Acromyrmex Species Research Articles

Chromosome Mapping of Ribosomal DNA Clusters in Four Leaf-cutting Ant Species of the Genus Acromyrmex, with Description of a Triploid Individual in A. laticeps

The increase of cytogenetic data in the leaf-cutting genus Acromyrmex has shown interesting contributions to the evolutionary and taxonomic approaches, including the creation of a new genus and the description of chromosomal patterns allowing comparative studies within Acromyrmex as well as between this genus and other similar taxa. The conserved chromosome number in Acromyrmex is a notable feature, with some distinguishable chromosomes among the species. In this study, we describe the diploid karyotype of Acromyrmex laticeps in addition to an individual with all the metaphases triploids, suggesting a triploid individual. To the best of our knowledge, this is the first report of polyploidy in a fungus-growing ant. Furthermore, we mapped 18S ribosomal DNA on the chromosomes of A. laticeps and three other Acromyrmex species. Among them, three species showed the ribosomal DNA cluster pattern observed in other previously studied members of this genus, with a terminal location in the largest subtelocentric pair. Meanwhile, Acromyrmex balzani had these ribosomal DNA clusters in its entire short arm. The role of triploidy in ant evolution has also been discussed. The increase in chromosomal data of leaf-cutting ants may provide significant insights into the evolution of this peculiar ant group.

Repellent potential of agroecological extracts: Exploring their efficacy against the leaf‐cutting ant Acromyrmex subterraneus in the laboratory

AbstractThe increasing restrictions on commercial toxic baits for controlling leaf‐cutting ants have started a “gold rush” for alternative compounds, with agroecological extracts emerging as a promising option. Here, we tested for the first time the repellent effects of concentrated and diluted ant extracts and cow urine solution on laboratory colonies of Acromyrmex subterraneus (Forel) (Hymenoptera: Formicidae). The agroecological extracts preparation and application followed the information provided by peasants from the Movimento dos Trabalhadores Rurais Sem Terra in Brazil who carry out agroecology. First, we applied the agroecological extracts in the middle segment of the upper path of a foraging trail to determine whether they act as a chemical barrier. All agroecological extracts induced the deviation of the foraging flow from the upper path to the lateral path. However, only the concentrated ant extract reduced the leaf delivery rate. Also, we applied the agroecological extracts directly on the leaf disks and conducted a paired selection test. The proportion of transported leaf disks with concentrated ant extract and cow urine solution was lower than that of control disks. We suggest that the observed repellent efficacy of the concentrated ant extract may stem from its content of chemical compounds belonging to the same chemical group as the alarm pheromones found by other Acromyrmex species. As for the cow urine solution, we attribute its repellent effect to the presence of urea. Our findings highlight a significant repellent potential of the concentrated ant extract and cow urine solution for leaf‐cutting ants, which may be applied both on foraging trails and on leaves of cultivated plants.

The interplay between leaf-cutter ants behaviour and social organization

AbstractLeaf-cutter ants (Atta and Acromyrmex species) exhibit complex social organizations that have fascinated scientists for decades. The leaf-cutter ants belong to a subgroup of fungus-growing ants, which live with fungus inside their nests. The behaviours exhibited by these ants are closely linked to their social organization, which involves intricate division of labour, caste systems, and cooperative tasks. This review article provides an overview of the behaviors associated with the social organization of leaf-cutter ants. It explores various aspects of their social organization, including foraging behavior, hitchhiking behaviour, hygienic behaviour, social organization and environmental influences. The leaf cutter ants have the instinct to forage, as they walk around the nest to cut leaf fragments from plants, then transport those fragments with their jaws and go back to the nest to cultivate a special fungus garden within the colony. By synthesizing existing knowledge, this review highlights the intricate relationships between behaviors and the social structure of leaf-cutter ants, shedding light on the underlying mechanisms and evolutionary implications. Furthermore, it identifies research gaps and proposes future directions for studying leaf-cutter ant behaviors, including emerging techniques and interdisciplinary approaches. The behaviors correlating with the social organization of leaf-cutter ants showcase the intricate mechanisms underlying their highly organized societies. In conclusion, this review contributes to our understanding of the fascinating behaviors exhibited by leaf-cutter ants and their implications in the broader context of social insect societies.

Entomopathogenic Strains of the Fungus Purpureocillium lilacinum Damage the Fungus Cultivar of Pest Leaf-Cutter Ants.

Purpureocillium lilacinum (Hypocreales: Ophiocordycipitaceae) is a cosmopolitan fungus not only pathogenic to insect and nematode hosts but also to other fungi. Although having one organism with multiple effects would be desirable in a biocontrol strategy, few studies have looked at the multiple roles one strain could play. This work shows how three strains of P. lilacinum, previously proven to be entomopathogenic to leaf-cutter ants (LCA), could degrade several strains of Leucoagaricus sp., the fungus cultivated by LCA as their food source. We isolated four strains of Leucoagaricus sp. from Acromyrmex and Atta LCA species, and we determined their species molecularly, as well as their clade identity (Leucoagaricus gongylophorus, clade-A). We observed the effects on growth rates on Petri dishes and the interaction of microscopic structures of both fungi on slides. All three P. lilacinum strains inhibited the growth of L. gongylophorus. They also degraded all L. gongylophorus isolated from the Acromyrmex species, causing hyphae expansion and degradation of the cell wall. However, only one of them succeeded in degrading the L. gongylophorus strain isolated from the Atta species. The results confirm the damage to the hyphae of ant cultivars and highlight the need for future studies that reveal whether such behavior is due to P. lilacinum's mycoparasitic behavior. Using a single P. lilacinum strain with a dual function that includes the degradation of the cultivar of LCA of both genera would be a very promising strategy for the biocontrol of one of the worst herbivore pests in the Neotropics.

Decoupled evolution of mating biology and social structure in Acromyrmex leaf-cutting ants

Insect societies vary greatly in their social structure, mating biology, and life history. Polygyny, the presence of multiple reproductive queens in a single colony, and polyandry, multiple mating by females, both increase the genetic variability in colonies of eusocial organisms, resulting in potential reproductive conflicts. The co-occurrence of polygyny and polyandry in a single species is rarely observed across eusocial insects, and these traits have been found to be negatively correlated in ants. Acromyrmex leaf-cutting ants are well-suited for investigating the evolution of complex mating strategies because both polygyny and polyandry co-occur in this genus. We used microsatellite markers and parentage inference in five South American Acromyrmex species to study how different selective pressures influence the evolution of polygyny and polyandry. We show that Acromyrmex species exhibit independent variation in mating biology and social structure, and polygyny and polyandry are not necessarily negatively correlated within genera. One species, Acromyrmex lobicornis, displays a significantly lower mating frequency compared to others, while another species, A. lundii, appears to have reverted to obligate monogyny. These variations appear to have a small impact on average intra-colonial relatedness, although the biological significance of such a small effect size is unclear. All species show significant reproductive skew between patrilines, but there was no significant difference in reproductive skew between any of the sampled species. We find that the evolution of social structure and mating biology appear to follow independent evolutionary trajectories in different species. Finally, we discuss the evolutionary implications that mating biology and social structure have on life history evolution in Acromyrmex leaf-cutting ants.Significance statementMany species of eusocial insects have colonies with multiple queens (polygyny), or queens mating with multiple males (polyandry). Both behaviors generate potentially beneficial genetic diversity in ant colonies as well as reproductive conflict. The co-occurrence of both polygyny and polyandry in a single species is only known from few ant species. Leaf-cutting ants have both multi-queen colonies and multiply mated queens, providing a well-suited system for studying the co-evolutionary dynamics between mating behavior and genetic diversity in colonies of eusocial insects. We used microsatellite markers to infer the socio-reproductive behavior in five South American leaf-cutter ant species. We found that variation in genetic diversity in colonies was directly associated with the mating frequencies of queens, but not with the number of queens in a colony. We suggest that multi-queen nesting and mating frequency evolve independently of one another, indicating that behavioral and ecological factors other than genetic diversity contribute to the evolution of complex mating behaviors in leaf-cutting ants.

Cytogenetic studies on the social parasite Acromyrmex ameliae (Formicidae: Myrmicinae: Attini) and its hosts reveal chromosome fusion in Acromyrmex

The ant Acromyrmex ameliae is a social parasite of two leaf-cutting ant subspecies: Acromyrmex subterraneus subterraneus and A. subterraneus brunneus. Cytogenetic data are available for 14 species of Acromyrmex and all of them possess 2n = 38 chromosomes. In this study, chromosome number, heterochromatin detection, and detection of AT and GC-rich blocks of colonies of A. ameliae and its hosts were carried out. Additionally, the detection of nucleolus organizer regions and 18S rDNA clusters in chromosomes of the parasite and physical mapping of telomeres were undertaken. The same chromosome number and morphology were detected for the hosts 2n = 38 (10m + 14sm + 12st + 2a), while the females and males of the social parasite A. ameliae presented 2n = 36 (10m + 16sm + 8st + 2a) and n = 18 (5m + 8sm + 4st + 1a). In both A. ameliae and its hosts, the terminal region on the short arm of the largest subtelocentric pair is heterochromatic GC-rich, and this region corresponded to the 18S rDNA clusters in the parasite. The short arms of several chromosomes were heterochromatin-rich. The telomeric probe hybridized telomeres on all chromosomes of the parasite and was not detected in intrachromosomal regions. Through a comparative cytogenetic analysis, we hypothesize that the karyotype of A. ameliae (2n = 36) originated from a chromosomal rearrangement that reduced the number of chromosomes from 38 to 36; as available data on the genus Acromyrmex show that all other species possess 38 chromosomes, representing 45% of the 33 valid species in this genus. The mechanism of the chromosome rearrangement is discussed. Thus, the chromosome number observed in A. ameliae is a derivation from the genus. Our data show variation in the chromosomal number in Acromyrmex and suggest that analyses of the karyotypes of parasite species can yield novel insights with regards to the evolution of this genus.

Economic Injury Level and Economic Threshold as required by Forest Stewardship Council for management of leaf‐cutting ants in forest plantations

Abstract The most commonly used method to control insect pests, such as leaf‐cutting ants (LCA) in forest plantations, is the application of chemical insecticides, but their use was forbidden in plantations certified by the Forest Stewardship Council because of their negative effect on the environment. A 5‐year derogation of their prohibition was obtained in 2016 in Argentina, on the condition that forestry companies reduce pesticide doses by 5% per year and inform the Economic Injury Level (EIL). The EIL and the Economic Threshold (ET) were estimated for the first time for the Acromyrmex genus in a Salicaceae forestation. As a model, we used the damage caused by the most detrimental LCA species of Argentina (Acromyrmex lundii) in the first, most critical year of a willow plantation (Salix nigra) in the lower basin of the Paraná River. The EIL (8–15 nest/ha) was useful to demonstrate the real magnitude of the damage caused by A. lundii, which justified the need for its control. The ET for profitable production (∼3 nests/ha) would not be operatively applicable in a long‐term management program in our study site. This methodology could be extrapolated to other Neotropical forest plantations attacked by Acromyrmex species.

Comparative FISH-mapping of TTAGG telomeric sequences to the chromosomes of leafcutter ants (Formicidae, Myrmicinae): is the insect canonical sequence conserved?

Telomeric sequences are conserved across species. The most common sequence reported among insects is (TTAGG)n, but its universal occurrence is not a consensus because other canonical motifs have been reported. In the present study, we used fluorescence in situ hybridization (FISH) using telomeric probes with (TTAGG)6 repeats to describe the telomere composition of leafcutter ants. We performed the molecular cytogenetic characterization of six Acromyrmex Mayr, 1865 and one Atta Fabricius, 1804 species (Acromyrmex ambiguus (Emery, 1888), Ac. crassispinus (Forel, 1909), Ac. lundii (Guérin-Mèneville, 1838), Ac. nigrosetosus (Forel, 1908), Ac. rugosus (Smith, 1858), Ac. subterraneus subterraneus (Forel, 1893), and Atta sexdens (Linnaeus, 1758)) and described it using a karyomorphometric approach on their chromosomes. The diploid chromosome number 2n = 38 was found in all Acromyrmex species, and the karyotypic formulas were as follows: Ac. ambiguus 2K = 14M + 12SM + 8ST + 4A, Ac. crassispinus 2K = 12M + 20SM + 4ST + 2A, Ac. lundii 2K = 10M + 14SM + 10ST + 4A, Ac. nigrosetosus 2K = 12M + 14SM + 10ST + 2A, and Ac. subterraneus subterraneus 2K = 14M + 18SM + 4ST + 2A. The exact karyotypic formula was not established for Ac. rugosus. FISH analyses revealed the telomeric regions in all the chromosomes of the species studied in the present work were marked by the (TTAGG)6 sequence. These results reinforce the premise that Formicidae presents high homology between their genera for the presence of the canonical sequence (TTAGG)n.

Dillapiole in Piper holtonii as an Inhibitor of the Symbiotic Fungus Leucoagaricus gongylophorus of Leaf-Cutting Ants.

Plants of the Piperaceae family are studied for their diverse secondary metabolism with a vast array of compounds that act as chemical defense agents against herbivores. Of all the agricultural pests, the management of insects is a highly significant challenge in the Neotropics, and ants of the Attini tribe pose a major problem. Due to their symbiotic association with the fungus Leucoagaricus gongylophorus (Möller) Singer (Agaricaceae), the species of Atta and Acromyrmex have exhaustive foraging activity which has intensified as deforestation and monoculture farming have increased. The control of leaf-cutting ants is still carried out with synthetic products with negative consequences to the environment and human health. In search for natural and sustainable alternatives to synthetic pesticides, Piper holtonii C. DC. was selected among other plant species after field observations of the foraging activity of Atta cephalotes, which revealed that P. holtonii was never chosen by ants. In vitro evaluation of an ethanol extract of the leaves of P. holtonii resulted in promising inhibitory activity (IC50 102 ppm) against L. gongylophorus. Subsequently, bioassay-guided fractionation led to the isolation of the phenylpropanoid dillapiole, which was also detected in the essential oil. This compound demonstrated inhibition of the fungus with an IC50 of 38 ppm. Considering the symbiotic relationship between the Attini ants and L. gongylophorus, the negative effect on the survival of one of the organisms will affect the survival of the other, so dillapiole or standardized essential oil extracts of P. holtonii containing this active principle could be a unique and useful source as a control agent for leaf cutting-ants.

Diversidad de micobiota en diferentes especies de hormigas cortadoras de hojas y sitios de Argentina

Attine ants are a monophyletic group comprising more than 230 species, distributed exclusively in the New World. All higher Attini ants depend on the cultivation of fungus gardens for food, and those gardens are continuously exposed to alien microorganisms. The present work describes, for the first time, the composition, relative frequency and the richness of the fungal species comprising the mycobiota from the garden of the most abundant Acromyrmex species from different regions of Argentina. We also compared, using a multivariate approach, the mycobiota composition with the purpose of testing two hypotheses: 1) whether mycobiota is defined by the ant species, or 2) whether mycobiota is determined by the geographical region. We found that each fungal community had a particular composition and abundance: the species that were highly frequent in a community or site, could be rare or altogether absent in another community or site. Species richness, as well as the theoretical maximum richness, also changed with locations. Furthermore, we found that different ant species at the same site had similar mycobiotas, whereas the same ant species in distant locations had only a few fungal species in common. Therefore, we concluded that mycobiota composition changed considerably with geographical site and was not dependent on the ant species. Our results provide an additional understanding of the leaf cutting-ants system, confirming that the garden of leaf cutting ants nests is a complex and dynamic fungal community which is dependent on the site where it was located.DOI: https://doi.org/10.25260/EA.17.27.3.0.488

Insights into the biodiversity and causes of distribution of potential entomopathogens associated with leaf-cutting ants

To our knowledge, this work is the first large-scaled, systematic survey of potential entomopathogens associated with worker ants of several Acromyrmex species. The study was performed at nine sites located in five Phytogeographical Provinces across Argentina. We recorded 28 species of fungi with entomopathogenic behaviour, which infected 24.3% of the 4737 collected ants from 94 colonies. Fusarium oxysporum and F. solani were the most widely distributed, followed by Purpureocillium lilacinum and Beauveria bassiana. The occurrence of species across nests within the same site varied from null to 98%. We did not detect any systematic association between fungi and site, Phytogeographical Province or ant species. Instead the microhabitats that surround each nest appear to play an important role in defining entomopathogen communities. We found that climatic variables like maximum temperature, dew point, and relative humidity helped to account for the distribution of these fungi at the site scale. Besides, colonies from undisturbed sites showed higher abundance of infections with entomopathogens than those from disturbed ones. These results greatly improve the knowledge of the ecology of the filamentous fungi associated with leaf-cutting ants. In addition, we proposed that the combination of the entomopathogen virulence and the resistance of ant colonies may be an important but overlooked effect influencing the diversity of entomopathogens.

Geographical Distribution Patterns and Niche Modeling of the Iconic Leafcutter Ant Acromyrmex striatus (Hymenoptera: Formicidae).

Ants are considered one of the most successful groups in the planet’s evolutionary history. Among them highlights the fungus-farming ants of the genera Atta and Acromyrmex that occur throughout most of the Americas. Within the Acromyrmex genus, the species A. striatus distinguishes from other Acromyrmex species as its morphology and karyotype differ from its congeners. This species is found in open environments of dry climate in the southern States of Brazil, Argentina, Paraguay and Uruguay; however, little is known about the current distribution of the species. This article aimed to investigate the current distribution of the species by compiling its known distribution and discussing its distributional range. To achieve this, published and unpublished data obtained through a literature search and active collections in various locations were compiled. Published and unpublished data revealed that 386 colonies were recorded, distributed across four countries where its occurrence is known. Environmental factors, such as temperature, humidity, soil type and vegetation, as well as historical geological and climate events that have modified Earth’s surface may have influenced species distribution patterns. In the Neotropics, the environmental factors that most impacted the distribution of species were the glaciation periods that occurred in the Quaternary, leading to a great migratory process. These factors may have contributed to the current geographical distribution of A. striatus.

Waste of leaf-cutting ants: disposal, nest structure, and abiotic soil factors around internal waste chambers

Leaf-cutting ants produce large quantities of waste that harbor bacteria and fungi that are harmful to the colony. To be protected from these pathogens, the workers of Atta species present a sophisticated organization to manage harmful material, which can be deposited outside the nest or in internal chambers. However, little is known about the behavior of Acromyrmex species in handling and disposal of waste. Due to some observations, we assume that the same species of Acromyrmex can deposit waste outside the nest and into internal chambers and raise the following question: what determines the occurrence of internal waste chambers in Acromyrmex? To address this question, we verified whether nest depth influences the waste-chamber occurrence. We also verified the nest structure and the abiotic factors of soil beside each waste-chamber: pH and water content of the soil. For this, eight nests were excavated for Acromyrmex balzani and Acromyrmex rugosus rugosus. We verified that not only can the same leaf-cutting ant species deposit debris both outside and inside the nest but also the same nest can present internal chambers and external waste deposit. The soil beside the waste chamber always presented an acidic pH, while the humidity varied widely. Our results showed that the nest depth was highly correlated with the depth of the waste chamber (p = 0.0003) and probably has some influence on waste disposal. The characteristics of the nest and the role of depth in the choice of waste chamber location are discussed.

Differences in Forage-Acquisition and Fungal Enzyme Activity Contribute to Niche Segregation in Panamanian Leaf-Cutting Ants

The genera Atta and Acromyrmex are often grouped as leaf-cutting ants for pest management assessments and ecological surveys, although their mature colony sizes and foraging niches may differ substantially. Few studies have addressed such interspecific differences at the same site, which prompted us to conduct a comparative study across six sympatric leaf-cutting ant species in Central Panama. We show that foraging rates during the transition between dry and wet season differ about 60 fold between genera, but are relatively constant across species within genera. These differences appear to match overall differences in colony size, especially when Atta workers that return to their nests without leaves are assumed to carry liquid food. We confirm that Panamanian Atta specialize primarily on tree-leaves whereas Acromyrmex focus on collecting flowers and herbal leaves and that species within genera are similar in these overall foraging strategies. Species within genera tended to be spaced out over the three habitat categories that we distinguished (forest, forest edge, open grassland), but each of these habitats normally had only a single predominant Atta and Acromyrmex species. We measured activities of twelve fungus garden decomposition enzymes, belonging to the amylases, cellulases, hemicellulases, pectinases and proteinases, and show that average enzyme activity per unit of fungal mass in Atta gardens is lower than in Acromyrmex gardens. Expression profiles of fungal enzymes in Atta also appeared to be more specialized than in Acromyrmex, possibly reflecting variation in forage material. Our results suggest that species- and genus-level identities of leaf-cutting ants and habitat-specific foraging profiles may give predictable differences in the expression of fungal genes coding for decomposition enzymes.

Parasitoids of Acromyrmex (Hymenoptera: Formicidae) Leaf-Cutting Ants in Continuous and Fragmented Atlantic Forest

Fragmentation of the habitat is a major threat to biodiversity in Atlantic forest. Parasitoids seem to be particularly susceptible to habitat fragmentation. This study evaluated whether habitat fragmentation affected the interactions between phorid parasitoids and their Acromyrmex leaf-cutting ant host. Host density, and parasitoid species richness, abundance and proportion of nests with phorids were compared for a fragmented landscape and a well-preserved continuous forest in the Atlantic forest in Rio de Janeiro. Five Acromyrmex species and seven species of phorid parasitoids were found, most of them attacking exclusively Acromyrmex niger (Smith). Host nest density was similar in continuous and fragmented forests, and host species density was higher in fragmented forest. Parasitoid species richness, abundance and proportion of ant nests with phorids were higher in the continuous forest. This work showed for the first time the negative effect that forest fragmentation has on parasitoid species of Acromyrmex ants, apparently due to phorid inability to reach fragments. However, even when phorid abundance was considerably reduced in forest fragments, phorids of some species were able to parasitize ants there. In addition, the quantitative interactions among Acromyrmex ants and their parasitoids in Atlantic forest are described for the first time.

Ensambles de ácaros oribátidos en hormigueros de Acromyrmex spp. (Hymenoptera, Formicidae)

Nests of Acromyrmex spp. exhibit aboveground elevations (tumulus) which maintain a stable microclimate, offering an attractive environment for oribatids and other soil invertebrates. We studied assemblages of Oribatid mites (Acari: Oribatida) in tumuli of two species of Acromyrmex in Integral Reserve Laguna de Los Padres, Buenos Aires province. Samples from nests of Acromyrmex lundii and A. ambiguus, and from their surrounding soil revealed the presence of 1440 oribatids belonging to 18 families, 25 genera and 34 species. Three of the species had not been previously described. Oribatid mites was the most abundant group and were more abundant on tumulus than in the surrounding soil. Conversely, its richness, diversity and evenness did not differ between sites or between ant species. The species composition varied between sites but the most abundant species were the same in tumuli and soil. We found an inverse relationship between density of substrate (tumulus or soil) and density of Oribatida. Potential myrmecophilous species were identified. Nests of Acromyrmex spp. exhibit aboveground elevations (tumulus) which maintain a stable microclimate, offering an attractive environment for oribatids and other soil invertebrates. We studied assemblages of Oribatid mites (Acari: Oribatida) in tumuli of two species of Acromyrmex in Integral Reserve Laguna de Los Padres, Buenos Aires province. Samples from nests of Acromyrmex lundii and A. ambiguus, and from their surrounding soil revealed the presence of 1440 oribatids belonging to 18 families, 25 genera and 34 species. Three of the species had not been previously described. Oribatid mites was the most abundant group and were more abundant on tumulus than in the surrounding soil. Conversely, its richness, diversity and evenness did not differ between sites or between ant species. The species composition varied between sites but the most abundant species were the same in tumuli and soil. We found an inverse relationship between density of substrate (tumulus or soil) and density of Oribatida. Potential myrmecophilous species were identified.

Attaichnus kuenzelii revisited: A Miocene record of fungus-growing ants from Argentina

Attaichnus kuenzelii is a nest of leaf-cutting ants from the Miocene of La Pampa, Argentina, described by Laza (1982) in a pioneer paper for insect palaeoichnology. The type locality was revisited to update morphological description, affinities, and palaeoenvironmental conditions. In present day outcrops, A. kuenzelii shows no mounds or high concentration of interconnected chambers as originally described, but well separated and regularly distributed chambers, which at most may compose groups of two or three. This updated morphology suggests that A. kuenzelii can be attributed to Attini other than Atta, probably species of Acromyrmex or Trachymyrmex. Associated trace fossils are large vertebrate burrows, rhizoliths, meniscate burrows, and scarce Coprinisphaera. Other specimens of dung beetle and bee trace fossils found in the same palaeosols were probably constructed by younger producers on soils developed on post-Miocene sediments, which are mostly eroded today. Evidence for this assumption is the lack of carbonate cementation and presence of organic remains in these traces. The reinterpretation of A. kuenzelii as a nest of species of Acromyrmex or Trachymyrmex modifies the original assumption of a northern subtropical palaeoenvironment during the Miocene for this area based on the distribution of Atta species. The new affinities proposed in combination with the studied palaeosols interpreted as mollisols, phytoliths, and previous palaeontological, and sedimentological evidence suggest an open palaeoenvironment under a temperate climate, dominated by grasses.

Cytogenetic and Molecular Analyses Reveal a Divergence between Acromyrmex striatus (Roger, 1863) and Other Congeneric Species: Taxonomic Implications

The leafcutter ants, which consist of Acromyrmex and Atta genera, are restricted to the New World and they are considered the main herbivores in the neotropics. Cytogenetic studies of leafcutter ants are available for five species of Atta and 14 species of Acromyrmex, both including subspecies. These two ant genera have a constant karyotype with a diploid number of 22 and 38 chromosomes, respectively. The most distinct Acromyrmex species from Brazil is A. striatus, which is restricted to the southern states of Santa Catarina and Rio Grande do Sul. Several cytogenetic and phylogenetic studies have been conducted with ants, but the karyotypic characterization and phylogenetic position of this species relative to leafcutter ants remains unknown. In this study, we report a diploid number of 22 chromosomes for A. striatus. The phylogenetic relationship between A. striatus and other leafcutter ants was estimated based on the four nuclear genes. A. striatus shared the same chromosome number as Atta species and the majority of metacentric chromosomes. Nuclear data generated a phylogenetic tree with a well-supported cluster, where A. striatus formed a different clade from other Acromyrmex spp. This combination of cytogenetic and molecular approaches provided interesting insights into the phylogenetic position of A. striatus among the leafcutter ants and the tribe Attini.

Interactions between Ants and Non‐myrmecochorous Fruits in Miconia (Melastomataceae) in a Neotropical Savanna

AbstractThe aim of this study was to evaluate the role of ants as secondary seed dispersers of six primarily bird‐dispersed Miconia species in the cerrados of southeastern Brazil. Vertebrate exclosure and seed germination experiments were performed for M. albicans, M. alborufescens, M. corallina, M. ferruginata, M. ibaguensis, and M. irwinii. Excluding vertebrates did not significantly alter fruit removal rate for any of the Miconia species relative to open controls. Fruits on stalks and fallen fruits were removed and transported to nests mainly by species of Atta, Acromyrmex, and Ectatomma (dispersal distance ranging from 0.1 to 45.2 m), while Camponotus ants tended to be observed removing the fruit pulp (seed cleaning) where the fruits were found. Seed manipulation by Atta decreased germination of M. irwinii, but not M. ferruginata. Germination did not occur in intact fruits, and thus seed cleaning was an important service provided by the ants. Ant nest soils did not inhibit germination of any of the Miconia species, suggesting they are a good substrate for long‐lived Miconia seeds. We conclude that ant activity could have important effects on the fate of Miconia seeds adapted for bird dispersal.

Generation of Nutrients and Detoxification: Possible Roles of Yeasts in Leaf-Cutting Ant Nests.

The possible roles played by yeasts in attine ant nests are mostly unknown. Here we present our investigations on the plant polysaccharide degradation profile of 82 yeasts isolated from fungus gardens of Atta and Acromyrmex species to demonstrate that yeasts found in ant nests may play the role of making nutrients readily available throughout the garden and detoxification of compounds that may be deleterious to the ants and their fungal cultivar. Among the yeasts screened, 65% exhibited cellulolytic enzymes, 44% exhibited pectinolytic activity while 27% and 17% possess enzyme systems for the degradation of protease and amylase, respectively. Galacturonic acid, which had been reported in previous work to be poorly assimilated by the ant fungus and also to have a negative effect on ants’ survival, was assimilated by 64% and 79% of yeasts isolated from nests of A. texana and Acromyrmex respectively. Our results suggest that yeasts found in ant nests may participate in generation of nutrients and removal of potentially toxic compounds, thereby contributing to the stability of the complex microbiota found in the leaf-cutting ant nests.