Leaf-cutting Ant Acromyrmex Research Articles

Brood adoption in the leaf-cutting ant Acromyrmex echinatior: adaptation or recognition noise?

The ability to discriminate between nestmates and non-nestmates is an important prerequisite for the evolution of eusociality. Indeed, social insect workers are typically able to discriminate between nestmate and non-nestmate workers. Adult non-nestmate workers are readily detected and rejected from the colony. Whether social insects can discriminate between nestmate and non-nestmate brood, however, is less clear. Here, we show that workers of the leaf-cutting ant Acromyrmex echinatior discriminate between nestmate and non-nestmate brood, and among brood of different stages. Initially, non-nestmate brood is attacked, but it is adopted after a delay. Adoption could occur due to inefficiency of the recognition system, or it could be adaptive because it is an inexpensive way to increase the workforce. Our results suggest that brood adoption may occur accidentally. We also report how workers replace fungal hyphae on the brood’s surface before transporting the brood into their fungus garden.

Growth and Conidiation Response of Escovopsis weberi (Ascomycota: Hypocreales) Against the Fungal Cultivar of <I>Acromyrmex lundii</I> (Hymenoptera: Formicidae)

ABSTRACT Leaf-cutter ants (Acromyrmex and Atta spp.) exhibit ancient and complex interactions with the symbiotic fungus Leucoagaricus (Basidiomycetes: Agaricales) from which they feed, and with the virulent and specific fungus Escovopsis weberi J.J. Muchovej & Delia Lucia (Ascomycetes: Hypocreales) that attacks the ants' fungal gardens. This system offers a unique opportunity to study possible avenues for replacing polluting pesticides with a biological control agent against the ants. We isolated both Leucoagaricus sp. and E. weberi from the gardens of Acromyrmex lundii Guerin-Meneville colonies and confronted them with each other by growing Leucoagaricus on petri dishes and placing inoculated pieces of agar inoculated with E. weberi at the edges. Here we present growth curves of Leucoagaricus sp. and E. weberi in the absence of each other, as a baseline to which we compare the effect of the fungi on each other. As expected, we found a negative effect of E. weberi on the ant cultivar from different coloni...

The Dynamics of Plant Cell-Wall Polysaccharide Decomposition in Leaf-Cutting Ant Fungus Gardens

The degradation of live plant biomass in fungus gardens of leaf-cutting ants is poorly characterised but fundamental for understanding the mutual advantages and efficiency of this obligate nutritional symbiosis. Controversies about the extent to which the garden-symbiont Leucocoprinus gongylophorus degrades cellulose have hampered our understanding of the selection forces that induced large scale herbivory and of the ensuing ecological footprint of these ants. Here we use a recently established technique, based on polysaccharide microarrays probed with antibodies and carbohydrate binding modules, to map the occurrence of cell wall polymers in consecutive sections of the fungus garden of the leaf-cutting ant Acromyrmex echinatior. We show that pectin, xyloglucan and some xylan epitopes are degraded, whereas more highly substituted xylan and cellulose epitopes remain as residuals in the waste material that the ants remove from their fungus garden. These results demonstrate that biomass entering leaf-cutting ant fungus gardens is only partially utilized and explain why disproportionally large amounts of plant material are needed to sustain colony growth. They also explain why substantial communities of microbial and invertebrate symbionts have evolved associations with the dump material from leaf-cutting ant nests, to exploit decomposition niches that the ant garden-fungus does not utilize. Our approach thus provides detailed insight into the nutritional benefits and shortcomings associated with fungus-farming in ants.

Evolutionary patterns of proteinase activity in attine ant fungus gardens.

BackgroundAttine ants live in symbiosis with a basidiomycetous fungus that they rear on a substrate of plant material. This indirect herbivory implies that the symbiosis is likely to be nitrogen deprived, so that specific mechanisms may have evolved to enhance protein availability. We therefore hypothesized that fungal proteinase activity may have been under selection for efficiency and that different classes of proteinases might be involved.ResultsWe determined proteinase activity profiles across a wide pH range for fungus gardens of 14 Panamanian species of fungus-growing ants, representing eight genera. We mapped these activity profiles on an independently obtained molecular phylogeny of the symbionts and show that total proteinase activity in lower attine symbionts peaks at ca. pH 6. The higher attine symbionts that have no known free-living relatives had much higher proteinase activities than the lower attine symbionts. Their total in vitro proteinase activity peaked at pH values around 5, which is close to the pH that the ants maintain in their fungus gardens, suggesting that the pH optimum of fungal proteinases may have changed after the irreversible domestication of evolutionary more derived fungal symbionts. This notion is also supported by buffering capacities of fungus gardens at pH 5.2 being remarkably high, and suggests that the fungal symbiont actively helps to maintain garden acidity at this specific level. Metalloproteinases dominated the activity profiles of lower attine gardens and may thus represent the ancestral type of proteinase production, whereas serine proteinase activity dominated the activity profiles of the higher attine gardens reared by Trachymyrmex and Sericomyrmex, suggesting that there may be trade-offs in the production of these enzyme classes. Remarkably, the single symbiont that is shared by species of the crown group of Atta and Acromyrmex leaf-cutting ants mostly showed metalloproteinase activity, suggesting that recurrent changes in enzyme production may have occurred throughout the domestication history of fungus-garden symbionts.ConclusionsProteinase pH optima and buffering capacities of fungal symbionts appear to have evolved remarkable adaptations to living in obligate symbiosis with farming ants. Although the functional roles of serine and metalloproteinases in fungus gardens are unknown, the differential production of these classes of proteolytic enzymes suggest that substrate specificity may be important and that trade-offs may prevent the simultaneous upregulation of both classes of enzymes.

Potential impact of the leaf-cutting ant Acromyrmex lobicornis on conifer plantations in northern Patagonia, Argentina

1 The economic losses associated with crop damage by invasive pests can be minimized by recognizing their potential impact before they spread into new areas or crops. 2 We experimentally evaluated the preferences of the leaf-cutting ant Acromyrmex lobicornis (Hymenoptera: Formicidae) for the most common conifer species commercially planted in northern Patagonia, Argentina. The areas of potential forest interest in this region and the geographical range of this ant overlap. We performed field preference tests and monitored the level of ant herbivory on planted conifer seedlings next to nests. 3 Acromyrmex lobicornis preferred some conifer species and avoided foraging on others. Pseudotsuga menziesii and Austrocedrus chilensis were the less preferred species, Pinus ponderosa and Pinus contorta were the most preferred by A. lobicornis. 4 The item mostly selected by ants was young needles from P. contorta. This species was also the pine mostly defoliated. Seedlings without ant-exclusion showed a mean±SE of 60±5% defoliation during the sampling period. Pinus ponderosa was less defoliated; control seedlings showed a mean±SE of 8.5±1% of leaf damage in the sampling period. 5 The present study shows how the use of simple field tests of leaf-cutting ant preferences could allow an improved selection of appropriate conifer species for future plantations in areas where leaf-cutting ants are present.

Leaf-cutting ant fungi produce cell wall degrading pectinase complexes reminiscent of phytopathogenic fungi

BackgroundLeaf-cutting (attine) ants use their own fecal material to manure fungus gardens, which consist of leaf material overgrown by hyphal threads of the basidiomycete fungus Leucocoprinus gongylophorus that lives in symbiosis with the ants. Previous studies have suggested that the fecal droplets contain proteins that are produced by the fungal symbiont to pass unharmed through the digestive system of the ants, so they can enhance new fungus garden growth.ResultsWe tested this hypothesis by using proteomics methods to determine the gene sequences of fecal proteins in Acromyrmex echinatior leaf-cutting ants. Seven (21%) of the 33 identified proteins were pectinolytic enzymes that originated from the fungal symbiont and which were still active in the fecal droplets produced by the ants. We show that these enzymes are found in the fecal material only when the ants had access to fungus garden food, and we used quantitative polymerase chain reaction analysis to show that the expression of six of these enzyme genes was substantially upregulated in the fungal gongylidia. These unique structures serve as food for the ants and are produced only by the evolutionarily advanced garden symbionts of higher attine ants, but not by the fungi reared by the basal lineages of this ant clade.ConclusionsPectinolytic enzymes produced in the gongylidia of the fungal symbiont are ingested but not digested by Acromyrmex leaf-cutting ants so that they end up in the fecal fluid and become mixed with new garden substrate. Substantial quantities of pectinolytic enzymes are typically found in pathogenic fungi that attack live plant tissue, where they are known to breach the cell walls to allow the fungal mycelium access to the cell contents. As the leaf-cutting ant symbionts are derived from fungal clades that decompose dead plant material, our results suggest that their pectinolytic enzymes represent secondarily evolved adaptations that are convergent to those normally found in phytopathogens.

Polymorphism and Division of Labor During Foraging Cycles in the Leaf-cutting Ant Acromyrmex octospinosus (Formicidae; Attini)

While division of labor within leaf-cutting ant nests has been well-characterized in the context of the collection and processing of leaf material, environmental factors such as day-night cycles and heavy rainfall limit the time during which leaf-cutting ant workers leave the nest to gather forage. Using a novel “flat panel” nest design, we studied how patterns of within-nest task performance changed when a colony of the leaf-cutting ant Acromyrmex octospinosus was and was not provided access to forage. We conducted scan samples of individuals working within the nest under both conditions and compared task allocation patterns across provisioning regimes and between workers of different sizes. When labor was compared between worker size groups, “minor” workers (head width ≤2.0 mm) and “major” workers (head width >2.0 mm) showed significantly different task performance patterns when forage was available: minors performed mostly brood-care and garden maintenance, while majors were mostly involved in the handling of freshly-cut leaf fragments. In contrast, when the colony was deprived of forage, the task performance patterns of minor and major workers converged and did not significantly differ. Marked major workers known to be foragers tended to remain idle within the nest when the colony was deprived of forage, while non-foragers of similar head width engaged in a variety of within-nest tasks, suggesting polyethism in majors may be based on factors other than size.

Multiple gains and losses of Wolbachia symbionts across a tribe of fungus‐growing ants

Although the intracellular bacterium Wolbachia is ubiquitous in insects, it has a unique relationship with New World ants on which particular bacterial strains have specialized. However, data are from distantly related hosts and detailed phylogenetic information which could reveal transmission dynamics are lacking. Here, we investigate host-Wolbachia relationships in the monophyletic fungus-growing ant tribe Attini, screening 23 species and using multilocus sequence typing to reliably identify Wolbachia strains. This technique reduces the significant problem of recombination seen using traditional single gene techniques. The relationship between Wolbachia and the fungus-growing ants appears complex and dynamic. There is evidence of co-cladogenesis, supporting vertical transmission; however, this is incomplete, demonstrating that horizontal transmission has also occurred. Importantly, the infection prevalence is frequently different between closely related taxa, with the Acromyrmex leaf-cutting ants appearing particularly prone to infection and there being no consistent relationship with any of the major life history transitions. We suggest that infection loss and horizontal transmission have driven epidemics or selective sweeps of Wolbachia, resulting in multiple gains and losses of infection across the fungus-growing ants.

The Thermoregulatory Function of Thatched Nests in the South American Grass-Cutting Ant,Acromyrmex heyeri

The construction of mound-shaped nests by ants is considered as a behavioral adaptation to low environmental temperatures, i.e., colonies achieve higher and more stables temperatures than those of the environment. Besides the well-known nests of boreal Formica wood-ants, several species of South American leaf-cutting ants of the genus Acromyrmex construct thatched nests. Acromyrmex workers import plant fragments as building material, and arrange them so as to form a thatch covering a central chamber, where the fungus garden is located. Thus, the degree of thermoregulation attained by the fungus garden inside the thatched nest largely depends on how the thatch affects the thermal relations between the fungus and the environment. This work was aimed at studying the thermoregulatory function of the thatched nests built by the grass-cutting ant Acromyrmex heyeri Forel (Hymenoptera: Formicidae: Myrmicinae). Nest and environmental temperatures were measured as a function of solar radiation on the long-term. The thermal diffusivity of the nest thatch was measured and compared to that of the surrounding soil, in order to assess the influence of the building material on the nest's thermoregulatory ability. The results showed that the average core temperature of thatched nests was higher than that of the environment, but remained below values harmful for the fungus. This thermoregulation was brought about by the low thermal diffusivity of the nest thatch built by workers with plant fragments, instead of the readily-available soil particles that have a higher thermal diffusivity. The thatch prevented diurnal nest overheating by the incoming solar radiation, and avoided losses of the accumulated daily heat into the cold air during the night. The adaptive value of thatching behavior in Acromyrmex leaf-cutting ants occurring in the southernmost distribution range is discussed.

Workers of Acromyrmex echinatior leafcutter ants police worker-laid eggs, but not reproductive workers

Nonreproductive workers of many eusocial Hymenoptera ‘police’ the colony, that is, they attack reproductive sister workers or destroy their eggs (unfertilized; developing into haploid males). Several ultimate causes of policing have been proposed, including (1) an increase in colony productivity, applicable if reproductive workers work less, or (2) an increase in worker-to-male relatedness, applicable if within-colony relatedness is low. To explain the distribution of policing across taxa, the explanatory power of these and other potential ultimate causes should be assessed separately. One of the few species for which this can be done is the leafcutter ant Acromyrmex echinatior. We previously demonstrated that colony productivity incentives (and sex ratio incentives) are minimal here, while relatedness incentives are strong because queens are highly multiply mated. Overcoming technical difficulties peculiar to leafcutter ants, we introduced reproductive versus nonreproductive workers and batches of queen-laid versus worker-laid eggs into experimental colony fragments and observed their fate. Our main finding was that workers policed by selectively destroying worker-laid eggs, but without attacking reproductive workers. We infer that relatedness incentives are the most likely ultimate cause of the evolutionary maintenance of worker–egg policing in A. echinatior.

Variation inPseudonocardiaantibiotic defence helps govern parasite‐induced morbidity inAcromyrmexleaf‐cutting ants

Host-parasite associations are potentially shaped by evolutionary reciprocal selection dynamics, in which parasites evolve to overcome host defences and hosts are selected to counteract these through the evolution of new defences. This is expected to result in variation in parasite-defence interactions, and the evolution of resistant parasites causing increased virulence. Fungus-growing ants maintain antibiotic-producing Pseudonocardia (Actinobacteria) that aid in protection against specialized parasites of the ants' fungal gardens, and current evidence indicates that both symbionts have been associated with the ants for millions of years. Here we examine the extent of variation in the defensive capabilities of the ant-actinobacterial association against Escovopsis (parasite-defence interactions), and evaluate how variation impacts colonies of fungus-growing ants. We focus on five species of Acromyrmex leaf-cutting ants, crossing 12 strains of Pseudonocardia with 12 strains of Escovopsis in a Petri plate bioassay experiment, and subsequently conduct subcolony infection experiments using resistant and non-resistant parasite strains. Diversity in parasite-defence interactions, including pairings where the parasites are resistant, suggests that chemical variation in the antibiotics produced by different actinobacterial strains are responsible for the observed variation in parasite susceptibility. By evaluating the role this variation plays during infection, we show that infection of ant subcolonies with resistant parasite strains results in significantly higher parasite-induced morbidity with respect to garden biomass loss. Our findings thus further establish the role of Pseudonocardia-derived antibiotics in helping defend the ants' fungus garden from the parasite Escovopsis, and provide evidence that small molecules can play important roles as antibiotics in a natural system.

Resource allocation among worker castes of the leaf-cutting ants Acromyrmex subterraneus subterraneus through trophallaxis

The division of labor between the different worker castes of leaf-cutting ants may reflect in their capacity to exchange liquids by trophallaxis. The crop capacity of and trophallactic exchanges between different size classes of worker leaf-cutting ants of the sub-species Acromyrmex subterraneus subterraneus were investigated. Size classes were defined from head capsule widths and crop capacity of each class was determined following ad libitum feeding on dye solution. Experiments were carried out to investigate trophallactic exchanges between donor ants and recipient ants of each class size combination on a one to one basis. An experiment was also performed to investigate dye distribution within mini-colonies following introduction of three classes of donor ants. Worker ants were categorized into four size classes from their head capsule widths (C1 = 0.8–1.0 mm; C2 = 1.2–1.5 mm; C3 = 1.6–2.0 mm; C4 = 2.1–2.4 mm). C1 ants crop capacity was 0.13 μL; C2: 0.21 μL; C3: 0.52 μL; C4: 1.03 μL. Ants of each class previously fed on the dye solution (donors) were placed individually with an unfed ant of each class (recipients) and the presence of dye solution, passed from the donor to the recipient by oral trophallaxis was observed after 1 h. Results showed that all classes of donor ants performed trophallactic exchanges with all recipient classes. However, statistically fewer exchanges were seen for C2 donor ants when placed with C3 recipient ants. Ten donor ants of each of three classes (C2, C3 and C4) were introduced into mini-colonies without queen ants. It was observed that C1 and C2 ants were poor recipients, whilst C3 and C4 received the highest percentages of dye. Within 10 h of introducing the donor ants, 14 to 20% of their nest-mates had received dye solution, with 58 to 77% of dye passed to recipients. These studies show the altruistic nature of “food-laden” leaf-cutters and indicate that ants involved in garden maintenance activity are less likely to receive liquids from foraging workers.

Waste management in the leaf-cutting ant Acromyrmex echinatior: the role of worker size, age and plasticity

Division of labour is the hallmark of the success of many social animals. It may be especially important with regard to waste management because waste often contains pathogens or hazardous toxins and worker specialisation can reduce the number of group members exposed to it. Here we examine waste management in a fungus-farming, leaf-cutting ant, Acromyrmex echinatior, in which waste management is necessary to protect their vulnerable fungal crop. By marking ants with task-specific paint colours, we found clear division of labour between workers that engage in waste management and those that forage, at least during the fine timescale of the 3-day marking period. This division of labour was influenced by both age and size, with waste management workers tending to be smaller and younger than foragers. The role of preventing contaminated ants from entering the colony was fulfilled mainly by medium-sized workers. When the level of waste was experimentally increased, most of the ants that responded to remove the waste were workers previously engaged in tasks inside the nest rather than external waste workers or foragers. These responding workers tended to be young and medium-sized. Surprisingly, the responding ants were subsequently able to revert back to working within the fungus garden, but the probability of them doing so depended on their age and the length of time they were exposed to waste. The results demonstrate the importance of division of labour with regard to waste management in A. echinatior and show that this is adaptable to changing needs.

Ecological engineering by a native leaf-cutting ant increases the performance of exotic plant species

Numerous mechanisms are proposed to explain why exotic plants successfully invade natural communities. However, the positive effects of native engineers on exotic plant species have received less consideration. We tested whether the nutrient-rich soil patches created by a native ecological engineer (refuse dumps from the leaf-cutting ant Acromyrmex lobicornis) increase the performance of exotic more than native plants. In a greenhouse experiment, individuals from several native and exotic species were planted in pots with refuse dumps (RDs) and non-nest soils (NNSs). Total plant biomass and foliar nutrient content were measured at the end of the experiment. We also estimated the cover of exotic and native plant species in external RDs from 54 field ant nests and adjacent areas. Greenhouse plants showed more biomass and foliar nutrient content in RDs than in NNS pots. Nevertheless, differences in the final mean biomass among RD and NNS plants were especially great in exotics. Accordingly, the cover of exotic plants was higher in field RDs than in adjacent, non-nest soils. Our results demonstrated that plants can benefit from the enhanced nutrient content of ant RDs, and that A. lobicornis acts as an ecosystem engineer, creating a substrate that especially increases the performance of exotics. This supports the fluctuating resource hypothesis as a mechanism to promote biological invasions, and illustrates how this hypothesis may operate in nature. Since ant nests and exotic plants are more common in disturbed than in pristine environments, the role of ant nests in promoting biological invasions might be of particular interest. Proposals including the use of engineer species to restore disturbed habitats should be planned with caution because of their potential role in promoting invasions.

Isolation and characterization of actinobacteria ectosymbionts from Acromyrmex subterraneus brunneus (Hymenoptera, Formicidae)

The ectosymbiont actinobacterium Pseudonocardia was isolated from the integument of Acromyrmex leaf-cutter ants and seems to play a crucial role in maintaining asepsis of the nest . Currently, there has been an intensive search for Pseudonocardia associated with several attine species, but few studies have indicated that other actinobacteria may be associated with these ants as well. We therefore characterized the culturable actinobacteria community associated with the integument of the fungus-growing ant Acromyrmex subterraneus brunneus Forel, 1893 (Hymenoptera: Formicidae) . Ectosymbionts were isolated using four different media and characterized by morphological and molecular (16S rDNA) methods. A total of 20 strains were isolated, of which 17 were characterized as Streptomyces spp., and one isolate each as Pseudonocardia, Kitassatospora and Propionicimonas. Unlike other Acromyrmex species, A. subterraneus brunneus is associated with a diversity of actinobacteria. Even though Pseudonocardia is present on this leaf-cutting ant’s integument, the number and diversity of Streptomyces spp. found differs from those of previous studies with other attine ants and suggest that different culturing approaches are needed to characterize the true diversity of microbes colonizing the integument of attine ants. Moreover, understanding the diversity of the culturable actinobacteria associated with A. subterraneus brunneus should increase our knowledge of the evolutionary relationship of this intricate symbiotic association.

The effects of age and social interactions on innate immunity in a leaf-cutting ant

Both developmental and environmental factors shape investment in costly immune defences. Social insect workers have different selection pressures on their innate immune system compared to non-social insects because workers do not reproduce and their longevity affects the fitness of relatives. Furthermore, hygienic behavioural defences found in social insects can result in increased survival after fungal infection, although it is not known if there is modulation in physiological immune defence associated with group living vs. solitary living. Here we investigated whether physiological immune defence is affected by both age and the short-term presence or absence of nestmates in the leaf-cutting ant Acromyrmex octospinosus. We predicted that older ants would show immune senescence and that group living may result in prophylactic differences in immune defence compared to solitarily kept ants. We kept old and young workers alone or in nestmate groups for 48 h and assayed a key innate immune system enzyme, expressing phenoloxidase (PO) and its stored precursor (proPO), a defence that acts immediately, i.e. it is constitutive. Short-term solitary living did not affect PO or proPO levels relative to group living controls and we found no evidence for immunosenescence in proPO. However, we found a significant increase in active PO in older workers, which is consistent with two non-mutually exclusive explanations: it could be an adaptive response or indicative of immunosenescence. Our results suggest that future studies of immunosenescence should consider both active immune effectors in the body, such as PO, and the stored potential to express immune defences, such as proPO.

Presence of Multiparasite Infections Within Individual Colonies of Leaf-Cutter Ants

Host-parasite dynamics can be altered when a host is infected by multiple parasite genotypes. The different strains of parasite are expected to compete for the limited host resources, potentially affecting the survival and reproduction of the host as well as the infecting parasites. Fungus-growing ants, including the well-known leaf-cutters, are an emerging model system for studying the evolution and ecology of symbiosis and host-parasite dynamics. We examine whether the fungus gardens of leaf-cutter ants can be simultaneously infected by multiple strains of the fungal pathogen Escovopsis. Intensive sampling of Escovopsis was conducted from individual gardens, as well as between different garden chambers within individual colonies of leaf-cutting ants. Isolates obtained were genotyped by DNA sequencing. We found that, minimally, 67% of the individual colonies of the leaf-cutter ant genera Atta and Acromyrmex and 50% of the At. colombica garden chambers studied were simultaneously infected by multiple distinct Escovopsis strains. Experimental challenges showed that different Escovopsis strains do not exhibit obvious antagonism toward each other, suggesting that coinfecting strains of the parasite do not engage in interference competition, although interactions were not studied at the cellular level. Further research is needed to understand interparasite interactions between coinfecting Escovopsis strains and to understand the impact of multiparasite infections on the survival of leaf-cutter ant gardens.

Symbiont Interactions in a Tripartite Mutualism: Exploring the Presence and Impact of Antagonism between Two Fungus-Growing Ant Mutualists

Mutualistic associations are shaped by the interplay of cooperation and conflict among the partners involved, and it is becoming increasingly clear that within many mutualisms multiple partners simultaneously engage in beneficial interactions. Consequently, a more complete understanding of the dynamics within multipartite mutualism communities is essential for understanding the origin, specificity, and stability of mutualisms. Fungus-growing ants cultivate fungi for food and maintain antibiotic-producing Pseudonocardia actinobacteria on their cuticle that help defend the cultivar fungus from specialized parasites. Within both ant-fungus and ant-bacterium mutualisms, mixing of genetically distinct strains can lead to antagonistic interactions (i.e., competitive conflict), which may prevent the ants from rearing multiple strains of either of the mutualistic symbionts within individual colonies. The success of different ant-cultivar-bacterium combinations could ultimately be governed by antagonistic interactions between the two mutualists, either as inhibition of the cultivar by Pseudonocardia or vice versa. Here we explore cultivar-Pseudonocardia antagonism by evaluating in vitro interactions between strains of the two mutualists, and find frequent antagonistic interactions both from cultivars towards Pseudonocardia and vice versa. To test whether such in vitro antagonistic interactions affect ant colonies in vivo, we performed sub-colony experiments using species of Acromyrmex leaf-cutting ants. We created novel ant-fungus-bacterium pairings in which there was antagonism from one, both, or neither of the ants' microbial mutualists, and evaluated the effect of directional antagonism on cultivar biomass and Pseudonocardia abundance on the cuticle of workers within sub-colonies. Despite the presence of frequent in vitro growth suppression between cultivars and Pseudonocardia, antagonism from Pseudonocardia towards the cultivar did not reduce sub-colony fungus garden biomass, nor did cultivar antagonism towards Pseudonocardia reduce bacteria abundance on the cuticle of sub-colony workers. Our findings suggest that inter-mutualist antagonism does not limit what combinations of cultivar and Pseudonocardia strains Acromyrmex fungus-growing ants can maintain within nests.

Architecture of nests of Acromyrmex ( Moellerius ) balzani (Formicidae: Myrmicini: Attini) in pasture

Leaf-cutter ants Acromyrmex ( M. ) balzani (Emery), specialized in the cutting of gramineae are very common in pastures and agricultural crops of the South West of Bahia, with high density of nests. The objective of this work was to study aspects of nest architecture of A. balzani , located in pastures in the Southwest of Bahia, in order to search for the improvement of pest control strategies. Ten nests located in the cities of Vitoria da Conquista, Itapetinga, Tremedal and Itambe, BA were selected for study. Externally, the area, the volume of loose soil and the distance between the mount of loose soil and the tower were evaluated. The excavation was complete, opening gutters and cuttings in the soil profile. Afterwards, the measure of the height, width, depth, and the chambers height, regarding to the soil surface and to the coordinates of an orthogonal axis previously established were performed. The area and volume varied from 325 cm2 to 1880 cm2 and from 0.15 L to 5.88 L respectively. The distances between the loose soil mount and the tower varied from 2 to 37 cm. The total number of chambers per nest varied from 3 to 14, predominantly of chambers containing fungus, adult ants, and young forms. The higher concentration of chambers was at the first 30 cm and near the soil mount, but not under its projection.

Genetic polyethism in leaf-cutting ants

Females mating with multiple males (polyandry) is taxonomically widespread but is hard to explain in many animals in which it has no obvious direct benefits. In some, of which social insects are the best example, it is suggested that females benefit from the increased genetic diversity of their offspring. Social insect colonies express division of labor, and if genotypes behave differently (genetic polyethism), then genetically diverse colonies may be fitter. However, unequivocal evidence of genetic polyethism is only known from honey bees. Here, we show that such a genetic influence on behavior is also present in the leaf-cutting ant Acromyrmex echinatior. In 2 of the 3 colonies examined, we found that the offspring of some fathers (patrilines) were more likely to engage in waste management, whereas other patrilines were more likely to engage in foraging. When we increased the stimulus for waste management, the representation of patrilines in the ants that responded was the same as normally engaged in waste management in 2 colonies but differed in the third colony. The leaf-cutting ant A. echinatior therefore shows genetic polyethism. Although other factors such as disease resistance may have also played a role in the evolution of polyandry, the results suggest that genetic polyethism may be widespread in social insects and potentially of general importance in the evolution of polyandry.