Plant species differ in their ability to reduce allocation to non-beneficial arbuscular mycorrhizal fungi

Theory suggests that cheaters threaten the persistence of mutualisms, but that sanctions to prevent cheating can stabilize mutualisms. In the arbuscular mycorrhizal symbiosis, reports of parasitism suggest that reductions in plant carbon allocation are not universally effective. I asked whether plant species differences in mycorrhizal responsiveness would affect both their susceptibility to parasitism and their reduction in allocation to non-beneficial arbuscular mycorrhizal fungi (AMF) in high-phosphorus soils. In a greenhouse experiment, I found that two C3 grasses, Bromus inermis and Elymus repens, effectively suppressed root colonization and AMF hyphal abundance. Increases in soil phosphorus did not reduce the degree to which AMF increased plant biomass. In contrast, two C4 grasses, Andropogon gerardii and Schizachyrium scoparium, more weakly reduced root colonization and failed to suppress AMF hyphal abundance. Consequently, they experienced strong declines in their response to AMF, and one species suffered parasitism. Thus, species differ in susceptibility to parasitism and their reduction in allocation to non-beneficial AMF. These differences may affect the distribution and abundance of plants and AMF, as well as the stability of the mutualism.     

Mycorrhizal benefit differs among the sexes in a gynodioecious species

Both plant sex and arbuscular mycorrhizal (AM) symbiosis influence resource acquisition and allocation in plants, but the interaction between these two components is not well established. As the different plant sexes differ in their resource needs and allocation patterns, it is logical to presume that they might differ in their relationship with AM as well. We investigate whether the association with AM symbiosis is different according to the host plant sex in the gynodioecious Geranium sylvaticum, of which, besides female and hermaphrodite plants, intermediate plants are also recognized. Specifically, we examine the effects of two different AM fungi in plant mass allocation and phosphorus acquisition using a factorial greenhouse/common garden experiment. Cloned G. sylvaticum material was grown in symbiosis with AM fungi or in non-mycorrhizal condition. We evaluated both the symbiotic plant benefit in terms of plant mass and plant P content and the fungal benefit in terms of AM colonization intensity in the plant roots and spore production. Our results suggest that G. sylvaticum plants benefit from the symbiosis with both AM fungal species tested but that the benefits gained from the symbiosis depend on the sex of the plant and on the trait investigated. Hermaphrodites suffered most from the lack of AM symbiosis as the proportion of flowering plants was dramatically reduced by the absence of AM fungi. However, females and intermediates benefited from the symbiosis relatively more than hermaphrodites in terms of higher P acquisition. The two AM fungal species differed in the amount of resources accumulated, and the fungal benefit was also dependent on the sex of the host plant. This study provides the first evidence of sex-specific benefits from mycorrhizal symbiosis in a gynodioecious plant species.     

Construction of chimaeric gardens through fungal intercropping: a symbiont choice experiment in the leafcutter ant Atta texana (Attini, Formicidae)

Interspecies or intraspecies cooperation can be stabilized evolutionarily if choosing partners favor beneficial partners and discriminate against non-beneficial partners. We quantified such partner choice (symbiont choice) in the leafcutter ant Atta texana (Attini, Formicidae) by presenting the ants in a cafeteria-style preference assay with genotypically distinct fungal cultivars from A. texana and Acromyrmex versicolor. Symbiont choice was measured as the ants' tendency to choose one or more cultivar(s) from several pure (axenic) cultivar fragments and convert a given fungal fragment into a garden. Microsatellite DNA fingerprinting enabled us to identify the cultivars chosen by the ants for their gardens. In 91% of the choice tests, A. texana workers combined multiple cultivars into a single intercropped, chimaeric garden, and the cultivars coexisted in such chimaeric gardens for as long as 4 months. Coexistence of distinct fungal genotypes in chimaeric gardens appears to contradict a recent model of cultivar competition postulating that each cultivar secretes incompatibility compounds harming other cultivars, which presumably would preclude the intercropped polyculture observed in our experiments. Although we found no clear evidence of novel, recombinant genotypes in the experimental chimaeric gardens, the intercropping of cultivar genotypes may occasionally lead under natural conditions to exchange of genetic material between coexisting cultivars, thus introducing novel cultivar genotypes into the leafcutter symbiosis. Symbiont choice by ants and any competition between coexisting cultivar strains in chimaeric gardens do not appear to operate fast enough in our laboratory assay to convert chimaeric gardens into the monocultures observed for A. texana under natural conditions.     

Distinct mycorrhizal communities on new and established hosts in a transitional tropical plant community

The extent to which interspecific plants share mycorrhizal fungal communities depends on the specificity of the symbiosis. For tropical forest tree seedlings, colonization by mycorrhizal fungi associated with established vegetation could have important consequences for survival and growth. I used a novel molecular technique to assess the potential for sharing of mycorrhizas in forest and pasture in southern Costa Rica, by identifying arbuscular mycorrhizal (AM) fungi in roots of the forest canopy tree species Terminalia amazonia, pasture grasses Urochloa ruziziensis and U. decumbens, and seedlings of T. amazonia planted into experimental reforestation plots. I tested the hypotheses that experimental seedlings were colonized either by the AM fungal community of the forest T. amazonia (suggesting host specificity) or of Urochloa (suggesting absence of specificity/importance of local environment). After two years, pasture-grown T. amazonia seedlings were colonized by neither community, but rather by a species of Glomus that was rarely observed on the other plants. These results suggest that conspecific seedlings planted into existing vegetation generate a distinct mycorrhizal community that may influence competitive interactions and the relative costs and benefits of the AM fungal symbiosis at early stages in the life cycle of tropical trees.     

The economics of mutualisms: optimal utilization of mycorrhizal mutualistic partners by plants

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

The influence of symbiont type on photosynthetic carbon flux in a model cnidarian–dinoflagellate symbiosis

We measured the relationship between symbiont diversity, nutritional potential, and symbiotic success in the cnidarian–dinoflagellate symbiosis, by infecting aposymbiotic (i.e. symbiont-free) specimens of the model sea anemone Aiptasia sp. with a range of Symbiodinium types. Four cultured heterologous Symbiodinium types (i.e. originally isolated from other host species) were used, plus both cultured and freshly isolated homologous zooxanthellae (i.e. from Aiptasia sp.). Rates of photosynthesis, respiration, and symbiont growth were measured during symbiosis establishment and used to estimate the contribution of the zooxanthellae to the animal’s respiratory carbon demands (CZAR). Anemones containing Symbiodinium B1 (both homologous and heterologous) tended to attain higher CZAR values and hence benefit most from their symbiotic partners. This was despite Symbiodinium B1 not achieving the highest cell densities, though it did grow more quickly during the earliest stages of the infection process. Rather, the heterologous Symbiodinium types A1.4, E2, and F5.1 attained the highest densities, with populations of E2 and F5.1 also exhibiting the highest photosynthetic rates. This apparent success was countered, however, by very high rates of symbiosis respiration that ultimately resulted in lower CZAR values. This study highlights the impact of symbiont type on the functionality and autotrophic potential of the symbiosis. Most interestingly, it suggests that certain heterologous symbionts may behave opportunistically, proliferating rapidly but in a manner that is energetically costly to the host. Such negative host–symbiont interactions may contribute to the host–symbiont specificity seen in cnidarian–dinoflagellate symbioses and potentially limit the potential for partner switching as an adaptive mechanism.     

Ecological implications of anti-pathogen effects of tropical fungal endophytes and mycorrhizae

We discuss studies of foliar endophytic fungi (FEF) and arbuscular mycorrhizal fungi (AMF) associated with Theobroma cacao in Panama. Direct, experimentally controlled comparisons of endophyte free (E-) and endophyte containing (E+) plant tissues in T. cacao show that foliar endophytes (FEF) that commonly occur in healthy host leaves enhance host defenses against foliar damage due to the pathogen (Phytophthora palmivora). Similarly, root inoculations with commonly occurring AMF also reduce foliar damage due to the same pathogen. These results suggest that endophytic fungi can play a potentially important mutualistic role by augmenting host defensive responses against pathogens. There are two broad classes of potential mechanisms by which endophytes could contribute to host protection: (1) inducing or increasing the expression of intrinsic host defense mechanisms and (2) providing additional sources of defense, extrinsic to those of the host (e.g., endophyte-based chemical antibiosis). The degree to which either of these mechanisms predominates holds distinct consequences for the evolutionary ecology of host-endophyte-pathogen relationships. More generally, the growing recognition that plants are composed of a mosaic of plant and fungal tissues holds a series of implications for the study of plant defense, physiology, and genetics.     

丛枝菌根真菌对羊草凋落物降解作用的研究

丛枝菌根真菌（Abuscular mycorrhizal fungi）能够影响植物生长及养分含量,从而影响凋落物的降解。采用根袋的方法研究了接种两种丛枝真菌Glomus mosseae和Glomus claroideum对羊草（Leymus chinensis）地上部及根系凋落物降解的影响。结果表明,随时间的延长,凋落物的重量逐渐减少,凋落物氮、磷含量均表现出先下降后逐渐升高的趋势。接种对地上部凋落物的养分含量及降解速度未产生显著性影响,但显著降低了根系氮磷含量及降解系数。接种Glomus mosseae和Glomus claroideum羊草根系氮、磷含量均显著低于CK;接种与未接种相比羊草根系k值显著降低;根系C：N未接种处理显著低于接种处理。说明丛枝菌根真菌可能间接影响草原生态系统中有机物质的分解和养分释放。     

Studies on the influence of mycorrhiza on the growth and physiology of Vigna unguiculata and Abelmoschus esculentus

Diverse forms of microorganisms present in the soil and near the roots of plants, which play a vital role in numerous physiological processes, have attracted the attention of scientists. The dynamic microbial associations may be saprophytic, pathogenic, or symbiotic. The most widespread symbiosis of plants is the mycorrhizal association between root-inhabiting fungi and the feeder roots of plants. The present study was conducted to study the effects of arbuscular mycorrhizal fungi on mineral nutrition of Vigna unguiculata and Abelmoschus esculentus. The experiment comprised of uninoculated seedlings and seedlings inoculated with Glomus mosseae. The chlorophyll a, chlorophyll b, total chlorophyll, carotenoid, protein, nitrate, nitrogen, and phosphorus content showed an increase in vesicular arbuscular mycorrhiza fungus-treated seedlings compared to non-mycorrhizal plants. The total soluble sugars and soluble starch content in leaves of all selected plant species in the present study showed a decrease in mycorrhizal seedlings compared to non-mycorrhizal seedlings.     

White locoweed toxicity is facilitated by a fungal endophyte and nitrogen-fixing bacteria

Mutualistic interactions with fungal endophytes and dinitrogen-fixing bacteria are known to exert key biological influences on the host plant. The influence of a fungal endophyte alkaloid on the toxicity of a plant has been documented in Oxytropis sericea. Oxytropis sericea is a perennial legume responsible for livestock poisoning in western North America. Livestock poisoning is attributed to the alkaloid swainsonine, which is synthesized inside the plant by the fungal endophyte Embellisia sp. In this study, the ability of Oxytropis sericea to form a dinitrogen-fixing symbiosis with Rhizobium and the effects of this symbiosis on the production of swainsonine by Embellisia sp. were evaluated in a greenhouse environment. Seeds of O. sericea were grown in plastic containers. Twenty-week-old O. sericea seedlings were inoculated with four strains of Rhizobium. Twenty weeks after inoculation, plant growth and root nodulation by Rhizobium were measured. Dinitrogen fixation was confirmed using an acetylene reduction assay (ARA) on excised root nodules. Dry leaves were analyzed for swainsonine content. A second set of plants was treated with fungicide to evaluate the effect of reduced fungal endophyte infection on plant growth and swainsonine production. All inoculated plants produced indeterminate nodules. The ARA indicated that 98% of the excised nodules were fixing dinitrogen. Rhizobium-treated plants had greater swainsonine concentrations than the non-inoculated controls. Plants that established from seeds treated with fungicide had lower biomass than non-fungicide-treated controls and plants treated with foliar fungicide. Plants treated with foliar fungicide and the controls had greater swainsonine concentrations than the plants that received seed fungicide. This greenhouse study is the first report of nodulation and dinitrogen fixation in O. sericea. It also demonstrates that dinitrogen fixation increases the production of swainsonine in O. sericea plants infected with Embellisia sp. Results from this study suggest that dinitrogen fixation affects swainsonine production and has the potential to support the symbiosis between Embellisia sp. and O. sericea when soil nitrogen is limited. Oxytropis sericea competitiveness appears to be facilitated by an ability to simultaneously associate with Rhizobium and a fungal symbiont.     

There are many ways to be a mutualist: endophytic fungus reduces plant survival but increases population growth

One of the challenges to quantifying the costs and benefits of symbiosis is that symbionts can influence different components of host fitness. To improve understanding of the ecology of inherited symbionts, we developed general theory for a perennial host-hereditary symbiont interaction, in which symbionts can have independent and potentially opposing effects on host regeneration and survival. The model showed that negative effects on one component of fitness may be outweighed by positive effects on another, leading to a net positive impact of symbiosis on population growth. Model predictions depended on the availability of suitable patches, which influenced the relative contributions of survival vs. regeneration to host fitness. We then used experimental symbiont removal to quantify effects of a hereditary, fungal endophyte on a grass host. Endophyte presence strongly reduced host survival but increased regeneration. Application of the model revealed that negative effects on plant survival were overwhelmed by beneficial effects on regeneration, resulting in stable endophyte persistence at 100% frequency, consistent with field observations. Our work demonstrates the utility of a demographic perspective for predicting the dynamics of symbioses and supports the hypothesis that symbionts function as mutualists when host and symbiont fitness are coupled through vertical transmission.     

Disrupting mycorrhizal mutualisms: a potential mechanism by which exotic tamarisk outcompetes native cottonwoods

The disruption of mutualisms between plants and mycorrhizal fungi is a potentially powerful mechanism by which invasives can negatively impact native species, yet our understanding of this mechanism's role in exotic species invasion is still in its infancy. Here, we provide several lines of evidence indicating that invasive tamarisk (Tamarix sp.) negatively affects native cottonwoods (Populus fremontii) by disrupting their associations with arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi. At a field site in the early stages of tamarisk invasion, cottonwoods with tamarisk neighbors had reduced EM colonization and altered EM fungal community composition relative to cottonwoods with native neighbors, leading to reductions in EM propagule abundance in the soil beneath tamarisk. Similarly, AM colonization of cottonwoods was reduced with a tamarisk neighbor, but there were no significant changes in AM fungal spore communities or propagule abundance. Root colonization by nonmycorrhizal fungi, including potential pathogens, was higher in cottonwoods with tamarisk neighbors. A greenhouse experiment in which AM and EM inoculation and plant neighbor were manipulated in a fully factorial design showed that cottonwoods benefited from mycorrhizas, especially EM, in terms of shoot biomass when grown with a conspecific, but shoot biomass was similar to that of nonmycorrhizal controls when cottonwoods were grown with a tamarisk neighbor. These results are partially explained by a reduction in EM but not AM colonization of cottonwoods by a tamarisk neighbor. Tamarisk neighbors negatively affected cottonwood specific leaf area, but not chlorophyll content, in the field. To pinpoint a mechanism for these changes, we measured soil chemistry in the field and the growth response of an EM fungus (Hebeloma crustuliniforme) to salt-amended media in the laboratory. Tamarisk increased both NO3- concentrations and electrical conductivity 2.5-fold beneath neighboring cottonwoods in the field. Salt-amended media did not affect the growth of H. crustuliniforme. Our findings demonstrate that a nonnative species, even in the early stages of invasion, can negatively affect a native species by disrupting its mycorrhizal symbioses. Some of these changes in mycorrhizal fungal communities may remain as legacy effects of invasives, even after their removal, and should be considered in management and restoration efforts.     

Floral scents: their roles in nursery pollination mutualisms

Mutualisms are interspecies interactions in which each participant gains net benefits from interacting with its partner. In nursery pollination mutualisms, pollinators reproduce within the inflorescence they pollinate. In these systems, each partner depends directly on the other for its reproduction. Therefore, the signal responsible for partner encounter is crucial in these horizontally transmitted mutualisms, in which the association between specific partners must be renewed at each generation. As in many other interspecies interactions, chemical signals are suspected to be important in the functioning of these mutualisms. We synthesized and compared the published data available on the role of floral scents in the functioning of the 16 known independently evolved nursery pollination mutualisms. So far, attraction of pollinators to their specific hosts has been investigated in only seven of these systems, and the majority of the studies have been conducted on one of them, fig/fig wasp interactions. While such unevenness of the information limits the potential for meta-analysis, some patterns emerge from this review concerning the role of flower volatiles in maintaining the specificity of pollinator attraction, in signaling the appropriate phenological stage for pollinator visit, in attracting the pollinator toward the rewardless sex in dioecious plant species and in aiding the location and exploitation of resources by parasites and predators associated with these mutualisms. Finally, we highlight new perspectives on the evolution of signals in these diversified systems depending on the age and the degree of specificity of the interaction, and on the effect of phylogenetic inertia on the evolutionary dynamics of plant signals.     

A multi-mutualist simulation: Applying biological market models to diverse mycorrhizal communities

We present a cellular automaton that simulates the interaction between a host tree and multiple potential mycorrhizal symbionts and generates testable hypotheses of how processes at the scale of individual root tips may explain mycorrhizal community composition. Existing theoretical biological market models imply that a single host is able to interact with and select from multiple symbionts to organize an optimal symbiont community. When evaluating the tree–symbiont interaction, two scales must be considered simultaneously: the scale of the entire host plant at which carbon utilization and nutrient demands operate, and the scale of the individual root tip, at which colonization and carbon-nutrient trade occurs. Three strategies that may be employed by the host tree for optimizing carbon use and nutrient acquisition through mycorrhizal symbiont communities are simulated: (1) carbon pool adjustment, in which the plant controls only the total amount of carbon to be distributed uniformly throughout the root system, (2) symbiont selection, wherein the plant opts either for or against the interaction at each fine root tip, and (3) selective carbon allocation, wherein the plant adjusts the amount of carbon allocated to each root tip based on the cost of nutrients. Strategies were tested over various nutrient availabilities (the amount of inorganically and organically bound nutrients). Success was defined on the basis of minimizing carbon expended for nutrient acquisition because this would allow more carbon to be utilized for growth and reproduction. In all cases, the symbiont selection and selective carbon allocation strategies were able to meet the nutritional requirements of the plant, but did not necessarily optimize carbon use. The carbon pool adjustment strategy is the only strategy that does not operate at the individual root tip scale, and the strategy was not successful when inorganic nutrients were scarce since there is no mechanism to exclude suboptimal symbionts. The combination of the symbiont selection strategy and the carbon pool adjustment resulted in optimal carbon use and nutrient acquisition under all environmental conditions but result in monospecific symbiont assemblages. On the other hand, the selective carbon allocation strategy is the only strategy that maintained successful, multi-symbiont communities. The simulations presented here thus imply clear hypotheses about the effect of nutrient availability on symbiont selection and mycorrhizal community richness and composition.     

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

Sensory traps are signal mimics that elicit out-of-context behaviors by exploiting the adaptive, neural responses of signal receivers. Sensory traps have long been invoked in studies of mate and prey attraction, but the possible roles of sensory traps in mutualisms (cooperation between species) have yet to be thoroughly examined. Our review identifies four candidate roles for sensory traps in the evolution of mutualistic interactions: reassembly, error reduction, enforcement, and cost reduction. A key consequence of sensory traps is that they limit the applicability of partner choice and biological market models of mutualism. We conclude by suggesting that an important research topic in the evolution of cooperation should be to identify any mechanisms that increase the truthfulness of communication between cooperating species.     

Biological markets: supply and demand determine the effect of partner choice in cooperation,mutualism and mating

The formation of collaborating pairs by individuals belonging to two different classes occurs in the contexts of reproduction and intea-specific cooperation as well as of inter-specific mutualism. There is potential for partner choice and for competition for access to preferred partners in all three contexts. These selective forces have long been recognised as important in sexual selection, but their impact is not yet appreciated in cooperative and mutualistic systems. The formation of partnerships between members of different classes has much in common with the conclusion of trade agreements in human markets with two classes of traders, like producers and consumers, or employers and employees. Similar game-theoretical models can be used to predict the behaviour of rational traders in human markets and the evolutionarily stable strategies used in biological markets. We present a formal model in which the influence of the market mechanism on selection is made explicit. We restrict ourselves to biological markets in which: (1) Individuals do not compete over access to partners in an agonistic manner, but rather by outcompeting each other in those aspects that are preferred by the choosing party. (2) The commodity the partner has to offer cannot be obtained by the use of force, but requires the consent of the partner. These two restrictions ensure a dominant role for partner choice in the formation of partnerships. In a biological market model the decision to cooperate is based on the comparison between the offers of several potential partners, rather than on the behaviour of a single potential partner, as is implicitly assumed in currently accepted models of cooperation. In our example the members of one class A offer a commodity of fixed value in exchange for a commodity of variable value supplied by the other class, B. We show that when the B-class outnumbers the A-class sufficiently and the cost for the A-class to sample the offers of the B-class are low, the choosiness of the A-class will lead to selection for the supply of high value commodities by the B-class (Fig. 3a). Under the same market conditions, but with a high sampling cost this may still be the evolutionariy stable outcome, but another pair of strategies proves to be stable too: relaxed choosiness of class A coupled with low value commodities supplied by class B (Fig. 3b). We give a number of examples of mating, cooperative and mutualistic markets that resemble the low sampling cost situation depicted in Fig. 3a.     

Orchid-fungus fidelity: a marriage meant to last?

The characteristics of plant-mycorrhizae associations are known to vary in both time and space, but the ecological consequences of variation in the dynamics of plant-fungus interactions are poorly understood. For example, do plants associate with single fungi or multiple fungi simultaneously, and do the associations persist through a plant's lifetime or do plants support a succession of different fungi? We investigated these and other questions related to plant-fungus interactions in Goodyera pubescens, an evergreen terrestrial orchid of the eastern United States, that interacts with closely related fungi in the genus Tulasnella. Unlike the mycorrhizal associations of other plants, orchid-mycorrhizal associations only benefit the orchid, based on current evidence. Many terrestrial orchids have been found to associate with specific groups of fungi. This characteristic could potentially limit orchids to relatively narrow ranges of environmental conditions and may be a contributing factor in the decline of many orchids in the face of changing environmental conditions. We found that G. pubescens protocorms (developing embryos prior to leaf production) and adults associated with only one fungal individual at a time. The orchid-fungus association persists for years, but during a drought period that was associated with the death of many plants, surviving plants were able to switch to new fungal individuals. These results suggest that G. pubescens interacts with the same fungal partner during periods of modest environmental variation but is able to switch to a different fungal partner. We hypothesize that the ability to switch fungi allows G. pubescens to survive more extreme environmental perturbations. However, laboratory experiments suggest that switching fungi has potential costs, as it increases the risk of mortality, especially for smaller individuals. Our findings indicate that it is unlikely that switching fungi is a common way to improve tolerance of less severe environmental fluctuations and disturbances. These findings may have important implications for plant responses to severe climatic events or to more gradual environmental changes such as global warming.     

丛枝菌根对土壤-植物系统中重金属迁移转化的影响

丛枝菌根真菌(arbuscular mycorrhizal fungi, AMF)是一类在自然和农业生态系统中广泛存在并能与多数陆生植物形成共生关系的土壤真菌,在重金属污染土壤中对宿主植物的生长及吸收累积重金属具有重要影响,因而对污染土壤的生物修复具有潜在应用价值。以重金属从根际土壤进入植物并在植物体内再分配过程为主线,介绍丛枝菌根在这一过程中对重金属环境行为,特别是根际土壤中重金属赋存形态及植物吸收重金属的影响。最后,对丛枝菌根影响植物重金属耐性机制研究前沿和菌根修复技术的应用前景进行展望。     

The influence of social experience on cooperative behaviour of rats (Rattus norvegicus): direct vs generalised reciprocity

Cooperation among non-kin has been attributed sometimes to reciprocal altruism: Two or more individuals exchange behaviour that benefits the respective partner. According to direct reciprocity, cooperation is based on past behaviour of a known partner. In contrast, in generalised reciprocity, cooperation is based on anonymous social experience where the identity of the partner is irrelevant. In a previous study, female Norway rats (Rattus norvegicus) were found to cooperate according to a generalised reciprocity mechanism. In this study, we tested whether Norway rats would also cooperate as predicted by a direct reciprocity mechanism and whether direct reciprocity would cause a higher propensity to cooperate than generalised reciprocity. Focal animals were experimentally manipulated to receive social experience from known or unknown, helpful or defecting partners in an instrumental cooperative task. Our first experiment shows that rats are more helpful towards a partner from which they had received help before than towards a partner that had not helped (i.e. direct reciprocity). Our second experiment revealed that after receiving help by others, rats were more helpful towards a partner from which they had received help before than towards a new partner (i.e. direct reciprocity generated a higher cooperation propensity than generalised reciprocity). We conclude that in female Norway rats, the tendency to cooperate is influenced by partner-specific information. To our knowledge, this is the first study to demonstrate direct reciprocity in rodents, and it is the first study testing direct vs generalised reciprocity in animals.     

The mutualism-parasitism continuum in ectomycorrhizas: a quantitative assessment using meta-analysis

Context dependency is deemed to position the outcomes of species interactions along a continuum of mutualism to parasitism. Thus, it is imperative to understand which factors determine where a particular interspecific interaction falls along the continuum. Over the past 20 years research on the ectomycorrhizal symbiosis has resulted in sufficient independent studies to now generalize about the factors and mechanisms that affect host response to ectomycorrhizas. Using meta-analysis we quantitatively evaluated the role of biotic (partner identity and colonization levels of ectomycorrhizal fungi) and abiotic (phosphorus levels) factors in determining host biomass, height, and shoot:root responses to ectomycorrhizal associations. On average, seedlings across multiple host genera increased in total biomass when inoculated with ectomycorrhizal fungi regardless of the identity of the fungal associate; host genera differed in the magnitude of response for both total biomass and shoot:root ratio. Association with different fungal genera modified only host allocation of biomass to shoots and roots. Neither level of colonization on inoculated seedlings nor the level of contamination on control seedlings relative to colonization levels by target fungi on inoculated seedlings was important in explaining variation in effect sizes for any growth response. None of our proposed factors (identity of partners, colonization level, magnitude of contamination, or duration of association) explained variation in effect sizes for shoot height, although in general seedlings were taller when inoculated with ectomycorrhizal fungi. Phosphorus additions did not influence effect sizes. Although the general trend across studies was for a positive response of hosts to ectomycorrhizal inoculation, publication bias and methodological issues effectively reduce and distort the spectrum on which we evaluate host responses to ectomycorrhizal inoculation. Our results indicate that the variation in ectomycorrhizal fungi perceived by the host may be of a discrete (presence/absence of ectomycorrhizal fungi) rather than continuous nature (variation in identity or abundance of ectomycorrhizal fungi).