of cooperation. I conclude by identifying tolerance as part of a more general phenomenon of co-adaptation in mutualism and parasitism that drives the evolution of the cost/benefit ratio from the interaction.

Function of bright coloration in the wasp spider Argiope bruennichi (Araneae: Araneidae)

There are two major competing explanations for the counter-intuitive presence of bright coloration in certain orb-web spiders. Bright coloration could lure insect prey to the web vicinity, increasing the spider's foraging success. Alternatively, the markings could function as disruptive camouflage, making it difficult for the insect prey to distinguish spiders from background colour variation. We measured the prey capture rates of wasp spiders, Argiope bruennichi, that were blacked out, shielded from view using a leaf fragment, or left naturally coloured. Naturally coloured spiders caught over twice the number of prey as did either blacked-out or leaf-shielded spiders, and almost three times as many orthopteran prey. Spectrophotometer measurements suggest that the bright yellow bands on the spider's abdomen are visible to insect prey, but not the banding on the legs, which could disguise the spider's outline. Thus, our results provide strong support for the hypothesis that bright coloration in the wasp spider acts as a visual lure for insect prey and weak support for the hypothesis that the arrangement of the banding pattern across the spider's body disguises the presence of the spider on the web.

Nest site selection and induced response in a dominant arboreal ant species

It is well known that arboreal ants, both territorially dominant species and plant ants (e.g., species associated with myrmecophytes or plants housing them in hollow structures), protect their host trees from defoliators. Nevertheless, the presence of an induced defense, suggested by the fact that the workers discovering a leaf wound recruit nestmates, is only known for plant ants. Based on the results from a field study, we show here (1) that colonies of Azteca chartifex, a territorially dominant, neotropical arboreal ant species, mostly selected Goupia glabra (Goupiaceae) trees in which to build their principal carton nests and (2) that plant signals induced workers to recruit nestmates, which patrol the leaves, likely providing the plant with a biotic defense. Furthermore, the number of recruited workers was clearly higher on G. glabra, their most frequently selected host tree species, than on other tree species. These results show that contrary to what was previously believed, induced responses are also found in territorially dominant arboreal ants and so are not limited to the specific associations between myrmecophytes and plant ants.