Phylogenetic patterns of ant–fungus associations indicate that farming strategies,not only a superior fungal cultivar,explain the ecological success of leafcutter ants

To elucidate fungicultural specializations contributing to ecological dominance of leafcutter ants, we estimate the phylogeny of fungi cultivated by fungus‐growing (attine) ants, including fungal cultivars from (i) the entire leafcutter range from southern South America to southern North America, (ii) all higher‐attine ant lineages (leafcutting genera Atta, Acromyrmex; nonleafcutting genera Trachymyrmex, Sericomyrmex) and (iii) all lower‐attine lineages. Higher‐attine fungi form two clades, Clade‐A fungi (Leucocoprinus gongylophorus, formerly Attamyces) previously thought to be cultivated only by leafcutter ants, and a sister clade, Clade‐B fungi, previously thought to be cultivated only by Trachymyrmex and Sericomyrmex ants. Contradicting this traditional view, we find that (i) leafcutter ants are not specialized to cultivate only Clade‐A fungi because some leafcutter species ranging across South America cultivate Clade‐B fungi; (ii) Trachymyrmex ants are not specialized to cultivate only Clade‐B fungi because some Trachymyrmex species cultivate Clade‐A fungi and other Trachymyrmex species cultivate fungi known so far only from lower‐attine ants; (iii) in some locations, single higher‐attine ant species or closely related cryptic species cultivate both Clade‐A and Clade‐B fungi; and (iv) ant–fungus co‐evolution among higher‐attine mutualisms is therefore less specialized than previously thought. Sympatric leafcutter ants can be ecologically dominant when cultivating either Clade‐A or Clade‐B fungi, sustaining with either cultivar‐type huge nests that command large foraging territories; conversely, sympatric Trachymyrmex ants cultivating either Clade‐A or Clade‐B fungi can be locally abundant without achieving the ecological dominance of leafcutter ants. Ecological dominance of leafcutter ants therefore does not depend primarily on specialized fungiculture of L. gongylophorus (Clade‐A), but must derive from ant–fungus synergisms and unique ant adaptations.     

Evolutionarily advanced ant farmers rear polyploid fungal crops

Innovative evolutionary developments are often related to gene or genome duplications. The crop fungi of attine fungus‐growing ants are suspected to have enhanced genetic variation reminiscent of polyploidy, but this has never been quantified with cytological data and genetic markers. We estimated the number of nuclei per fungal cell for 42 symbionts reared by 14 species of Panamanian fungus‐growing ants. This showed that domesticated symbionts of higher attine ants are polykaryotic with 7–17 nuclei per cell, whereas nonspecialized crops of lower attines are dikaryotic similar to most free‐living basidiomycete fungi. We then investigated how putative higher genetic diversity is distributed across polykaryotic mycelia, using microsatellite loci and evaluating models assuming that all nuclei are either heterogeneously haploid or homogeneously polyploid. Genetic variation in the polykaryotic symbionts of the basal higher attine genera Trachymyrmex and Sericomyrmex was only slightly enhanced, but the evolutionarily derived crop fungi of Atta and Acromyrmex leaf‐cutting ants had much higher genetic variation. Our opposite ploidy models indicated that the symbionts of Trachymyrmex and Sericomyrmex are likely to be lowly and facultatively polyploid (just over two haplotypes on average), whereas Atta and Acromyrmex symbionts are highly and obligatorily polyploid (ca. 5–7 haplotypes on average). This stepwise transition appears analogous to ploidy variation in plants and fungi domesticated by humans and in fungi domesticated by termites and plants, where gene or genome duplications were typically associated with selection for higher productivity, but allopolyploid chimerism was incompatible with sexual reproduction.     

The molecular phylogenetics of Trachymyrmex Forel ants and their fungal cultivars provide insights into the origin and coevolutionary history of ‘higher‐attine’ ant agriculture

The fungus‐growing ants and their fungal cultivars constitute a classic example of a mutualism that has led to complex coevolutionary dynamics spanning c. 55–65 Ma. Of the five agricultural systems practised by fungus‐growing ants, higher‐attine agriculture, of which leaf‐cutter agriculture is a derived subset, remains poorly understood despite its relevance to ecosystem function and human agriculture across the Neotropics and parts of North America. Among the ants practising higher‐attine agriculture, the genus Trachymyrmex Forel, as currently defined, shares most‐recent common ancestors with both the leaf‐cutter ants and the higher‐attine genera Sericomyrmex Mayr and Xerolitor Sosa‐Calvo et al. Although previous molecular‐phylogenetic studies have suggested that Trachymyrmex is a paraphyletic grade, until now insufficient taxon sampling has prevented a full investigation of the evolutionary history of this group and limited the possibility of resolving its taxonomy. Here we describe the results of phylogenetic analyses of 38 Trachymyrmex species, including 27 of the 49 described species and at least 11 new species, using four nuclear markers, as well as phylogenetic analyses of the fungi cultivated by 23 species of Trachymyrmex using two markers. We generated new genetic data for 112 ants (402 new gene sequences) and 95 fungi (153 new gene sequences). Our results corroborate previous findings that Trachymyrmex, as currently defined, is paraphyletic. We propose recognizing two new genera, Mycetomoellerius gen.n. and Paratrachymyrmex gen.n. , and restricting the continued use of Trachymyrmex to the clade of nine largely North American species that contains the type species [Trachymyrmex septentrionalis (McCook)] and that is the sister group of the leaf‐cutting ants. Our fungal cultivar phylogeny generally corroborates previously observed broad patterns of ant–fungus association, but it also reveals further violations of those patterns. Higher‐attine fungi are divided into two groups: (i) the single species Leucoagaricus gongylophorus (Möller); and (ii) its sister clade, consisting of multiple species, recently referred to as Leucoagaricus Singer ‘clade B’. Our phylogeny indicates that, although most non‐leaf‐cutting higher‐attine ants typically cultivate species in clade B, some species cultivate L. gongylophorus, whereas still others cultivate fungi typically associated with lower‐attine agriculture. This indicates that the attine agricultural systems, which are currently defined by associations between ants and fungi, are not entirely congruent with ant and fungal phylogenies. They may, however, be correlated with as yet poorly understood biological traits of the ants and/or of their microbiomes.     

FACULTATIVE SEX ALLOCATION BIASING BY WORKERS IN SOCIAL HYMENOPTERA

A single-locus two-allele model is analyzed to determine the invasion conditions for facultative biasing of colony sex allocation by hymenopteran workers in response to queen mating frequency, for a situation in which colonies have a single queen mated to one or two males. Facultative biasing of sex allocation towards increased male production in double mated colonies and increased female production in single mated colonies can both invade when the population sex allocation ratio is at the worker optimum. However, when the population sex allocation ratio is more male biased than the worker optimum, plausibly due to mixed queen and worker control, it is likely that only increased female allocation in colonies perceived by the workers to have single mated queens can invade. In this case, the frequency of mistakes made by workers in assessing queen mating frequency is an important constraint on the invasion of facultative male biasing in colonies perceived to have a double mated queen. When the population sex allocation ratio is not between the optima for workers in single and double mated colonies, plausibly due to strong queen control, then facultative biasing cannot invade. In this situation, workers in all colonies should attempt to bias allocation towards increased females. Worker male production in queenright colonies (provided not all males are worker-derived), unequal sperm use by double mated queens, and the amount of facultative biasing, do not alter these results.     

Polygynous supercolonies of the acacia‐ant Pseudomyrmex peperi,an inferior colony founder

In ant–plant protection mutualisms, plants provide nesting space and nutrition to defending ants. Several plant–ants are polygynous. Possessing more than one queen per colony can reduce nestmate relatedness and consequently the inclusive fitness of workers. Here, we investigated the colony structure of the obligate acacia‐ant Pseudomyrmex peperi, which competes for nesting space with several congeneric and sympatric species. Pseudomyrmex peperi had a lower colony founding success than its congeners and thus, appears to be competitively inferior during the early stages of colony development. Aggression assays showed that P. peperi establishes distinct, but highly polygynous supercolonies, which can inhabit large clusters of host trees. Analysing queens, workers, males and virgin queens from two supercolonies with eight polymorphic microsatellite markers revealed a maximum of three alleles per locus within a colony and, thus, high relatedness among nestmates. Colonies had probably been founded by one singly mated queen and supercolonies resulted from intranidal mating among colony‐derived males and daughter queens. This strategy allows colonies to grow by budding and to occupy individual plant clusters for time spans that are longer than an individual queen’s life. Ancestral states reconstruction indicated that polygyny represents the derived state within obligate acacia‐ants. We suggest that the extreme polygyny of Pseudomyrmex peperi, which is achieved by intranidal mating and thereby maintains high nestmate relatedness, might play an important role for species coexistence in a dynamic and competitive habitat.     

Fitness consequences of nest infiltration by the mutualist‐exploiter Megalomyrmex adamsae

1. Fungus‐growing ants are obligate mutualists. Their nutrient‐rich fungus garden provides a valuable food store that sustains the ant hosts, but can also attract social parasites. 2. The ‘guest ant' Megalomyrmex adamsae Longino parasitises the fungus‐growing Trachymyrmex zeteki Weber queen just after nest founding. The parasitic queen infiltrates the incipient nest, builds a cavity in the fungal garden, and lays eggs that develop into workers and reproductive males and females. 3. This study compared young parasitised and non‐parasitised laboratory colonies by measuring garden growth and biomass, and the number of host workers and reproductives. Host queen survival and parasite colony growth were also monitored. 4. Parasitised Trachymyrmex colonies had reduced host worker and alate numbers, as well as lower garden biomass, compared with non‐parasitised control colonies, confirming that M. adamsae is a xenobiotic social parasite. Host queen survival was not significantly different between parasitised and control colonies. 5. This is the first study that experimentally infects host colonies with a xenobiotic social parasite to measure fitness cost to the host. The natural history of M. adamsae and the fungus‐growing ant mutualism are evaluated in the context of three general predictions of (Bronstein, Ecology Letters, 4 , 277–287, 2001a) regarding the cost of mutualism exploiters.     

Phylogenomic species delimitation and host‐symbiont coevolution in the fungus‐farming ant genus Sericomyrmex Mayr (Hymenoptera: Formicidae): ultraconserved elements (UCEs) resolve a recent radiation

Ants in the Neotropical genus Sericomyrmex Mayr cultivate fungi for food. Both ants and fungi are obligate, coevolved symbionts. The taxonomy of Sericomyrmex is problematic because the morphology of the worker caste is generally homogeneous across all of the species within the genus, species limits are vague, and the relationships between them are unknown. We used ultraconserved elements (UCEs) as genome‐scale markers to reconstruct evolutionary history and to infer species boundaries in Sericomyrmex. We recovered an average of ～990 UCE loci for 88 Sericomyrmex samples from across the geographical range of the genus as well as for five outgroup taxa. Using maximum likelihood and species‐tree approaches, we recovered nearly identical topologies across datasets with 50–95% matrix completeness. We identify nine species‐level lineages in Sericomyrmex, including two new species. This is less than the previously described 19 species, even accounting for two species for which we had no UCE samples, which brings the total number of Sericomyrmex species to 11. Divergence‐dating analyses recovered 4.3 Ma as the crown‐group age estimates for Sericomyrmex, indicating a recent, rapid radiation. We also sequenced mitochondrial cytochrome oxidase subunit I (COI) for 125 specimens. Resolution and support for clades in our COI phylogeny are weak, indicating that COI is not an appropriate species‐delimitation tool. However, taxa within species consistently cluster together, suggesting that COI is useful as a species identification (‘DNA barcoding’) tool. We also sequenced internal transcribed spacer (ITS) and large subunit (LSU) for 32 Sericomyrmex fungal cultivars. The fungal phylogeny confirms that Sericomyrmex fungi are generalized higher‐attine cultivars, interspersed with Trachymyrmex‐associated fungal species, indicating cultivar sharing and horizontal transfer between these two genera. Our results indicate that UCEs offer immense potential for delimiting and resolving relationships of problematic, recently diverged species.     

Sex allocation conflict between queens and workers in Formica pratensis wood ants predicts seasonal sex ratio variation

Sex allocation theory predicts parents should adjust their investment in male and female offspring in a way that increases parental fitness. This has been shown in several species and selective contexts. Yet, seasonal sex ratio variation within species and its underlying causes are poorly understood. Here, we study sex allocation variation in the wood ant Formica pratensis. This species displays conflict over colony sex ratio as workers and queens prefer different investment in male and female offspring, owing to haplodiploidy and relatedness asymmetries. It is unique among Formica ants because it produces two separate sexual offspring cohorts per season. We predict sex ratios to be closer to queen optimum in the early cohort but more female‐biased and closer to worker optimum in the later one. This is because the power of workers to manipulate colony sex ratio varies seasonally with the availability of diploid eggs. Consistently, more female‐biased sex ratios in the later offspring cohort over a three‐year sampling period from 93 colonies clearly support our prediction. The resulting seasonal alternation of sex ratios between queen and worker optima is a novel demonstration how understanding constraints of sex ratio adjustment increases our ability to predict sex ratio variation.     

Sex investment ratios in eusocial Hymenoptera support inclusive fitness theory

Inclusive fitness theory predicts that sex investment ratios in eusocial Hymenoptera are a function of the relatedness asymmetry (relative relatedness to females and males) of the individuals controlling sex allocation. In monogynous ants (with one queen per colony), assuming worker control, the theory therefore predicts female‐biased sex investment ratios, as found in natural populations. Recently, E.O. Wilson and M.A. Nowak criticized this explanation and presented an alternative hypothesis. The Wilson–Nowak sex ratio hypothesis proposes that, in monogynous ants, there is selection for a 1 : 1 numerical sex ratio to avoid males remaining unmated, which, given queens exceed males in size, results in a female‐biased sex investment ratio. The hypothesis also asserts that, contrary to inclusive fitness theory, queens not workers control sex allocation and queen–worker conflict over sex allocation is absent. Here, I argue that the Wilson–Nowak sex ratio hypothesis is flawed because it contradicts Fisher's sex ratio theory, which shows that selection on sex ratio does not maximize the number of mated offspring and that the sex ratio proposed by the hypothesis is not an equilibrium for the queen. In addition, the hypothesis is not supported by empirical evidence, as it fails to explain ‘split’ (bimodal) sex ratios or data showing queen and worker control and ongoing queen–worker conflict. By contrast, these phenomena match predictions of inclusive fitness theory. Hence, the Wilson–Nowak sex ratio hypothesis fails both as an alternative hypothesis for sex investment ratios in eusocial Hymenoptera and as a critique of inclusive fitness theory.     

Sex-ratio conflict between queens and workers in eusocial Hymenoptera: mechanisms, costs, and the evolution of split colony sex ratios

Because workers in the eusocial Hymenoptera are more closely related to sisters than to brothers, theory predicts that natural selection should act on them to bias (change) sex allocation to favor reproductive females over males. However, selection should also act on queens to prevent worker bias. We use a simulation approach to analyze the coevolution of this conflict in colonies with single, once-mated queens. We assume that queens bias the primary (egg) sex ratio and workers bias the secondary (adult) sex ratio, both at some cost to colony productivity. Workers can bias either by eliminating males or by directly increasing female caste determination. Although variation among colonies in kin structure is absent, simulations often result in bimodal (split) colony sex ratios. This occurs because of the evolution of two alternative queen or two alternative worker biasing strategies, one that biases strongly and another that does not bias at all. Alternative strategies evolve because the mechanisms of biasing result in accelerating benefits per unit cost with increasing bias, resulting in greater fitness for strategies that bias more and bias less than the population equilibrium. Strategies biasing more gain from increased biasing efficiency whereas strategies biasing less gain from decreased biasing cost. Our study predicts that whether queens or workers evolve alternative strategies depends upon the mechanisms that workers use to bias the sex ratio, the relative cost of queen and worker biasing, and the rates at which queen and worker strategies evolve. Our study also predicts that population and colony level sex allocation, as well as colony productivity, will differ diagnostically according to whether queens or workers evolve alternative biasing strategies and according to what mechanism workers use to bias sex allocation.     

Sex ratio conflicts, mating frequency, and queen fitness in the ant Formica truncorum

We examined the effect of facultative sex allocation by workerson queen fitness in a Furnish population of the ant Formicatruncorum. Workers rear female-biased broods in colonies headedby a singly mated queen and male-biased broods in colonies headedby a multiply mated queen. As a result, multiply mated queenshave a 37% fitness advantage over singly mated queens. Neitherreproductive output nor worker population of colonies variedwith queen mating frequency. We suggest that singly mated queenspersist in the population because fitness benefits to multiplymated queens via sex allocation are balanced by costs of additionalmatings. Alternatively, singly mated queens may persist simplybecause some queens lack opportunities to mate multiply or becausemale control sometimes prevents additional matings by queens.     

Energetics of newly-mated queens and colony founding in the fungus-gardening ants Cyphomyrmex rimosus and Trachymyrmex septentrionalis (Hymenoptera: Formicidae)

Abstract.  The energetics of colony founding is investigated in the fungus gardening ants (Attini) Trachymyrmex septentrionalis and Cyphomyrmex rimosus . Similar to most ants, inseminated queens of these two species found nests independently unaccompanied by workers (haplometrosis). Whereas most ant founding queens seal themselves in a chamber and do not feed when producing a brood entirely from metabolic stores (claustral founding), the majority of fungus gardening ants must forage during the founding phase (semiclaustral founding). Laboratory-reared T. septentrionalis individuals comprise 84 dealate females collected after mating flights in June 2004. Twenty are immediately killed to obtain values for queen traits and another 20 after worker emergence for queen, fungus garden and worker traits. Cyphomyrmex rimosus comprise 22 dealate females collected in June 2005; ten of which are immediately killed and similarly prepared. Newly-mated T. septentrionalis queens have 25% of their dry weight as fat; whereas newly-mated C. rimosus queens contain 11% fat. These amounts are 50–75% less than most independently founding ant species. Trachymyrmex septentrionalis queens lose merely 5% of their energetic content during colony founding, whereas the total energetic content of their brood is more than three-fold the amount lost by the queen. Incipient T. septentrionalis colonies produce approximately half as much ant biomass per gram of fungus garden as do mature colonies. Similar to most ants, T. septentrionalis produces minim workers that are approximately 40% lighter than workers from mature colonies. Regardless of their size, T. septentrionalis workers contain much lower fat than do workers of claustral species. These data indicate that fungus gardening is adaptive because colonies can produce much cheaper offspring, making colony investment much lower.     

The success of alternative reproductive tactics in monogyne populations of the ant Solenopsis invicta: significance for transitions in social organization

Newly produced queens from monogyne (single-queen) coloniesof the ant Solenopsis invicta usually initiate reproductionindependently, that is, without worker assistance. However,some recently mated queens attempt to bypass this risky phaseof new colony foundation by entering established nests to reproduce,although it is unclear how often these queens are successfulin natural populations. We surveyed a mature monogyne populationof S. invicta in both 1995 and 1996 for colonies headed by queensincapable of independent colony founding (diploid-male-producingqueens) in order to estimate the frequency of colonies thatare headed by queens that initiated reproduction within establishednests (adopted queens). Using the frequency of diploid-male-producingqueens among the recently mated queens in this population, weestimated that the overall rate of queen replacement by adoptedqueens is about 0.7% per colony per year. Although theory suggeststhat a change to a novel queen reproductive tactic could beassociated with a fundamental change in social organization(queen number), this does not appear to be the case in monogyneS. invicta. However, the evolution of nest-infiltrating reproductivetactics by queens in a monogyne population and the evolutionof multiple-queen societies may result from similar ecologicalpressures facing newly mated queens. We therefore incorporatethis strategy into an existing theoretical framework that wasdeveloped to explain the evolution of alternative social organizationsin ants, providing testable predictions regarding the distributionand frequency of queen adoption in other single-queen ant societies.     

A heritable component in sex ratio and caste determination in a Cardiocondyla ant

Studies on sex ratios in social insects provide among the most compelling evidence for the importance of kin selection in social evolution. The elegant synthesis of Fisher's sex ratio principle and Hamilton's inclusive fitness theory predicts that colony-level sex ratios vary with the colonies' social and genetic structures. Numerous empirical studies in ants, bees, and wasps have corroborated these predictions. However, the evolutionary optimization of sex ratios requires genetic variation, but one fundamental determinant of sex ratios - the propensity of female larvae to develop into young queens or workers ("queen bias") - is thought to be largely controlled by the environment. Evidence for a genetic influence on sex ratio and queen bias is as yet restricted to a few taxa, in particular hybrids. Because of the very short lifetime of their queens, ants of the genus Cardiocondyla are ideal model systems for the study of complete lifetime reproductive success, queen bias, and sex ratios. We found that lifetime sex ratios of the ant Cardiocondyla kagutsuchi have a heritable component. In experimental single-queen colonies, 22 queens from a genetic lineage with a highly female-biased sex ratio produced significantly more female-biased offspring sex ratios than 16 queens from a lineage with a more male-biased sex ratio (median 91.5% vs. 58.5% female sexuals). Sex ratio variation resulted from different likelihood of female larvae developing into sexuals (median 50% vs. 22.6% female sexuals) even when uniformly nursed by workers from another colony. Consistent differences in lifetime sex ratios and queen bias among queens of C. kagutsuchi suggest that heritable, genetic or maternal effects strongly affect caste determination. Such variation might provide the basis for adaptive evolution of queen and worker strategies, though it momentarily constrains the power of workers and queens to optimize caste ratios.     

Self-restraint and sterility in workers of Acromyrmex and Atta leafcutter ants

Summary. Queens of leafcutter ants (Acromyrmex and Atta) are highly multiply mated, resulting in a potential queenworker and worker-worker conflict over who should produce the males in the colony. We studied whether this conflict is expressed, by determining the amount of reproductive egg-laying by workers in queenright colonies of Acromyrmex echinatior, Acromyrmex octospinosus, Atta cephalotes, and Atta sexdens through ovary dissections. Worker sons are absent or rare in queenright Acromyrmex colonies, but can be produced in orphaned colonies. In Atta, most workers have rudimentary ovaries that never produce eggs, but a few (mostly small and medium workers that form a retinue around the queen) lay many trophic eggs for consumption by the queen. These eggs are large, flaccid, and lacking in yolk compared to queen-laid eggs, and appear to be always inviable. In Acromyrmex, many workers (especially young large workers) lay eggs that are similar in size to queen-laid eggs, but mostly with a reduced amount of yolk. Trophic eggs appear to be an important source of food for larvae in Acromyrmex (especially in Ac. echinatior), but not in Atta. Five (0.8) of 616 dissected Ac. echinatior workers but no Ac. octospinosus workers (n = 552), had ready-to-lay reproductive eggs. Old workers in all four species are incapable of laying eggs due to ovary resorption. We conclude that Atta workers are sterile, while Acromyrmex workers display reproductive self-restraint, possibly representing an earlier stage in the evolution towards worker sterility. Worker selfrestraint in Acromyrmex may be maintained by a queen or worker policing mechanism, but individual cost-benefit explanations may also apply.Received 1 March 2004; revised 28 June 2004; accepted 1 July 2004.     

Sex‐allocation conflict and sexual selection throughout the lifespan of eusocial colonies

Models of sex‐allocation conflict are central to evolutionary biology but have mostly assumed static decisions, where resource allocation strategies are constant over colony lifespan. Here, we develop a model to study how the evolution of dynamic resource allocation strategies is affected by the queen‐worker conflict in annual eusocial insects. We demonstrate that the time of dispersal of sexuals affects the sex‐allocation ratio through sexual selection on males. Furthermore, our model provides three predictions that depart from established results of classic static allocation models. First, we find that the queen wins the sex‐allocation conflict, while the workers determine the maximum colony size and colony productivity. Second, male‐biased sex allocation and protandry evolve if sexuals disperse directly after eclosion. Third, when workers are more related to new queens, then the proportional investment into queens is expected to be lower, which results from the interacting effect of sexual selection (selecting for protandry) and sex‐allocation conflict (selecting for earlier switch to producing sexuals). Overall, we find that colony ontogeny crucially affects the outcome of sex‐allocation conflict because of the evolution of distinct colony growth phases, which decouples how queens and workers affect allocation decisions and can result in asymmetric control.     

Reduced biological control and enhanced chemical pest management in the evolution of fungus farming in ants

To combat disease, most fungus-growing ants (Attini) use antibiotics from mutualistic bacteria (Pseudonocardia) that are cultured on the ants'' exoskeletons and chemical cocktails from exocrine glands, especially the metapleural glands (MG). Previous work has hypothesized that (i) Pseudonocardia antibiotics are narrow-spectrum and control a fungus (Escovopsis) that parasitizes the ants'' fungal symbiont, and (ii) MG secretions have broad-spectrum activity and protect ants and brood. We assessed the relative importance of these lines of defence, and their activity spectra, by scoring abundance of visible Pseudonocardia for nine species from five genera and measuring rates of MG grooming after challenging ants with disease agents of differing virulence. Atta and Sericomyrmex have lost or greatly reduced the abundance of visible bacteria. When challenged with diverse disease agents, including Escovopsis, they significantly increased MG grooming rates and expanded the range of targets. By contrast, species of Acromyrmex and Trachymyrmex maintain abundant Pseudonocardia. When challenged, these species had lower MG grooming rates, targeted primarily to brood. More elaborate MG defences and reduced reliance on mutualistic Pseudonocardia are correlated with larger colony size among attine genera, raising questions about the efficacy of managing disease in large societies with chemical cocktails versus bacterial antimicrobial metabolites.     

Interactions between Fungus-Growing Ants (Attini), Fruits and Seeds in Cerrado Vegetation in Southeast Brazil1

We surveyed the material collected for fungus culturing by attine ants in the cerrado vegetation of southeast Brazil. Six genera of the so-called lower attines (Cyphomyrmex, Mycetarotes, Mycocepurus, Myrmicocrypta, Sericomyrmex and Trachymyrmex) collect a wide variety of plant material as fungal substrate. Plant diaspores of nonmyrmecochorous species comprise a large portion of the items brought to the nest, especially in the rainy season. Removal experiments using fruits of selected plant species revealed that attine ants (including the leaf-cutters Atta and Acromyrmex) not only actively clean the seeds (remove fruit pulp), but also carry them up to 12 m in the cerrado. Germination tests showed that removal of fruit pulp by attine ants increases germination rate in Ocotea pulchella (Lauraceae), Prunus sellowii (Rosaceae), Ouratea spectabilis (Ochnaceae), Rapanea umbellata (Myrsinaceae) and Psychotria stachyoides (Rubiaceae). For P. stachyoides, however, ants had no effect on germination if seeds had already passed the digestive tract of birds. Aril removal by attines also increases germination success of Copaifera langsdorffii (Leguminosae) and Virola sebifera (Myristicaceae) seeds. The results indicate that attine-fruit/seed interactions are particularly conspicuous in the cerrado, suggesting that fungus-growing ants may play a relevant role in fruit/seed biology in this vegetation type. Potential ant-derived benefits to diaspores of nonmyrmecochorous plants in the cerrado would include secondary seed dispersion and/or increased germination success by ant-handled seeds.     

Mating system evolution and worker caste diversity in Pheidole ants

The efficiency of social groups is generally optimized by a division of labour, achieved through behavioural or morphological diversity of members. In social insects, colonies may increase the morphological diversity of workers by recruiting standing genetic variance for size and shape via multiply mated queens (polyandry) or multiple‐breeding queens (polygyny). However, greater worker diversity in multi‐lineage species may also have evolved due to mutual worker policing if there is worker reproduction. Such policing reduces the pressure on workers to maintain reproductive morphologies, allowing the evolution of greater developmental plasticity and the maintenance of more genetic variance for worker size and shape in populations. Pheidole ants vary greatly in the diversity of worker castes. Also, their workers lack ovaries and are thus invariably sterile regardless of the queen mating frequency and numbers of queens per colony. This allowed us to perform an across‐species study examining the genetic effects of recruiting more patrilines on the developmental diversity of workers in the absence of confounding effects from worker policing. Using highly variable microsatellite markers, we found that the effective mating frequency of the soldier‐polymorphic P. rhea (avg. me_N = 2.65) was significantly higher than that of the dimorphic P. spadonia (avg. me_N = 1.06), despite a significant paternity skew in P. rhea (avg. B = 0.10). Our findings support the idea that mating strategies of queens may co‐evolve with selection to increase the diversity of workers. We also detected patriline bias in the production of different worker sizes, which provides direct evidence for a genetic component to worker polymorphism.