Mutualistic Nature Research Articles

The network structure of plant–arbuscular mycorrhizal fungi

Ecological network theory predicts that in mutualistic systems specialists tend to interact with a subset of species with which generalists interact (i.e. nestedness). Approaching plant-arbuscular mycorrhizal fungi (AMF) association using network analyses will allow the generality of this pattern to be expanded to the ubiquitous plant-AMF mutualism. Based on certain plant-AMF specificity recently suggested, networks are expected to be nested as a result of their mutualistic nature, and modular, with certain species interacting more tightly than others. Network analyses were used to test for nestedness and modularity and to compare the different contribution of plant and AMF to the overall nestedness. Plant-AMF networks share general network properties with other mutualisms. Plant species with few AMFs in their roots tend to associate with those AMFs recorded in most plant species. AMFs present in a few plant species occur in plant species sheltering most AMF (i.e. nestedness). This plant-AMF network presents weakly interlinked subsets of species, strongly connected internally (i.e. modularity). Both plants and AMF show a nested structure, although AMFs have lower nestedness than plants. The plant-AMF interaction pattern is interpreted in the context of how plant-AMF associations can be underlying mechanisms shaping plant community assemblages.

Searching for Evidence against the Mutualistic Nature of Hereditary Symbioses: A Comment on Faeth

Abstract: Associations between fungal endophytes and grasses have served as particularly useful systems for exploring the nature and significance of hereditary symbiosis. Here, we propose alternative explanations for recent work by Faeth, in which endophytes were proposed to function as reproductive parasites. Faeth argued that his data demonstrated a symbiosis-induced shift to earlier host reproduction that could generate parasitism through life-history trade-offs with growth/survival. We contend that identifying a symbiosis as mutualistic or parasitic requires studies that incorporate both correlations among demographic pathways and rates of symbiont transmission; such work can advance understanding of the ecology and evolution of symbiosis.

Mycorrhizal acquisition of inorganic phosphorus by the green-leaved terrestrial orchid Goodyera repens.

Mycorrhizal fungi play a vital role in providing a carbon subsidy to support the germination and establishment of orchids from tiny seeds, but their roles in adult orchids have not been adequately characterized. Recent evidence that carbon is supplied by Goodyera repens to its fungal partner in return for nitrogen has established the mutualistic nature of the symbiosis in this orchid. In this paper the role of the fungus in the capture and transfer of inorganic phosphorus (P) to the orchid is unequivocally demonstrated for the first time. Mycorrhiza-mediated uptake of phosphorus in G. repens was investigated using spatially separated, two-dimensional agar-based microcosms. External mycelium growing from this green orchid is shown to be effective in assimilating and transporting the radiotracer (33)P orthophosphate into the plant. After 7 d of exposure, over 10 % of the P supplied was transported over a diffusion barrier by the fungus and to the plants, more than half of this to the shoots. Goodyera repens can obtain significant amounts of P from its mycorrhizal partner. These results provide further support for the view that mycorrhizal associations in some adult green orchids are mutualistic.

Unraveling the signaling and signal transduction mechanisms controlling arbuscular mycorrhiza development

Arbuscular mycorrhiza (AM) are the most widespread symbiotic associations between plant roots and soil fungi. AM can contribute to increasing the survival and fitness of plants to limiting environments mostly due to their ability in improving nutrient uptake from the soil solution. Despite their ecological significance, the mechanisms controlling AM development and functioning are largely unknown. The obligate mutualistic nature of the arbuscular mycorrhizal fungi (AMF) has hampered the advances on the understanding and application of the symbiosis. Significant alterations in the genetic programs of both symbionts are required for the successful establishment of an AM, and complex signaling and signal transduction mechanisms are likely involved. The analyses of legume mutants affected in the development of nitrogen fixing nodules and AM suggest that part of the signal transduction pathways involved in the regulation of both symbioses are conserved. Even though the use of genomics of model plants has helped to advance our understanding of the regulatory mechanisms in AM, identifying the signal molecules involved in plant-AMF communication and determining their transduction pathways is still essential for its biotechnological application in agriculture.

The ultimate effect of being cleaned: does ectoparasite removal have reproductive consequences for damselfish clients?

The mutualistic nature of cleaning symbioses has long remained unconfirmed because of the difficulty in showing net benefits for clients. We have previously shown that cleaning gobies (Elacatinus spp.) within territories of Caribbean longfin damselfish (Stegastes diencaeus) reduce the number of gnathiid isopod ectoparasites on territory owners. We now investigate whether this benefit of being cleaned has reproductive consequences for male longfin damselfish. The mating success, rate of egg loss, and parental aggression of 40 nest-guarding males were assessed during six consecutive monthly reproductive periods. Ten males had cleaning stations within their territory, 10 males were without cleaning stations, and 20 males initially with a cleaning station had their cleaners removed half-way through the study. Ectoparasite loads on our focal fish were very low; however, damselfish with cleaning stations still had significantly fewer ectoparasites than did fish without cleaning stations. There was, however, no significant difference in the number of eggs, clutches, or area of clutches received, or in the number of eggs lost before hatching between damselfish with and without cleaners. We also found no difference in parental male aggression between damselfish with and without cleaners. We conclude that although ectoparasite removal appears to have no direct consequence for reproduction, at least for the levels of infestations observed on our study site, it may still affect other aspects of damselfish fitness such as survival.

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

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

The persistence of vertically transmitted fungi in grass metapopulations.

Theory predicts that (i) vertical transmission of parasites (i.e. when they are passed directly from a host to its offspring) selects for benign association with the host and that (ii) vertically transmitted parasites that lower their hosts' fitness cannot persist if they are not able to infect horizontally (i.e. contagiously) other host individuals in the population. In this paper, we develop a mathematical model to examine whether mutualism is a prerequisite for persistence of exclusively vertically transmitted (from maternal plant to offspring via seeds) fungal endophytes in structured grass metapopulations. Interestingly, endophyte survival does not require plant mutualism, even in a metapopulation consisting of qualitatively identical patches, if vertical transmission of the fungus is perfect, i.e. if all established seedlings in offspring of the endophyte-infected plant are infected. In more realistic situations, when the metapopulation consists of qualitatively different patches, endophyte-infected plants may persist at the metapopulation level even if the vertical transmission is imperfect (due to hyphae inviability or failure to grow into all seeds) and the endophyte decreases the host grass fitness in certain environments. These results have biological importance because they (i) question the requirement of a mutualistic nature in exclusively vertically transmitted symbionts and (ii) emphasize the importance of habitat diversity in relation to symbiont success in vertical transmission.

How Plants Obtain Predatory Mites as Bodyguards

Phytophagous mites are a serious threat to their host plants; in absence of predators they tend to overexploit their food source. To prevent such a crash and maintain as much leaf area as possible host plants may defend themselves in various ways, one of which is to increase the effectiveness of natural enemies of the phytophagous mites. Predatory mites are considered to be very important natural enemies of plant-feeding mites and there is evidence for a mutualistic interaction with plants. Examples of how plants obtain and arrest predatory mites as bodyguards are discussed. It is known for a long time that some plant species provide pollen that appear to be a very profitable food source for some species of predatory mites: it does not only promote survival, but also allows development and egg production. In doing so, plants ensure themselves of bodyguards even before any damage is inflicted. Recently, evidence has been obtained that plants under attack by spider mites provide information by releasing a blend of volatile chemicals that are helpful to predatory mites in locating their prey. Plant-predator interactions are not always of a mutualistic nature. Some plant species invest in a rigorous defence against spider mites, even though this may be to the detriment of the predators: glandular hairs of some plant species entrap not only spider mites, but also their predators. The evolutionary implications of these various plant-predator interactions are discussed.

The Gutless Marine Tubificid Phallodrilus planus, a Flattened Oligochaete with Symbiotic Bacteria. Results from Morphological and Ecological Studies

The anatomy of Phallodrilus planus Erseus, 1979, a relatively large interstitial tubificid from shallow subtidal sands off Bermuda, is characterized by an unequal development of the muscular layer which results in a flattening of the body shape. Like its relative P. leukodermatus, it lacks completely the digestive tract and nephridia, but is regularly associated with a thick subcuticular layer of two different gram‐negative bacteria which are involved in sulphide oxidation. This is shown by ultrastructural, histochemical and physiological data and affects the biological and ecological situation of these aberrant oligochaetes. The mutualistic nature of this bacteria/animal association is discussed.