Facilitated selfing offers reproductive assurance: a mutualism between a hemipteran and carnivorous plant

Reproductive assurance is frequently used to explain the evolution of selfing but has become controversial from lack of evidence. We studied the pollination system of the near carnivorous plant genus Roridula and showed that reproductive assurance is important in this system. Hemipterans have a digestive mutualism with Roridula and have been implicated in pollination but flowers show adaptations to hymenopteran pollination. We deduce that hemipterans are the primary pollinators of Roridula because seed set is significantly reduced when hemipterans are excluded from the flowers. Using allozyme electrophoresis, we show that hemipterans are responsible for mostly selfed progeny. Although bees still pollinate Roridula on very rare occasions, their exclusion does not affect seed set. The complicated floral structures that occur in Roridula most likely evolved as adaptations for bee pollination. Resident hemipterans facilitate selfing by Roridula, and this acts as a reproductive assurance mechanism because it increases seed production and ensures that plants still reproduce in the absence of more motile, outcrossing pollinators.     

Infiltration of a facultative ant–plant mutualism by the introduced Argentine ant: effects on mutualist diversity and mutualism benefits

1. Ant–plant mutualisms have been the focus of considerable empirical research, but few studies have investigated how introduced ants affect these interactions. Using 2 years of survey data, this study examines how the introduced Argentine ant [Linepithema humile (Mayr)] differs from native ants with respect to its ability to protect the extrafloral nectary‐bearing coast barrel cactus (Ferocactus viridescens) in Southern California. 2. Eighteen native ant species visited cacti in uninvaded areas, but cacti in invaded areas were primarily visited by the Argentine ant. The main herbivore of the coast barrel cactus present at the study sites is a leaf‐footed bug (Narnia wilsoni). 3. Herbivore presence (the fraction of surveys in which leaf‐footed bugs were present on individual cacti) was negatively related to ant presence (the fraction of surveys in which ants were present on individual cacti). Compared with cacti in uninvaded areas, those in invaded areas were less likely to have herbivores and when they did had them less often. 4. Seed mass was negatively related to herbivore presence, and this relationship did not differ for cacti in invaded areas versus those in uninvaded areas. 5. Although the Argentine ant might provide superior protection from herbivores, invasion‐induced reductions in ant mutualist diversity could potentially compromise plant reproduction. The cumulative number of ant species on individual cacti over time was lower in invaded areas and was associated with a shortened seasonal duration of ant protection and reduced seed mass. These results support the hypothesis that multiple partners may enhance mutualism benefits.     

A facultative mutualism between aphids and an invasive ant increases plant reproduction

1. The consequences to plants of ant–aphid mutualisms, particularly those involving invasive ants, are poorly studied. Ant–aphid mutualisms may increase or decrease plant fitness depending on the relative cost of herbivory by ant‐tended aphids versus the relative benefit of increased ant suppression of other (non‐aphid) herbivores. 2. We conducted field and greenhouse experiments in which we manipulated the presence and absence of cotton aphids (Aphis gossypii) on cotton plants to test the hypothesis that a mutualism between cotton aphids and an invasive ant, the red imported fire ant (Solenopsis invicta), benefits cotton plants by increasing fire ant suppression of caterpillars. We also manipulated caterpillar abundance to test whether the benefit of the mutualism varied with caterpillar density. 3. We found that more fire ants foraged on plants with cotton aphids than on plants without cotton aphids, which resulted in a significant reduction in caterpillar survival and caterpillar herbivory of leaves, flower buds, and bolls on plants with aphids. Consequently, cotton aphids indirectly increased cotton reproduction: plants with cotton aphids produced 16% more bolls, 25% more seeds, and 10% greater seedcotton mass than plants without aphids. The indirect benefit of cotton aphids, however, varied with caterpillar density: the number of bolls per plant at harvest was 32% greater on plants with aphids than on plants without aphids at high caterpillar density, versus just 3% greater at low caterpillar density. 4. Our results highlight the potential benefit to plants that host ant–hemipteran mutualisms and provide the first experimental evidence that the consequences to plants of an ant–aphid mutualism vary at different densities of non‐aphid herbivores.     

Flexibility of nutritional strategies within a mutualism: food availability affects algal symbiont productivity in two congeneric sea anemone species

Mutualistic symbioses are common, especially in nutrient-poor environments where an association between hosts and symbionts can allow the symbiotic partners to persist and collectively out-compete non-symbiotic species. Usually these mutualisms are built on an intimate transfer of energy and nutrients (e.g. carbon and nitrogen) between host and symbiont. However, resource availability is not consistent, and the benefit of the symbiotic association can depend on the availability of resources to mutualists. We manipulated the diets of two temperate sea anemone species in the genus Anthopleura in the field and recorded the responses of sea anemones and algal symbionts in the family Symbiodiniaceae to our treatments. Algal symbiont density, symbiont volume and photosynthetic efficiency of symbionts responded to changes in sea anemone diet, but the responses depended on the species of sea anemone. We suggest that temperate sea anemones and their symbionts can respond to changes in anemone diet, modifying the balance between heterotrophy and autotrophy in the symbiosis. Our data support the hypothesis that symbionts are upregulated or downregulated based on food availability, allowing for a flexible nutritional strategy based on external resources.     

Landscape genomics of an obligate mutualism: Concordant and discordant population structures between the leafcutter ant Atta texana and its two main fungal symbiont types

To explore landscape genomics at the range limit of an obligate mutualism, we use genotyping‐by‐sequencing (ddRADseq) to quantify population structure and the effect of host–symbiont interactions between the northernmost fungus‐farming leafcutter ant Atta texana and its two main types of cultivated fungus. Genome‐wide differentiation between ants associated with either of the two fungal types is of the same order of magnitude as differentiation associated with temperature and precipitation across the ant's entire range, suggesting that specific ant–fungus genome–genome combinations may have been favoured by selection. For the ant hosts, we found a broad cline of genetic structure across the range, and a reduction of genetic diversity along the axis of range expansion towards the range margin. This population‐genetic structure was concordant between the ants and one cultivar type (M‐fungi, concordant clines) but discordant for the other cultivar type (T‐fungi). Discordance in population‐genetic structures between ant hosts and a fungal symbiont is surprising because the ant farmers codisperse with their vertically transmitted fungal symbionts. Discordance implies that (a) the fungi disperse also through between‐nest horizontal transfer or other unknown mechanisms, and (b) genetic drift and gene flow can differ in magnitude between each partner and between different ant–fungus combinations. Together, these findings imply that variation in the strength of drift and gene flow experienced by each mutualistic partner affects adaptation to environmental stress at the range margin, and genome–genome interactions between host and symbiont influence adaptive genetic differentiation of the host during range evolution in this obligate mutualism.     

Assembling a mutualism: ant symbionts locate their host plants by detecting volatile chemicals

In most mutualisms, partners disperse independently of each other. For instance, in ant-plant symbioses, plants disperse as seeds, and ants disperse as winged queens. For an ant-plant mutualism to persist, therefore, queens must be able to locate and colonise host plant saplings. It has been suggested that host plants emit volatile chemical cues that attract dispersing queens, but this has never been demonstrated experimentally. We used a Y-tube olfactometry protocol to test this hypothesis in the tropical understorey antplant Cordia nodosa Lam. (Boraginaceae), which associates with two genera of ants, Azteca (Dolichoderinae) and Allomerus (Myrmicinae). Both genera show significant attraction to the volatiles of C. nodosa over control understorey plant species that do not associate with ants. These results support the hypothesis that ants are attracted to volatiles emitted by their host plant and suggest a key preadaptation that promoted the evolution of ant-plant symbioses. Received 1 July 2005; revised 2 November 2005; accepted 8 November 2005.     

Ant-plant-homopteran mutualism: how the third partner affects the interaction between a plant-specialist ant and its myrmecophyte host

By estimating relative costs and benefits, we explored the role of the homopteran partner in the protection mutualism between the myrmecophyte Leonardoxa africana T3, the ant Aphomomyrmex afer, and sap-sucking homopterans tended by ants in the tree''s swollen hollow twigs. The ants obtain nest sites and food from their host-plant (food is obtained either directly by extrafloral nectar or indirectly via homopterans). Aphomomyrmex workers patrol the young leaves of L. africana T3 and protect them against phytophagous insects. Because ants tended, either solely or primarily, coccids in some trees and pseudococcids in others, we were able to study whether the nature of the interaction was dependent on the identity of the third partner. First, the type of homopteran affects the benefits to the tree of maintaining a large ant colony. Larger colony size (relative to tree size) confers greater protection against herbivory; this relationship is more pronounced for trees whose ants tend pseudococcids than for those in which ants tend coccids. Second, for trees (and associated ant colonies) of comparable size, homopteran biomass was much larger in trees harbouring coccids than in trees with pseudococcids. Thus, the cost to the tree of maintaining ants may be greater when ants are associated with coccids. The net benefits to the plant of maintaining ants appear to be much greater with pseudococcids as the third partner. To explore how the type of homopteran affects functioning of the system, we attempted to determine which of the resources (nest sites, extrafloral nectar, and homopterans) is likely to limit ant colony size. In trees where ants tended coccids, ant-colony biomass was strongly dependent on the number of extrafloral nectaries. In contrast, in trees whose ants tended only pseudococcids, colony biomass was not related to the number of nectaries and was most strongly determined by the volume of available nest sites. We present hypotheses to explain how the type of homopteran affects functioning of this symbiosis, and discuss the implications of our study for the evolutionary ecology of ant–plant–homopteran relationships.     

Cheating on a mutualism: indirect benefits of ant attendance to a coccidophagous coccinellid

Coccinellids (Coleoptera: Coccinellidae) are generally unable to prey on ant-tended prey. However, particular coccinellid species have morphological, behavioral, or chemical characteristics that render them immune to ant attacks, and some species are even restricted to ant-tending areas. The benefit gained from living in close association with ants can be twofold: (1) gaining access to high-density prey areas and (2) gaining enemy-free space. Here, the myrmecophily of Azya orbigera Mulsant (Coleoptera: Coccinellidae), an important predator of the green coffee scale, Coccus viridis (Green) (Hemiptera: Coccidae), is reported. In this paper, three main questions were studied. (1) Are the waxy filaments of A. orbigera larvae effective as defense against attacks of the mutualistic ant partner of C. viridis, Azteca instabilis F. Smith (Hymenoptera: Formicidae)? (2) Does A. instabilis reduce the rate at which A. orbigera larvae prey on scales? (3) Do A. orbigera larvae gain enemy-free space by living in close association with A. instabilis? Laboratory and field experiments were conducted to answer these questions. We found that, because of the sticky waxy filaments of A. orbigera larvae, A. instabilis is incapable of effectively attacking them and, therefore, the predation rate of A. orbigera on C. viridis does not decrease in the presence of ants. Furthermore, A. instabilis showed aggressive behavior toward A. orbigera's parasitoids, and the presence of ants reduced the parasitism suffered by A. orbigera. This is the first time that this kind of indirect positive effect is reported for an ant and a coccidophagous coccinellid. Furthermore, this indirect positive effect may be key to the persistence of A. orbigera's populations.     

The Argentine ant persists through unfavorable winters via a mutualism facilitated by a native tree

Mutualisms and facilitations can fundamentally change the relationship between an organism's realized and fundamental niche. Invasive species may prove particularly suitable models for investigating this relationship as many are dependent on finding new partners for successful establishment. We conducted field-based experiments testing whether a native tree facilitates the successful survival of the invasive Argentine ant, Linepithema humile (Mayr), through unfavorable winter conditions in the southeastern United States. We found Argentine ant nests aggregated around the native loblolly pine, Pinus taeda L., during the winter months. The bark of this tree absorbed enough radiant solar energy to reach temperatures suitable for Argentine ant foraging even when ambient temperatures should have curtailed all foraging. Conversely, foraging ceased when the trunk was shaded. The sun-warmed bark of this tree gave the Argentine ant access to a stable honeydew resource. Argentine ants were not found on or near deciduous trees even though bark temperatures were warm enough to permit Argentine ant foraging on cold winter days. Augmenting deciduous trees with sucrose water through the winter months lead to Argentine ant nests remaining at their base and Argentine ants foraging on the tree. The Argentine ant requires both foraging opportunity and a reliable winter food source to survive through unfavorable winter conditions in the southeastern United States. The loblolly pine provided both of these requirements extending the realized niche of Argentine ants beyond its fundamental niche.     

Exploiting a mutualism: parasite specialization on cultivars within the fungus-growing ant symbiosis

Fungus-growing ants, their cultivated fungi and the cultivar-attacking parasite Escovopsis coevolve as a complex community. Higher-level phylogenetic congruence of the symbionts suggests specialized long-term associations of host-parasite clades but reveals little about parasite specificity at finer scales of species-species and genotype-genotype interactions. By coupling sequence and amplified fragment length polymorphism genotyping analyses with experimental evidence, we examine (i) the host specificity of Escovopsis strains infecting colonies of three closely related ant species in the genus Cyphomyrmex, and (ii) potential mechanisms constraining the Escovopsis host range. Incongruence of cultivar and ant relationships across the three focal Cyphomyrmex spp. allows us to test whether Escovopsis strains track their cultivar or the ant hosts. Phylogenetic analyses demonstrate that the Escovopsis phylogeny matches the cultivar phylogeny but not the ant phylogeny, indicating that the parasites are cultivar specific. Cross-infection experiments establish that ant gardens can be infected by parasite strains with which they are not typically associated in the field, but that infection is more likely when gardens are inoculated with their typical parasite strains. Thus, Escovopsis specialization is shaped by the parasite's ability to overcome only a narrow range of garden-specific defences, but specialization is probably additionally constrained by ecological factors, including the other symbionts (i.e. ants and their antibiotic-producing bacteria) within the coevolved fungus-growing ant symbiosis.     

A unique resource mutualism between the giant Bornean pitcher plant, Nepenthes rajah, and members of a small mammal community

The carnivorous pitcher plant genus Nepenthes grows in nutrient-deficient substrates and produce jug-shaped leaf organs (pitchers) that trap arthropods as a source of N and P. A number of Bornean Nepenthes demonstrate novel nutrient acquisition strategies. Notably, three giant montane species are engaged in a mutualistic association with the mountain treeshrew, Tupaia montana, in which the treeshrew defecates into the pitchers while visiting them to feed on nectar secretions on the pitchers' lids.Although the basis of this resource mutualism has been elucidated, many aspects are yet to be investigated. We sought to provide insights into the value of the mutualism to each participant. During initial observations we discovered that the summit rat, R. baluensis, also feeds on sugary exudates of N. rajah pitchers and defecates into them, and that this behavior appears to be habitual. The scope of the study was therefore expanded to assess to what degree N. rajah interacts with the small mammal community.We found that both T. montana and R. baluensis are engaged in a mutualistic interaction with N. rajah. T .montana visit pitchers more frequently than R. baluensis, but daily scat deposition rates within pitchers do not differ, suggesting that the mutualistic relationships are of a similar strength. This study is the first to demonstrate that a mutualism exists between a carnivorous plant species and multiple members of a small mammal community. Further, the newly discovered mutualism between R. baluensis and N. rajah represents only the second ever example of a multidirectional resource-based mutualism between a mammal and a carnivorous plant.     

Insect-microbe mutualism without vertical transmission: a stinkbug acquires a beneficial gut symbiont from the environment every generation

The broad-headed bug Riptortus clavatus (Heteroptera: Alydidae) possesses a number of crypts at a posterior midgut region, which house a dense population of a bacterial symbiont belonging to the genus Burkholderia. Although the symbiont is highly prevalent (95 to 100%) in the host populations, the symbiont phylogeny did not reflect the host systematics at all. In order to understand the mechanisms underlying the promiscuous host-symbiont relationship despite the specific and prevalent association, we investigated the transmission mode and the fitness effects of the Burkholderia symbiont in R. clavatus. Inspection of eggs and a series of rearing experiments revealed that the symbiont is not vertically transmitted but is environmentally acquired by nymphal insects. The Burkholderia symbiont was present in the soil of the insect habitat, and a culture strain of the symbiont was successfully isolated from the insect midgut. Rearing experiments by using sterilized soybean bottles demonstrated that the cultured symbiont is able to establish a normal and efficient infection in the host insect, and the symbiont infection significantly improves the host fitness. These results indicated that R. clavatus postnatally acquires symbiont of a beneficial nature from the environment every generation, uncovering a previously unknown pathway through which a highly specific insect-microbe association is maintained. We suggest that the stinkbug-Burkholderia relationship may be regarded as an insect analogue of the well-known symbioses between plants and soil-associated microbes, such as legume-Rhizobium and alder-Frankia relationships, and we discuss the evolutionary relevance of the mutualistic but promiscuous insect-microbe association.     

Parenchyma: a neglected plant tissue in the <Emphasis Type=Italic>Cecropia</Emphasis>/ant mutualism

The Cecropia/Azteca association is a well studied and perhaps the best known mutualistic system in the Neotropics. In this study we assessed the ultrastructure of the parenchymal tissue of the septum inside the internodes of two Cecropia species, Cecropia obtusifolia (a myrmecophytic species) and C. angustifolia (a non-myrmecophytic species), through the use of scanning electron microscopy. We found a series of large oval cavities in the parenchyma of C. obtusifolia, which were absent in the parenchyma of C. angustifolia, and which seemed to be spatially associated with vascular bundles. We also found two layers of fibers in C. obtusifolia, one on each side of the septum. Finally, the parenchymal cells of C. angustifolia were filled with several tiny oval bodies, which appeared to be plastids (perhaps serving as storage) and which were completely absent in C. obtusifolia. The structural differences between these two Cecropia species, as well as other evidence from the behavior of the ants, suggests that the parenchyma of myrmecophytic Cecropia species provides an additional source of nutrition for the Azteca queens during colonization of Cecropia saplings. Other possible uses of parenchymal tissue by the ant colony are also discussed.     

A new mite-plant association: mites living amidst the adhesive traps of a carnivorous plant

Non-antagonistic interactions between arthropods and leaves of insectivorous plants with adhesive traps so far have never been reported. The mites are common prey of such plants, but we have found a new subspecies of the mite Oribatula tibialis living on the leaves of Pinguicula longifolia. Because of its small size and the low glandular density of the host, the mite moves without being trapped by the mucilaginous droplets of the leaf surface. P. longifolia provides shelter and food for the mite, while the plant may also benefit because of its fungivorous and scavenging activities. This new interaction is another dramatic example of widespread miteplant associations.     

Experimental evidence of a tripartite mutualism: bacteria protect ant fungus gardens from specialized parasites

Symbioses shape all levels of biological organization. Although symbiotic interactions are typically viewed as bipartite associations, with two organisms interacting largely in isolation from other organisms, the presence and importance of additional symbionts is becoming increasingly more apparent. This study examines the importance of a third mutualist within the ancient symbiosis between leaf-cutting ants and their fungal cultivars. Specifically, we experimentally examine the role of a filamentous bacterium (actinomycete), which is typically carried on the cuticle of fungus-growing ants, in suppressing the growth of a specialized microfungal parasite ( Escovopsis ) of the fungus garden. We conducted two-by-two factorial design experiments crossing the presence/absence of actinomycete with the presence/absence of Escovopsis within small sub-colonies of Acromyrmex octospinosus . In these experiments, infection by Escovopsis became much more extensive within fungus gardens and had a greater impact on the health of gardens in those sub-colonies with the bacterium removed from workers as compared to gardens with the bacterium still present on the ants. We establish that the actinomycete bacterium is most abundant on those major workers tending the garden, providing further support that the bacterium is involved in garden hygiene. We also found a significantly higher abundance of actinomycete on workers in colonies experimentally infected with Escovopsis as compared to uninfected control colonies. We suggest that mutualisms between antibiotic-producing microbes and higher organisms may be common associations that are mostly overlooked and that the role of symbionts in reducing the impact of parasites is likely an important aspect in the cost-benefit assessment of mutualisms.     

Seed dispersal by hornets: An unusual insect‐plant mutualism

Vespicochory, seed dispersal by hornets, is a rare seed dispersal mechanism in angiosperms and, to date, there are few records of this phenomenon.Through field investigations and behavioral assays conducted in four populations of Stemona tuberosa from 2011–2016, we demonstrate that hornets are the primary seed dispersers of S. tuberosa and play an important role in long-distance seed dispersal in this species. Furthermore, some ant species act as secondary dispersers and may transport the seeds to safe sites.Hornets and ants provide complementary seed dispersal at different spatial scales. This unique example of insect-plant mutualism may be an underestimated but important strategy to ensure long-distance seed dispersal in other myrmecochorous plants.     

Control of plant development by limiting factors: A nutritional perspective

It is postulated that limiting nutritional factors play a major role in the regulation of some aspects of plant development, and can provide an alternative to mechanisms based on the concept of hormonal control. This hypothesis is consistent with experimental evidence of the role of water as a limiting factor in (1) seed maturation and viviparous germination, (2) the elongation and phototropism of hypocotyls and coleoptiles, (3) the NO₃⁻-induced germination of dormant seeds, and (4) the release of buds from correlative inhibition. Studies on the influence of nutrition on morphogenesis have shown that the relative amounts of nitrogen and carbohydrate can determine the path of bud development as a shoot or rhizome. There is also evidence that either NO₃⁻ or sugar can limit lateral root initiation, and it is postulated that they may influence this process by a combination of osmotic and nutritional effects. The close correlation between environmentally induced developmental responses and the associated changes in the water or nutritional status of the responsive tissues, together with increasing evidence of the role of water and nutrients as transmitted signals and as regulators of gene expression, are in good agreement with their postulated role as limiting factors in the regulation of plant development.     

The eroded genome of a Psychotria leaf symbiont: hypotheses about lifestyle and interactions with its plant host

Several plant species of the genus Psychotria (Rubiaceae) harbour Burkholderia sp. bacteria within specialized leaf nodules. The bacteria are transmitted vertically between plant generations and have not yet been cultured outside of their host. This symbiosis is also generally described as obligatory because plants devoid of symbionts fail to develop into mature individuals. We sequenced for the first time the genome of the symbiont of Psychotria kirkii in order to shed some light on the nature of their symbiotic relationship. We found that the 4?Mb genome of Candidatus Burkholderia kirkii (B.?kirkii) is small for a Burkholderia species and displays features consistent with ongoing genome erosion such as large proportions of pseudogenes and transposable elements. Reductive genome evolution affected a wide array of functional categories that may hinder the ability of the symbiont to be free-living. The genome does not encode functions commonly found in plant symbionts such as nitrogen fixation or plant hormone metabolism. Instead, a collection of genes for secondary metabolites' synthesis is located on the 140?kb plasmid of B.?kirkii and suggests that leaf nodule symbiosis benefits the host by providing protection against herbivores or pathogens.