Traditionally, rust fungi are understood to disperse primarily through wind or water, relying on the production of vast quantities of small, lightweight urediniospores adapted for aerial transport. However, recent field observations have challenged this singular view of abiotic dispersal. Researchers have documented Western honey bees, Apis mellifera, actively collecting urediniospores of A. psidii from infected host plants. These foragers store the spores in their pollen baskets and return them to the hive in a manner that is visually and behaviorally indistinguishable from standard pollen foraging. This behavior suggests a potential novel mutualistic interaction between two alien species, where the fungus gains a new mode of dispersal and the bee may obtain an alternative source of nutrition.

To validate this hypothesis, researchers investigated two critical conditions required for such a mutualism to function: whether the spores remain viable after entering the hive environment and whether they provide a nutritional benefit to the pollinator. The study focused on the persistence of A. psidii urediniospores within honey bee colonies and their nutritional composition. By testing these factors, the team aimed to determine if spore foraging is a viable strategy for honey bees rather than an aberration, and if this interaction could facilitate the long-distance dispersal of the pathogen.

The results indicated that A. psidii spores remain viable for at least nine days within honey bee colonies. In controlled trials, spores placed in hives maintained germination rates comparable to initial levels, showing no significant decline in viability over the nine-day period. This persistence is particularly concerning given that commercial hives are frequently transported over long distances for agricultural pollination services, typically within a three-to-seven-day window. The survival of viable spores during this timeframe suggests that managed hives could act as a significant vector for the pathogen, a pathway not currently incorporated into existing biosecurity frameworks.

Nutritional analysis revealed that A. psidii urediniospores match or exceed the nutritional quality of known high-protein pollens. The spores contained 22.5% protein, which is comparable to willow pollen and significantly higher than other fungal spores tested. Furthermore, the spores contained all ten essential amino acids required for honey bee development in amounts above the minimum thresholds for full protein absorption. These findings suggest that the spores are not merely contaminants but a potentially valuable dietary resource for the bees, especially in environments where floral resources are scarce due to drought, habitat loss, or the rust infection itself.

Artificial rearing trials further demonstrated that A. psidii spores support normal development and survival in honey bee brood. Larvae fed diets containing the rust spores developed at rates and achieved weights similar to those fed high-quality pollen sources. While survival rates were slightly lower than those on kiwifruit pollen, they remained well above the minimum thresholds for colony health. In contrast, larvae fed spores from a different rust species, Melampsora sp., showed reduced survival and lower body weight, indicating that not all fungal spores are equal in their nutritional value to bees. The specific amino acid balance in A. psidii spores appears to be key to their suitability as a food source.

This interaction exemplifies an “invasional mutualism,” a phenomenon where introduced species facilitate each other’s success, often exacerbating the speed and scale of invasion. In this case, the rust gains a new mode of urediniospore dispersal, while the bee obtains an alternative source of protein. This dynamic could create a positive ecological feedback loop: as myrtle rust reduces floral resources, bees may increasingly turn to fungal spores for nutrition, thereby enhancing the pathogen’s spread and further depleting the floral resources that support pollinator networks. Such a cycle could destabilize plant-pollinator interactions and forest regeneration, particularly in regions with high Myrtaceae endemism.

The implications for biosecurity are significant. Current national and regional strategies, such as the Australian Myrtle Rust Action Plan and New Zealand surveillance plans, do not yet include the potential of apiculture in the spread of plant pathogens. Recognizing pollinators as potential vectors is essential for developing more effective containment strategies. Integrating pollinator movement into management plans could help limit the geographic expansion of A. psidii and protect biodiversity in vulnerable areas. Future research should experimentally test the final condition for mutualism: successful infection of a new host following pollinator-mediated dispersal. Quantifying the relative contribution of insect-mediated versus windborne dispersal across different landscapes and seasons will be vital for forecasting future spread and designing evidence-based interventions.

Subject of Research: Honey bees as potential vectors of the invasive rust pathogen Austropuccinia psidii: nutritional mutualism and implications for pathogen spread

Article Title: Honey bees as potential vectors of the invasive rust pathogen Austropuccinia psidii : nutritional mutualism and implications for pathogen spread

Article References: Shin-Clayton, S., Mortensen, A. N., Beggs, J. R., Buxton, M. N., Hauxwell, C., Bateson, M. F., Jochym, M., Pegg, G. S., & Pattemore, D. E. (2026). Honey bees as potential vectors of the invasive rust pathogen Austropuccinia psidii: nutritional mutualism and implications for pathogen spread. NeoBiota, 106, 75-90. https://doi.org/10.3897/neobiota.106.169027

Image Credits: AI Generated

DOI: 10.3897/neobiota.106.169027

Keywords: Honey, bees, potential, vectors, invasive, rust, pathogen, Austropuccinia, psidii, nutritional, mutualism, implications

The research team, working with incipient colonies collected at the Federal University of São Carlos and experimented on at the Luiz de Queiroz College of Agriculture in Piracicaba, São Paulo, set out to quantify a behaviour that is easy to observe but hard to weigh. They housed miniaturised colonies in plaster-lined Petri dishes, each containing half a gram of symbiotic fungal garden, one larva, one pupa, and ten medium-sized ants, and let them acclimate for 48 hours. Three treatments followed: a Tween 80 control, a suspension of the entomopathogenic fungus Metarhizium anisopliae, and a suspension of Escovopsis phialicopiosa, the notorious garden parasite. Suspensions were adjusted to 10^8 conidia per millilitre and applied with an airbrush spray. Cameras recorded four hours of behaviour per replicate at 30-minute intervals, with five-minute focal sampling analysed in BORIS software, allowing the researchers to measure, in seconds, how much time workers spent on self-grooming versus allogrooming.

The behavioural data revealed a striking asymmetry. Under the control treatment, maximum allogrooming reached 1701 seconds, with a mean of 330 seconds. When Metarhizium anisopliae was present, those figures collapsed to a maximum of 169 seconds and a mean of just 54 seconds, suggesting that workers facing a virulent pathogen pull back from social contact, likely to avoid horizontal spore transmission between nestmates. Escovopsis phialicopiosa, by contrast, produced allogrooming values close to the control, with a maximum of 1639 seconds and a mean of 82 seconds. Self-grooming, meanwhile, remained relatively stable across treatments, with means hovering between 12 and 14.4 seconds. The largest difference among treatments appeared between the maximum and mean allogrooming values, hinting that social hygiene is the more behaviourally plastic of the two defences.

To translate these seconds into colony-scale consequences, the team built on a mathematical framework originally developed by Kang and colleagues in 2011 to describe biomass dynamics in Acromyrmex ants, a genus that typically cultivates its fungus in a single chamber. Two coupled differential equations track ant biomass and fungal biomass over time: ant growth depends on fungal biomass consumed through a Holling Type I functional response, while fungal growth depends on the labour ants allocate to harvesting and processing leaves, modelled with a Type III numerical response and a half-saturation constant. Mortality rates for both partners, plus a conversion rate of fungus into ant biomass, close the loop. The new study extended this single-chamber model into a two-chamber system, because Atta sexdens cultivates its symbiotic fungus across multiple underground chambers rather than one.

The extension introduced two crucial innovations. First, grooming was written directly into the mortality terms: pathogen-induced mortality for ants and fungus is scaled by the factor one minus the grooming time divided by its maximum, so the longer workers spend grooming, the lower the pathogen’s lethal intensity. The same structure was applied to allogrooming by substituting the allogrooming time. Second, a one-way transfer rule was added between chambers: when fungal biomass in chamber A exceeds its carrying capacity, a density-dependent flux moves ants and fungus to chamber B, mimicking how real Atta workers relocate brood and fungal biomass to newly excavated chambers as the colony expands. This directional flow reflects the colony’s developmental trajectory, in which older chambers act as sources of biomass for new compartments, and it embodies the organisational immunity that compartmentalised nests provide by limiting pathogen dissemination.

Simulations solved with the deSolve package in R, using the lsoda numerical method, produced sigmoidal growth curves for both partners, consistent with classical models of obligate mutualism. When the one-way transfer parameter was set to 0.1 per day, biomass decreased in the source chamber while increasing in the receiving chamber, and inflection points in the growth curves, identified from the second derivative of the biomass trajectories, were delayed. Those delayed inflection points mark a critical growth shift: the moment when accelerating growth gives way to deceleration. Under pathogen pressure, the timing of these shifts became diagnostic. In the Metarhizium treatment, ant and fungal biomasses dropped significantly compared with the control, and inflection points moved to higher values, producing a time-delayed effect. In the Escovopsis treatment, curves closely resembled the control, with only minor shifts in inflection values.

The most sobering result concerned Metarhizium anisopliae’s resilience to grooming. Even when the model was fed the maximum measured self-grooming and allogrooming durations, the pathogen still significantly reduced the biomasses of both ants and symbiotic fungus. The reason lies in the pathogen’s direct virulence to workers: Metarhizium infects insects through conidia that adhere to the cuticle, germinate, and penetrate the integument, so self-grooming alone may be insufficient to prevent infection even though it reduces spore quantity and viability. Infection may also impair worker activity, and empirical studies have shown that Metarhizium can disrupt essential tasks such as foraging, destabilising the social homeostasis that keeps a colony functioning. Escovopsis phialicopiosa, an opportunistic pathogen that attacks the cultivated fungus rather than the ants themselves, had minimal impact unless fungal mortality exceeded a critical threshold, roughly above 0.5 per day in the simulations, suggesting the strain’s lower virulence fails to trigger strong defensive responses.

Bifurcation diagrams, which map how equilibrium states change as parameters vary, exposed the system’s stability landscape. The proportion of workers allocated to fungus cultivation, denoted q, proved decisive: when q approached zero, too few workers tended the garden and biomass collapsed; when q approached one, workers neglected other essential tasks to the detriment of colony function. Intermediate values between 0.4 and 0.6 maximised stability by balancing competing demands. Ant and fungal mortality rates produced non-linear declines in most simulations, with a pronounced threshold near 0.5 per day for ant mortality, beyond which worker losses exceeded the colony’s replacement capacity and fungal biomass accelerated its decline. Self-grooming proved more effective than allogrooming in mitigating Metarhizium’s effects, likely reflecting the prioritisation of individual defences under high pathogen pressure, and ant biomass associated with self-grooming was significantly higher than biomass associated with allogrooming, although fungal biomass did not follow this pattern.

These findings carry practical weight well beyond ecology. Atta species are dominant herbivores in Neotropical biomes, capable of consuming up to 17 percent of available foliar biomass, and they inflict substantial damage on Eucalyptus and Pinus plantations as well as crops such as cassava and coffee. Biological control with entomopathogenic fungi has long struggled against these ants precisely because their multi-chambered nests and layered behavioural defences hinder uniform application of control agents. By identifying the behavioural and structural factors that limit or enhance pathogen effectiveness, the model suggests that targeted interventions could exploit critical mortality thresholds and worker allocation dynamics, minimising environmental impacts while optimising population control. The framework also provides a useful tool for understanding epidemiological dynamics in natural nests, integrating behavioural defences and spatial strategies to safeguard a mutualism that has persisted for 66 million years, and pointing toward future studies that could extend the two-chamber model into interconnected multi-chamber networks to reveal how full nest topology shapes ecological stability.

Subject of Research: Grooming behaviour and nest architecture regulating biomass growth of leafcutter ants and their symbiotic fungus under entomopathogenic fungal infection

Article Title: The role of grooming in regulating biomass growth of ants and symbiotic fungi under entomopathogenic fungal infection: experiments and mathematical modelling

Article References: Bueno, I., Toledo, J. V. M. F. D., Canuto, L. S., & Godoy, W. A. C. (2025). The role of grooming in regulating biomass growth of ants and symbiotic fungi under entomopathogenic fungal infection: experiments and mathematical modelling. BioRisk, 23, 17-43. https://doi.org/10.3897/biorisk.23.151538

Image Credits: AI Generated

DOI: 10.3897/biorisk.23.151538

Keywords: leafcutter ants, Atta sexdens, symbiotic fungus, grooming, Metarhizium anisopliae, Escovopsis phialicopiosa, entomopathogenic fungi, mathematical modelling, bifurcation analysis, mutualism, social immunity, biological control

The findings, published in Neotropical Biology and Conservation, are significant because arid valleys wedged between Andean ridges remain among the most poorly documented pollination systems in the Neotropics. These landscapes experience strongly seasonal rainfall concentrated between December and March, and the flowering that follows the wet season creates a brief, spatially concentrated window of floral abundance. Understanding which species carry the weight of pollination interactions during this window matters for conservation, because the loss of ecological interactions can degrade ecosystem function even before any species goes locally extinct.

Between March and August 2025, researchers completed 24 weekly surveys along a fixed transect of approximately three kilometers, accumulating about 120 observation hours. Each survey ran from 06:00 to 11:00, the interval when hummingbird foraging activity peaks. Using 10 x 42 binoculars and a superzoom camera, observers recorded every feeding event, perching event, and competitive encounter, noting the species involved, foraging strategy, interaction height, and effective feeding time. Feeding was defined as any event in which a hummingbird inserted its bill into a flower or accessed nectar directly.

The quantitative feeding network comprised 10 plant species, four hummingbird species, and 21 observed links out of 40 possible pairings. Network connectance reached 0.525, meaning more than half of all possible interactions among the species present were actually recorded, a sign of considerable floral resource sharing. Complementary specialization, measured with the H2′ index, came in at an intermediate 0.550, while modularity of Q = 0.386 suggested some internal organization into subsets of more strongly interacting species. Presence-absence nestedness was relatively high at NODF = 65.23, although weighted nestedness dropped when interaction frequencies were considered, indicating that nested organization was more evident in which interactions occurred than in how often they happened.

What stands out most is how unevenly visits were distributed. Tecoma fulva, a shrub with tubular orange flowers, absorbed 47.5 percent of all feeding records. Ligaria cuneifolia, a mistletoe-like hemiparasite, took 25.1 percent, and Nicotiana glauca, the tree tobacco, contributed 10.3 percent. Together these three species accounted for 83 percent of all nectar visits, while each remaining floral resource drew less than 7 percent. At the hummingbird level, the Peruvian sheartail Rhodopis vesper was the most frequent visitor, followed by the giant hummingbird Patagona gigas, the Peruvian piedtail Thaumastura cora, and the sparkling violetear Colibri coruscans. The three most frequent pairings, including the Tecoma fulva-Rhodopis vesper link at 25.1 percent of all records, jointly made up 63.2 percent of feeding activity.

Dependence analysis exposed striking asymmetries within these dominant interactions. Rhodopis vesper directed 73.7 percent of its visits to Tecoma fulva, while the plant received visits from multiple hummingbird species, producing a negative asymmetry value of -0.209 that indicates the bird depends more heavily on the plant than the plant does on the bird. The giant hummingbird showed even stronger reliance, with 76.3 percent of its visits aimed at Tecoma fulva and an asymmetry of -0.338. In contrast, the Ligaria cuneifolia-Thaumastura cora pairing was nearly balanced, with asymmetry close to zero. Rarely visited plants such as Tarasa capitata, Nicotiana paniculata, Mutisia acuminata, Cantua volcanica, and Tropaeolum tuberosum showed complete dependence on a single recorded visitor, though these may represent exploratory resource use rather than established mutualisms.

Centrality metrics, combining degree, species strength, betweenness, and closeness into a composite index, confirmed the structural roles suggested by raw visit counts. Rhodopis vesper scored highest among hummingbirds with a composite centrality of 1.184, reflecting its eight floral partners, high interaction frequency, and important position connecting different network sectors. Among plants, Tecoma fulva dominated with a composite index of 1.748, followed by Nicotiana glauca at 0.926 and Ligaria cuneifolia at 0.748. These species appear to contribute disproportionately to network connectivity, meaning their persistence may be critical for maintaining the interaction web in the valley.

Temporal dynamics proved equally revealing. Feeding records climbed from 17 in March to peaks of 68 in May and 82 in June, then collapsed to 17 in July and just seven in August. Unique interactions followed the same trajectory, rising from four in March to twelve in May before falling to three in August. Monthly network comparison using Bray-Curtis dissimilarity showed May and June were the most similar months, with a temporal similarity of 0.733, while August was the most differentiated, showing almost no resemblance to any other month. The identity of central species also rotated: Thaumastura cora led in March, Rhodopis vesper in April, Colibri coruscans in May, and Rhodopis vesper again from June onward. Tecoma fulva held the central plant role consistently from April through July before yielding to Nicotiana glauca in August.

Perhaps the most counterintuitive result concerns floral traits. Eight of the ten plants in the feeding network were classified as ornithophilous, bearing the tubular flowers, conspicuous colors, and accessible nectar classically associated with bird pollination. Yet neither nectar sugar concentration, measured in degrees Brix with a portable refractometer, nor floral length showed a significant association with visitation frequency. The Spearman correlation between nectar concentration and visits was essentially zero (rho = 0.018, p = 0.969), and exploratory count models found that the null model outperformed all trait-based alternatives. Tecoma fulva was the most visited resource despite not having the highest nectar concentration, while Nicotiana paniculata showed high sugar values but few visits. This aligns with recent evidence that nectar traits do not necessarily predict interaction frequency, and suggests that factors such as flower abundance, phenological overlap, and plant architecture likely govern resource use in this system.

The study also documented that plants serve functions far beyond nectar. Of the 349 records, 36.1 percent were perching events, with Schinus molle, the Peruvian peppertree, absorbing 43.7 percent of perch use despite playing almost no role in the feeding network. Tecoma fulva fulfilled a dual role as both the most visited floral resource and the second most used perch, hosting feeding, resting, vigilance, and territorial defense near its flowers. Twenty-one competitive events, including chases and displacements, clustered around the most visited plants, and even a nectar-robbing event by the cinereous conebill Conirostrum cinereum was recorded. For conservation in arid Andean landscapes, the message is clear: preserving heterogeneous vegetation matters not only for the flowers that feed hummingbirds but for the branches that structure their daily lives, and maintaining a small set of key plant species may buffer these interaction networks through the seasonal swings that define life between the Andes.

Subject of Research: Ecology

Article Title: Temporal dynamics and central species in a plant-hummingbird interaction network in an inter-Andean valley in the southern Peruvian Andes

Article References: Quispe-Puma, M. A., & Luque-Fernández, C. R. (2026). Temporal dynamics and central species in a plant-hummingbird interaction network in an inter-Andean valley in the southern Peruvian Andes. Neotropical Biology and Conservation, 21(3), 271-296. https://doi.org/10.3897/neotropical.21.e202837

Image Credits: AI Generated

DOI: 10.3897/neotropical.21.e202837

Keywords: hummingbirds, pollination, ecological networks, Peru, Andes, mutualism, seasonality, conservation, Temporal, dynamics, central, species

The phyllosphere, as the leaf environment is called, is a notoriously difficult place to make a living. It is exposed to ultraviolet radiation, fluctuating humidity, temperature swings and scarce nutrients, and it changes dramatically with the seasons. Yet despite these pressures, leaf surfaces host complex communities of bacteria, fungi, oomycetes and other eukaryotes that influence plant health, disease resistance and even ecosystem function. Understanding which interactions hold these communities together has been a long-standing puzzle, because most studies rely on computational predictions of who interacts with whom rather than direct experimental proof of the underlying mechanisms.

To crack the problem, the researchers launched a continental-scale field survey, collecting 347 wild Arabidopsis plants from 15 natural populations spanning Spain, France, Germany and Sweden. They sequenced bacterial, fungal, oomycete and broader eukaryotic marker genes from both whole-leaf samples and surface-sterilized endophytic samples, building a multikingdom picture of who lives where. Network analysis of the resulting data revealed that certain taxa, so-called hub microbes, occupied disproportionately central positions in the microbial community. Among the most prominent were the oomycete genus Albugo and the basidiomycete yeast Dioszegia, both of which scored highly on measures of closeness and betweenness centrality, indicating that they act as connective tissue within the leaf microbiome.

Albugo laibachii is an obligate biotrophic pathogen, meaning it can only survive on living host tissue. Such pathogens typically lose essential metabolic pathways during evolution because they can steal the missing nutrients from their hosts. Albugo’s genome, for example, lacks the entire pathway for making thiamine. But there is a catch: during the earliest stages of infection, before the pathogen has penetrated the plant, the leaf surface offers almost no resources. The researchers hypothesized that other leaf-dwelling microbes might be filling this gap, and that a positive interaction between Albugo and a co-occurring microbe could explain how the pathogen manages to establish itself in nature.

Testing this idea required moving from correlation to causation. The team adapted a gnotobiotic system in which sterile Arabidopsis seedlings are grown in plates and inoculated with defined microbes, allowing them to measure how individual strains affect Albugo colonization. Of twelve candidate interactors predicted from the network analysis, most either inhibited the pathogen or had no effect. One result was particularly instructive: two strains of the yeast Leucosporidium, predicted to interact positively with Albugo, actually suppressed infection, a reminder that computational associations do not always translate into direct functional outcomes. But one strain, a Dioszegia hungarica isolate designated EY, consistently and strongly promoted Albugo infection, exactly as the network had predicted.

The molecular culprit turned out to be a gene called DhPER1, which encodes a membrane permease for thiamine. Comparing the genomes of the Albugo-promoting EY strain and a non-promoting strain from Portugal, the researchers found that DhPER1 carried the highest density of single-nucleotide polymorphisms among all thiamine-related genes, and that its expression was more than twentyfold higher in EY than in the non-promoting strain. Using a newly developed CRISPR-based transformation system for Dioszegia, they knocked out DhPER1 in the EY strain and found that the mutant lost its ability to promote Albugo infection. Complementing the mutant with the EY version of the gene restored the phenotype, confirming that this single permease is the key to the interaction.

Enzyme-linked immunosorbent assays showed that strains expressing DhPER1 secreted thiamine into their surroundings, and adding purified thiamine to the gnotobiotic system boosted Albugo infection by up to 500 percent at low concentrations. Under the microscope, the researchers observed something remarkable: Albugo zoospores, the motile spores that initiate infection, actively swam toward Dioszegia EY cells rather than drifting freely in water, suggesting that the yeast both attracts and feeds the pathogen. Transcriptomic analysis two days after infection revealed that Albugo’s overall gene expression tripled in the presence of the EY strain, with 316 genes upregulated, many involved in metabolism and membrane transport, indicating that the yeast jump-starts the pathogen’s metabolic machinery during the critical early phase of colonization.

Evolutionary analyses added a deeper layer to the story. The DhPER1 gene shows signatures of strong purifying selection in Dioszegia, with a ratio of nonsynonymous to synonymous substitutions among the most conserved of all genes, and the RELAX algorithm revealed intensified selection specifically along the D. hungarica lineage. The EY strain also carries more than twice as many tRNA genes as other D. hungarica strains, and its thiamine-related genes show significantly higher predicted translation efficiency, suggesting that natural selection has optimized the yeast’s protein-production machinery to support the cross-feeding lifestyle. These findings echo the Black Queen Hypothesis, which proposes that leaky production of costly public goods drives stable metabolic dependencies between microbes.

What does the yeast get out of the deal? In long-term co-cultivation experiments lasting 32 days, roughly the span of Arabidopsis’s vegetative growth, Dioszegia populations gradually died out on non-sterile plants when Albugo was absent, overwhelmed by the native microbiome. But when Albugo was present, the yeast populations were rescued, increasing more than a hundredfold, and strains expressing DhPER1 fared significantly better than those without it. The benefit appears to be indirect: Albugo, known from earlier work to suppress plant immunity and inhibit certain competitors, effectively engineers the leaf environment, sheltering its metabolic partner from competitive exclusion. The result is an asymmetric mutualism in which the yeast supplies a vitamin and the pathogen supplies a niche.

The implications reach well beyond Arabidopsis. Vitamin auxotrophy is widespread among bacteria in oceans, soils and the human gut, and obligate plant pathogens such as downy mildews, rusts and arbuscular mycorrhizal fungi all depend on nutrients they cannot make themselves. If the colonization of such organisms depends on nutrient supplies from the microbiome rather than solely from the host, as this study suggests, then manipulating microbial nutrient flows could become a powerful tool. Researchers envision strategies that either starve harmful obligate pathogens by disrupting their microbial supply lines or bolster beneficial symbionts by engineering cross-feeding partners, opening a new frontier in microbiome engineering for agriculture and ecosystem management.

Subject of Research: Thiamine cross-feeding between a leaf yeast and an oomycete pathogen in the Arabidopsis phyllosphere microbiome

Article Title: Thiamine cross-feeding drives microbial coexistence in the leaf microbiome

Article References: Hu, Y., Bode, J., Gómez-Pérez, D., Guerreiro, M. A., Mari, A., Wang, K., Niemann, S., Mahmoudi, M., Kemen, A., Duran, P., Wacker, O., Straub, D., Nahnsen, S., Schwessinger, B., Roux, F., Alonso-Blanco, C., Ågren, J., Hacquard, S., Stukenbrock, E. H., & Kemen, E. (2026). Thiamine cross-feeding drives microbial coexistence in the leaf microbiome. Nature Microbiology. https://doi.org/10.1038/s41564-026-02499-w

Image Credits: AI Generated

DOI: 10.1038/s41564-026-02499-w

Keywords: thiamine, cross-feeding, phyllosphere, microbiome, Arabidopsis thaliana, Albugo laibachii, Dioszegia hungarica, mutualism, keystone taxa, oomycete, plant pathology, microbial ecology

Compiling the inventory required a fusion of classical and molecular techniques. The team combined careful morphological taxonomy, the traditional bedrock of ant systematics, with species delimitation based on COI DNA barcoding, a molecular approach that uses a standardized segment of the mitochondrial cytochrome c oxidase I gene to separate closely related or cryptic forms. This dual strategy matters on small islands, where worker ants are often morphologically variable and where a single overlooked species could distort the ecological picture. By cross-checking what the microscope revealed against what the barcode sequences suggested, the researchers were able to disentangle how geography, vegetation cover and human disturbance mold the composition of island ant assemblages. The work thus delivers not merely a checklist but an analytical framework, one that treats each island as a natural experiment in colonization, extinction and community assembly played out over a small and biologically depauperate landscape.

The headline numbers conceal some genuinely surprising patterns. Across the seven islands surveyed, ant species richness showed only marginally positive and statistically non-significant correlations with the distance to the nearest neighboring island. That outcome runs counter to classic island biogeography theory, which since the pioneering work of MacArthur and Wilson has predicted that isolation should strongly suppress species richness, because distant islands are harder for dispersing organisms to reach. On the Xisha Islands, isolation appears to matter far less than expected for ants, a group whose winged queens can travel considerable distances and whose colonies are notorious stowaways in human cargo. Even more striking, no measured environmental variable could significantly explain the overall composition of ant communities across the archipelago. In an era when ecologists increasingly seek tidy statistical relationships between environment and biodiversity, the Xisha data are a reminder that young, disturbed, human-penetrated island systems can defy textbook expectations.

What did emerge clearly was the fingerprint of anthropogenic disturbance. This factor stood out as the key driver boosting the colonization of exotic ant species across the archipelago. The contrast between islands could hardly be sharper. Highly disturbed Yongxing Island, the administrative and logistical hub of the archipelago, hosted three exotic ant species, including two of the world’s most notorious invaders, the fire ants Solenopsis invicta and Solenopsis geminata. By contrast, the islands with low levels of disturbance contained at most a single non-native ant species. The pattern aligns with a growing body of evidence that human infrastructure, supply shipments and habitat modification act as conveyor belts for tramp ant species, which thrive in the open, disturbed microhabitats that construction and cultivation create. For a fragile coral archipelago, the arrival of aggressive, competitively dominant fire ants is not a trivial addition to a species list but a potential restructuring force for the entire terrestrial food web.

Vegetation, by contrast, exerted a negligible influence on ant species richness and on the proportion of exotic species present. This finding is notable because plant community structure is often invoked as a central filter for insect assemblages, shaping microclimate, nesting sites and food availability. On the Xisha Islands, where native vegetation is naturally sparse and where coconut plantings and other introduced flora have altered much of the green cover, the ant communities appear relatively insensitive to these differences. The implication is that on such a young and inherently depauperate landscape, the sorting of ant species is governed less by the fine details of habitat than by which species manage to arrive, and whether disturbance has opened the door for them to establish. That interpretation strengthens the case for focusing management attention on pathways of introduction rather than solely on habitat restoration.

The ecological roles performed by the ants themselves add a further layer of significance. Most of the Xisha ant fauna consists of ground-dwelling omnivores, generalists that forage across the soil surface for a wide range of resources. Yet these generalists are far from ecologically inert. Up to seventy-five percent of the recorded ant species engage in mutualistic relationships with hemipteran insects, the sap-sucking bugs such as scale insects, mealybugs and aphids whose sugary honeydew ants harvest in exchange for protection. Such mutualisms can cascade through island ecosystems, influencing plant health both positively and negatively depending on the hemipteran species involved. In addition, many of the recorded taxa display potential pollination and seed dispersal functions, services that are critically important for the depauperate plant communities of coral islands, where every effective mutualist partner counts. On islands with short lists of native animals, ants may shoulder ecological duties that would be distributed among many more groups on the mainland.

Conspicuously absent from the inventory were any endemic ant species, a finding consistent with the archipelago’s young geological origin. Coral islands such as those of the Xisha chain are geologically ephemeral features, built up by reef organisms and reshaped by storms and sea-level fluctuations, and they have simply not existed long enough for distinctive endemic lineages to evolve. This youthfulness is precisely what makes the archipelago scientifically valuable and ecologically vulnerable at the same time. Because the native biota is drawn entirely from colonists, the system is unusually open to new arrivals, including the invasive ones. As climate change and sea-level rise squeeze the available habitat even further, the balance between native colonists and exotic invaders becomes ever more consequential. The new inventory therefore establishes vital baseline data against which future change can be measured, whether that change stems from warming, from further development, or from the continued spread of introduced species.

From these baselines flow concrete management recommendations. The researchers warn that preventing the spread of invasive fire ants from highly disturbed islands such as Yongxing toward the less disturbed, more intact islands of the archipelago should be treated as an urgent conservation priority. Fire ants are capable of displacing native ants, preying upon or outcompeting ground-nesting wildlife, and imposing agricultural and public health costs, so their containment on a chain of small islands where inter-island traffic is manageable is a realistic goal rather than a distant aspiration. Quarantine and inspection of goods moving between islands, together with targeted monitoring of ant communities on the least disturbed islets, emerge from the study as practical tools. The work also underscores the value of investing in biodiversity surveys on small tropical islands, which are often skipped by large-scale sampling programs precisely because they seem biologically modest.

The authors are candid about the limits of the current study and the directions it opens. Sampling constraints inherent to short, logistically demanding expeditions on remote islands mean that some species, particularly rare or seasonally active ones, may have gone unrecorded, and the reliance on a single genetic locus for molecular delimitation leaves room for refinement. The team points toward future multilocus genomic research, which would use many independent genetic markers to resolve species boundaries and population histories with far greater confidence, and toward field-based functional validation of the ecological roles inferred for the ants, from seed dispersal experiments to direct measurement of hemipteran mutualisms. Such follow-up work would convert this first inventory into a dynamic monitoring program. For now, the Xisha study stands as a timely demonstration that on the world’s smallest and most fragile islands, the fate of native biodiversity is decided less by distance and isolation than by what people choose to move, build and protect.

Subject of Research: Ant community structure and exotic species colonization on the Xisha Islands, a tropical coral archipelago in the South China Sea

Article Title: Ant diversity of the Xisha islands: Community structure, exotic species colonization, and ecological functions in a tropical coral archipelago

Article References: Ant diversity of the Xisha islands: Community structure, exotic species colonization, and ecological functions in a tropical coral archipelago. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: ants, Xisha Islands, South China Sea, invasive species, fire ants, island biogeography, biodiversity, coral islands, mutualism, seed dispersal, DNA barcoding, anthropogenic disturbance

The experimental design was elegantly simple in concept but rich in the behavioral data it generated. Two groups of captive brown capuchin monkeys, one containing 22 individuals and the other 18, were given access to an apparatus featuring five distinct options. At the center stood the Stag box, a cooperative device that required two monkeys to press levers simultaneously to release high-value rewards. Surrounding it were four Hare boxes, individually operable devices that dispensed lower-value rewards to any single monkey willing to work alone. Because the animals could move freely among all five options and select any partner they wished across six testing sessions, the setup captured the genuine social dynamics of partner choice rather than forcing arbitrary pairings on unwilling participants.

The results were striking in their breadth. Most individuals in both groups engaged in successful cooperative bouts at the Stag box, demonstrating that the majority of capuchins in the study were both willing and able to coordinate their actions with a partner when the payoff justified the effort. At the same time, the monkeys did not neglect the safer alternative, regularly obtaining rewards on their own from the Hare boxes. This pattern mirrors the fundamental tension built into the Stag Hunt paradigm: cooperating with a partner promises a bigger prize but carries the risk of failure if the partner does not pull their weight, while working alone guarantees a modest but reliable return.

To disentangle the factors driving cooperative decisions, the researchers turned to generalized linear mixed models, a statistical framework well suited to behavioral data in which many observations are nested within individuals and dyads. The analysis revealed that dyadic tolerance, a measure of how comfortably two particular monkeys could share space and resources without conflict, significantly predicted the likelihood of cooperation in both groups. In other words, capuchins were not choosing partners at random. They were systematically more likely to attempt the demanding joint task with partners they tolerated well, suggesting that the quality of a social relationship acts as a gatekeeper for cooperative ventures in this species.

Individual characteristics mattered as well. The models showed that more exploratory monkeys, those that readily investigated novel objects and environments, participated in more cooperative interactions than their less adventurous groupmates. Dyads involving juveniles also engaged in more cooperative bouts, hinting that younger animals may be especially motivated to experiment with the apparatus or that their playful dispositions make them attractive or available partners. These findings underscore that cooperation is not solely a property of relationships; the personalities and life stages of the individuals involved shape how often cooperative opportunities are taken up in the first place.

Perhaps the most technically intriguing results came from a finer-grained analysis of pressing behavior, which examined how the mere presence of a partner influenced how hard monkeys worked at each type of box. Here the two groups diverged in unexpected ways. In the East Group, the presence of a partner increased pressing at the Stag box while simultaneously lowering pressing at the Hare boxes, a pattern consistent with genuine social facilitation of cooperation and a devaluation of the solo option when a collaborator was available. In the West Group, by contrast, Stag-box pressing increased steadily across sessions regardless of whether a partner was present or absent, suggesting that learning and familiarity with the apparatus, rather than partner presence, drove much of the cooperative effort in that group.

This group-level divergence carries important methodological implications for the field of comparative cognition. It demonstrates that ostensibly identical experimental conditions can produce different behavioral signatures in different social groups, and that conclusions about partner sensitivity drawn from a single group may not generalize. The authors argue that this variability is precisely why paradigms allowing free partner choice are so valuable: they reveal the social dynamics underlying cooperation rather than smoothing them away under the constraints of forced pairing. When animals can vote with their feet, researchers gain access to the decision processes that matter in the real world where cooperation evolved.

The study also speaks to a long-standing debate about the evolutionary foundations of mutualism. Mutualistic cooperation with free partner choice is widely considered a key pillar in the evolution of cooperative behavior, including in humans, because it allows individuals to shop for reliable partners and abandon unreliable ones, creating selection pressure for trustworthiness and coordination. Yet many experimental studies of animal cooperation have restricted partner choice or offered few alternatives, limiting insight into the factors that actually shape cooperative decisions. By combining a cooperative option with multiple solo alternatives in an open social setting, the Stag Hunt apparatus used here brings experimental designs closer to the ecological realities in which cooperative psychology took shape.

The ethical design of the research deserves note as well. The study was entirely non-invasive and relied exclusively on voluntary participation: no animal was food-deprived, socially separated, restrained, or physically handled for the purposes of the experiment. Multiple apparatuses were installed simultaneously to reduce monopolization by dominant individuals, and zoo staff implemented scattered feeding during testing to maintain naturalistic foraging dispersion and minimize competition. All rewards formed part of the animals’ regular diet, and no violent or injurious interactions were observed in connection with the procedures. The work was approved by the School of Psychology and Neuroscience Ethics Committee at the University of St Andrews and conducted in close collaboration with zoo authorities at Living Links, Edinburgh Zoo.

Taken together, the findings highlight the importance of integrating social relationships and individual traits into any account of cooperative decision-making in primates. Tolerance between partners determines whether cooperation is attempted at all; exploratory tendencies and youth shape how often individuals engage; and the influence of a partner’s presence on effort can vary from group to group. For capuchins, at least, the road to the big prize runs through social tolerance, and the willingness to take that road depends on who is standing beside you. As researchers continue to probe the roots of cooperation across species, studies like this one demonstrate that the richest answers emerge not from forcing animals into prescribed partnerships, but from letting them choose for themselves.

Subject of Research: Cooperative behavior and partner choice in brown capuchin monkeys tested with a Stag Hunt cooperation task

Article Title: Cooperation in brown capuchin monkeys: social tolerance, individual traits, and partner sensitivity in a Stag Hunt task

Article References: Baruah, K. M., Call, J., & Seed, A. M. (2026). Cooperation in brown capuchin monkeys: social tolerance, individual traits, and partner sensitivity in a Stag Hunt task. Animal Cognition. https://doi.org/10.1007/s10071-026-02107-8

Image Credits: AI Generated

DOI: 10.1007/s10071-026-02107-8

Keywords: capuchin monkeys, cooperation, Stag Hunt, partner choice, social tolerance, animal cognition, primates, mutualism, exploration, behavioral ecology, comparative psychology, social behavior

Endosymbiosis, the phenomenon in which one organism lives inside the cells of another, underlies some of the most important transitions in the history of life. Mitochondria and chloroplasts, the energy-converting organelles of eukaryotic cells, are the descendants of free-living bacteria that took up residence inside ancestral host cells more than a billion years ago. Wolbachia, an alpha-proteobacterium related to the ancestors of mitochondria, is arguably the most successful animal-associated microbe on Earth, colonizing an estimated 50 to 60 percent of all insect and arthropod species as well as filarial nematodes. Depending on the host and the bacterial strain, the relationship ranges from reproductive parasitism to strict mutualism, in which the host can no longer survive without its passenger.

In filarial worms such as Wuchereria bancrofti, the cause of elephantiasis, and Onchocerca volvulus, the agent of river blindness, the mutualism has become so intimate that antibiotic killing of Wolbachia also kills the worm. In insects, the bacterium can supply essential metabolites including heme, purines, pyrimidines, flavin adenine dinucleotide and riboflavin, and it can suppress infections by pathogenic viruses, bacteria and protists. Yet modeling the early stages of such coevolution in insects or nematodes is slow and experimentally awkward, because adaptive changes in these animals take generations to accumulate. Yeast, with its rapid doubling time and fully sequenced, easily manipulated genome, offers an appealing alternative, particularly since recent surveys have revealed that wild yeasts, long considered axenic, actually harbor hidden communities of intracellular bacteria.

To build their model, the Mexican team exploited two deletion mutants of the laboratory strain By4742. The first, ΔRIB1, lacks GTP cyclohydrolase II, the enzyme that catalyzes the very first step of riboflavin biosynthesis. The second, ΔRIB4, lacks lumazine synthase, which manufactures the immediate precursor of the vitamin. When the researchers grew these strains in a synthetic lactate medium lacking riboflavin, both mutants failed to grow, while the wild-type strain reached measurable densities and grew slightly better when the vitamin was added. The auxotrophy was not an artifact of the carbon source: the mutants also failed to grow without riboflavin in dextrose-containing medium, confirming that the defect lay squarely in the vitamin’s synthesis pathway rather than in respiratory metabolism per se.

Biochemical measurements reinforced the picture. Using a Clark-type oxygen electrode, the team found that both mutants consumed oxygen at very low rates, consistent with their dependence on riboflavin-derived cofactors. Riboflavin is the substrate for the synthesis of flavin mononucleotide and flavin adenine dinucleotide, the two redox coenzymes that power oxidative metabolism and a host of other cellular reactions. Fluorescence-based quantification of intracellular flavins showed that the mutants contained roughly half the riboflavin of wild-type cells, and that exogenous supplementation raised their internal levels, with ΔRIB4 responding particularly strongly. The uncoupling agent CCCP increased oxygen consumption in all strains, indicating that the respiratory machinery itself remained responsive and that the bottleneck was cofactor supply.

The pivotal experiment came next. The researchers infected the yeast strains with Wolbachia strain wStr, originally isolated from the planthopper Laodelphax striatellus and maintained in an Aedes albopictus cell line. Infection was accomplished by co-centrifuging bacteria and yeast and was confirmed seven days later by reverse-transcription PCR amplification of the wsp gene, which encodes the bacterium’s major surface protein. A 650-base-pair product appeared in all infected strains but not in uninfected controls. Strikingly, when the infected ΔRIB1 and ΔRIB4 mutants were grown in medium lacking riboflavin, they grew to levels comparable to those achieved with vitamin supplementation, demonstrating that the endosymbiont had functionally complemented the missing biosynthetic steps.

Physiological assays supported the conclusion that the bacterium was genuinely provisioning its host. Oxygen consumption, which had been negligible in the uninfected auxotrophs, rose significantly after infection, with the largest gains seen in maximal, uncoupled respiration. Intracellular riboflavin concentrations increased in both infected mutants, more modestly in wΔRIB1 and more pronouncedly in wΔRIB4. The team also tested whether infection conferred broader physiological benefits by exposing cells to two harsh conditions: one molar sodium chloride, an ionic stress, and ten millimolar hydrogen peroxide, which generates reactive oxygen species. Infected mutants survived these challenges at substantially higher rates than their uninfected counterparts. Under salt stress, survival of ΔRIB1 rose from 34 to 71 percent and that of ΔRIB4 from 11 to 37 percent; under peroxide, ΔRIB1 improved from 41 to 75 percent and ΔRIB4 from 37 to 57 percent. The wild-type strain showed little change, suggesting that the benefit is most visible when the host is metabolically vulnerable.

The mechanism most likely centers on Wolbachia‘s own riboflavin pathway, which retains a conserved set of six genes, ribA, ribD, ribB, ribE, ribC and ribF, across diverse insect-associated strains. Parallel examples abound in nature. The bed bug Cimex lectularius depends on its Wolbachia for B-vitamin synthesis during blood digestion, the brown planthopper Nilaparvata lugens receives both biotin and riboflavin from its symbiont, and in the tomato psyllid, co-resident Liberibacter and Wolbachia jointly enhance host fitness by supplying arginine and riboflavin. Beyond nutrient transfer, the bacterium’s type IV secretion system and its ankyrin repeat-containing proteins may modulate host cell processes, a strategy documented both in mosquitoes and, notably, in S. cerevisiae itself in earlier work by the same group.

That earlier work, published in 2019, showed that Wolbachia pipientis strain wAlbB infecting yeast W303 deregulated mitochondrial oxidative phosphorylation and hastened host death, a clearly parasitic outcome. The new study therefore captures a different face of the same bacterium: in a host with a specific metabolic vulnerability, infection becomes advantageous rather than lethal, at least in the short term. The authors caution that the benefits are likely transient, since prolonged infection has been shown to induce mitochondrial dysregulation and early cell death in yeast, and they note that the iron supplementation used in their media may itself have supported both symbiont growth and mitochondrial function through heme and iron-sulfur cluster assembly. Even so, the system neatly recapitulates the trajectory from parasite to mutualist that Wolbachia follows in nature, and it does so in an organism whose genetics, metabolism and respiratory physiology are among the best characterized in biology.

The researchers argue that the artificial yeast-endosymbiont model could become a standard platform for dissecting the molecular details of metabolic complementation: which compartments participate in riboflavin trafficking, how nutrient exchange is regulated, and how bacterial effectors reshape host physiology. Because the host is a single-celled eukaryote with a short generation time, experiments that would take years in insects can be completed in days. In an era when Wolbachia is being deployed at continental scale to suppress mosquito-borne diseases such as dengue, understanding the spectrum of its interactions with host cells, from exploitation to essential provisioning, has practical as well as evolutionary significance. This study shows that the first steps of that spectrum can now be reproduced, observed and manipulated in a test tube, one vitamin at a time.

Subject of Research: Wolbachia-mediated rescue of riboflavin auxotrophy in Saccharomyces cerevisiae as a model of endosymbiotic mutualism

Article Title: In Saccharomyces cerevisiae ΔRIB1 and ΔRIB4 mutants, riboflavin auxotrophy is reverted by Wolbachia (wStr) infection

Article References: Mendez-Romero, O., Ricardez-García, C., & Uribe-Carvajal, S. (2026). In Saccharomyces cerevisiae ΔRIB1 and ΔRIB4 mutants, riboflavin auxotrophy is reverted by Wolbachia (wStr) infection. International Microbiology. https://doi.org/10.1007/s10123-026-00884-3

Image Credits: AI Generated

DOI: 10.1007/s10123-026-00884-3

Keywords: Wolbachia, Saccharomyces cerevisiae, riboflavin auxotrophy, endosymbiosis, mutualism, vitamin B2, flavin cofactors, mitochondrial respiration, oxidative stress, wStr strain, yeast model, symbiotic complementation