Search

Honeybees are known to have genetically diverse colonies because queens mate with many males and the recombination rate is extremely high. Genetic diversity among social insect workers has been hypothesized to improve general performance of large and complex colonies, but this idea has not been tested in other social insects. Here, we present a linkage map and an estimate of the recombination rate for Acromyrmex echinatior, a leaf-cutting ant that resembles the honeybee in having multiple mating of queens and colonies of approximately the same size. A map of 145 AFLP markers in 22 linkage groups yielded a total recombinational size of 2076 cM and an inferred recombination rate of 161 kb cM(-1) (or 6.2 cM Mb(-1)). This estimate is lower than in the honeybee but, as far as the mapping criteria can be compared, higher than in any other insect mapped so far. Earlier studies on A. echinatior have demonstrated that variation in division of labour and pathogen resistance has a genetic component and that genotypic diversity among workers may thus give colonies of this leaf-cutting ant a functional advantage. The present result is therefore consistent with the hypothesis that complex social life can select for an increased recombination rate through effects on genotypic diversity and colony performance.

In a recent study, Denny et al. (2004a) showed that queens of the army ant, Eciton burchellii, mate with multiple males and presented estimates suggesting that they mate with more males than queens of any other ant species so far investigated. They also inferred that data were consistent with queens being inseminated repeatedly throughout their life, which would be exceptional among the social Hymenoptera and contradictory to predictions from kin selection theory. In the present study, we reanalyze these data using new software and supplement them with similar microsatellite data from other colonies of the same species. Mating frequencies in E. burchellii are indeed very high (mean observed and effective queen-mating frequencies of 12.9 each) but considerably lower than the previous estimates. We show that the number of patrilines represented in the first worker offspring of a young queen is lower than in older queens but suggest that this may be due to initial sperm clumping in the queen's sperm storage organ, rather than to repeated inseminations. Moreover, we found no evidence for repeated mating by genotyping sequential worker generations produced by a single old queen, showing that she did not obtain new inseminations despite ample opportunities for mating.

Ant queens are among the most long-lived insects known. They mate early in adult life and maintain millions of viable sperm in their sperm storage organ until they die many years later. Because they never re-mate, the reproductive success of queens is ultimately sperm-limited, but it is not known what selective forces determine the upper limit to sperm storage. Here we show that sperm storage carries a significant cost of reduced immunity during colony founding. Newly mated queens of the leaf-cutting ant Atta colombica upregulate their immune response shortly after completing their nest burrow, probably as an adaptive response to a greater exposure to pathogens in the absence of grooming workers. However, the immune response nine days after colony founding is negatively correlated with the amount of sperm in the sperm storage organ, indicating that short-term survival is traded off against long-term reproductive success. The immune response was lower when more males contributed to the stored sperm, indicating that there might be an additional cost of mating or storing genetically different ejaculates.

Selfish gene and self-organisation approaches have revolutionised the study of social insects and have provided unparalleled insights into the highly sophisticated nature of insect social evolution. Here, we briefly review the core programs and interfaces with communication and recognition studies that characterise these fields today, and offer an interdisciplinary future perspective for the study of social insect evolutionary biology.

Sperm length is highly variable, both between and within species, but the evolutionary significance of this variation is poorly understood. Sexual selection on sperm length requires a significant additive genetic variance, but few studies have actually measured this. Here we present the first estimates of narrow sense heritability of sperm length in a social insect, the bumblebee Bombus terrestris. In spite of a balanced and straightforward rearing design of colonies, and the possibility to replicate measurements of sperm within single males nested within colonies, the analysis proved to be complex. Several appropriate statistical models were derived, each depending on different assumptions. The heritability estimates obtained ranged from h (2) = 0.197 +/- 0.091 to h (2) = 0.429 +/- 0.154. All our estimates were substantially lower than previous estimates of sperm length heritability in non-social insects and vertebrates.

In most social insects workers do not mate, but have retained the ability to produce haploid eggs that can develop into viable male offspring. Under what circumstances this reproductive potential is realized and how the ensuing worker-queen conflict over male production is resolved, is an area of active research in insect sociobiology. Here we present microsatellite data for 176 males from eight colonies of the African army ant Dorylus (Anomma) molestus. Comparison with worker genotypes and inferred queen genotypes from the same colonies show that workers do not or at best very rarely reproduce in the presence of the queen. Queens of D. (A.) molestus are known to be highly multiply mated. This implies that workers are on average more closely related to queen sons than to other workers' sons, so that our results are consistent with predictions from inclusive fitness theory. It remains unknown, however, whether worker sterility is maintained by active worker policing or by self-restraint.

Attine ants engage in a quadripartite symbiosis with fungi they cultivate for food, specialized garden parasites, and parasite-inhibiting bacteria. Molecular phylogenetic evidence supports an ancient host-pathogen association between the ant-cultivar mutualism and the garden parasite. Here we show that ants rear the antibiotic-producing bacteria in elaborate cuticular crypts, supported by unique exocrine glands, and that these structures have been highly modified across the ants' evolutionary history. This specialized structural evolution, together with the absence of these bacteria and modifications in other ant genera that do not grow fungus, indicate that the bacteria have an ancient and coevolved association with the ants, their fungal cultivar, and the garden parasite.

Polyandry is often difficult to explain because benefits of the behaviour have proved elusive. In social insects, polyandry increases the genetic diversity of workers within a colony and this has been suggested to improve the resistance of the colony to disease. Here we examine the possible impact of host genetic diversity on parasite evolution by carrying out serial passages of a virulent fungal pathogen through leaf-cutting ant workers of known genotypes. Parasite virulence increased over the nine-generation span of the experiment while spore production decreased. The effect of host relatedness upon virulence appeared limited. However, parasites cycled through more genetically diverse hosts were more likely to go extinct during the experiment and parasites cycled through more genetically similar hosts had greater spore production. These results indicate that host genetic diversity may indeed hinder the ability of parasites to adapt while cycling within social insect colonies.

Acromyrmex leaf-cutting ants maintain two highly specialized, vertically transmitted mutualistic ectosymbionts: basidiomycete fungi that are cultivated for food in underground gardens and actinomycete Pseudonocardia bacteria that are reared on the cuticle to produce antibiotics that suppress the growth of Escovopsis parasites of the fungus garden. Mutualism stability has been hypothesized to benefit from genetic uniformity of symbionts, as multiple coexisting strains are expected to compete and, thus, reduce the benefit of the symbiosis. However, the Pseudonocardia symbionts are likely to be involved in Red-Queen-like antagonistic co-evolution with Escovopsis so that multiple strains per host might be favoured by selection provided the cost of competition between bacterial strains is low. We examined the genetic uniformity of the Pseudonocardia symbionts of two sympatric species of Acromyrmex ants by comparing partial sequences of the nuclear Elongation Factor-Tu gene. We find no genetic variation in Pseudonocardia symbionts among nest mate workers, neither in Acromyrmex octospinosus, where colonies are founded by a single queen, nor in Acromyrmex echinatior, where mixing of bacterial lineages might happen when unrelated queens cofound a colony. We further show that the two ant species maintain the same pool of Pseudonocardia symbionts, indicating that horizontal transmission occasionally occurs, and that this pool consists of two distinct clades of closely related Pseudonocardia strains. Our finding that individual colonies cultivate a single actinomycete strain is in agreement with predictions from evolutionary theory on host-symbiont conflict over symbiont mixing, but indicates that there may be constraints on the effectiveness of the bacterial symbionts on an evolutionary timescale.

All-hybrid populations of the water frog, Rana esculenta, are exceptional in consisting of independently and to some extent sexually reproducing interspecific hybrids. In most of its range R. esculenta reproduces hemiclonally with one of the parental species, R. lessonae or R. ridibunda, but viable populations of diploid and triploid hybrids, in which no individuals of the parental species have been found, exist in the northern part of the range. We test the hypothesis that nonhybrids arise every year in these all-hybrid populations, but die during larval development. Microsatellite markers were used to determine the genotypes of adults and abnormal and healthy offspring in three all-hybrid populations of R. esculenta in Denmark. Of all eggs and larvae, 63% developed abnormally or died, with some being nonhybrid (genomes matching one of the parental species), many being aneuploid (with noninteger chromosome sets), a few being tetraploid, and many eggs possibly being unfertilized. The 37% surviving and apparently healthy froglets were all diploid or triploid hybrids. In all three populations, gametogenesis matched the pattern previously described for all-hybrid R. esculenta populations in which most triploid adults have two R. lessonae genomes. This pattern was surprising for the one population in which triploid adults had two R. ridibunda genomes, because here it leads to a deficiency of gametes with an R. lessonae genome and should compromise the stability of this population. We conclude that faulty gametogenesis and mating between frogs with incompatible gametes induce a significant hybrid load in all-hybrid populations of R. esculenta, and we discuss compensating advantages and potential evolutionary trajectories to reduce this hybrid load.

The medical application of antibiotics dramatically reduced human infant mortality in the previous century. A new study indicates that ground nesting wasps exploit Streptomyces strains that they rear in their antennae for the same purpose.

Leaf-cutting ants rear clonal fungi for food and transmit the fungi from mother to daughter colonies so that symbiont mixing and conflict, which result from competition between genetically different clones, are avoided. Here we show that despite millions of years of predominantly vertical transmission, the domesticated fungi actively reject mycelial fragments from neighboring colonies, and that the strength of these reactions are in proportion to the overall genetic difference between these symbionts. Fungal incompatibility compounds remain intact during ant digestion, so that fecal droplets, which are used for manuring newly grown fungus, elicit similar hostile reactions when applied to symbionts from other colonies. Symbiont control over new mycelial growth by manurial imprinting prevents the rearing of multiple crops in fungus gardens belonging to the same colony.

Pair formation in social insects mostly happens early in adult life and away from the social colony context, which precludes promiscuity in the usual sense. Termite males have continuous sperm production, but males of social Hymenoptera have fixed complements of sperm, except for a few species that mate before female dispersal and show male-fighting and lifelong sperm production. We develop an evolutionary framework for testing sexual selection and sperm competition theory across the advanced eusocial insects (ants, wasps, bees, termites) and highlight two areas related to premating sexual selection (sexual dimorphism and male mate number) that have remained understudied and in which considerable progress can be achieved with relatively simple approaches. We also infer that mating plugs may be relatively common, and we review further possibilities for postmating sexual selection, which gradually become less likely in termite evolution, but for which eusocial Hymenoptera provide unusual opportunities because they have clonal ejaculates and store viable sperm for up to several decades.

The natural distribution of honeybee subspecies in Europe has been significantly affected by human activities during the last century. Non-native subspecies of honeybees have been introduced and propagated, so that native black honeybee (Apis mellifera mellifera) populations lost their identity by gene-flow or went extinct. After previous studies investigated the remaining gene-pools of native honeybees in France and Spain, we here assess the genetic composition of eight northwest European populations of the black honeybee, using both mitochondrial (restriction fragment length polymorphisms of the intergenic transfer RNAleu-COII region) and nuclear (11 microsatellite loci) markers. Both data sets show that A. m. mellifera populations still exist in Norway, Sweden, Denmark, England, Scotland and Ireland, but that they are threatened by gene flow from commercial honeybees. Both Bayesian admixture analysis of the microsatellite data and DraI-RFLP (restriction fragment length polymorphism) analysis of the intergenic region indicated that gene-flow had hardly occurred in some populations, whereas almost 10% introgression was observed in other populations. The most introgressed population was found on the Danish Island of Laeso, which is the last remaining native Danish population of A. m. mellifera and the only one of the eight investigated populations that is protected by law. We discuss how individual admixture analysis can be used to monitor the restoration of honeybee populations that suffer from unwanted hybridization with non-native subspecies.

Large blue (Maculinea) butterflies are highly endangered throughout the Palaearctic region, and have been the focus of intense conservation research. In addition, their extraordinary parasitic lifestyles make them ideal for studies of life history evolution. Early instars consume flower buds of specific host plants, but later instars live in ant nests where they either devour the brood (predators), or are fed mouth-to-mouth by the adult ants (cuckoos). Here we present the phylogeny for the group, which shows that it is a monophyletic clade nested within Phengaris, a rare Oriental genus whose species have similar life histories. Cuckoo species are likely to have evolved from predatory ancestors. As early as five million years ago, two Maculinea clades diverged, leading to the different parasitic strategies seen in the genus today. Contrary to current belief, the two recognized cuckoo species show little genetic divergence and are probably a single ecologically differentiated species. On the other hand, some of the predatory morphospecies exhibit considerable genetic divergence and may contain cryptic species. These findings have important implications for conservation and reintroduction efforts.

Background: Parasite heterogeneity and within-host competition are thought to be important factors influencing the dynamics of host-parasite relationships. Yet, while there have been many theoretical investigations of how these factors may act, empirical data is more limited. We investigated the effects of parasite density and heterogeneity on parasite virulence and fitness using four strains of the entomopathogenic fungus, Metarhizium anisopliae var. anisopliae, and its leaf-cutting ant host Acromyrmex echinatior as the model system., Results: The relationship between parasite density and infection was sigmoidal, with there being an invasion threshold for an infection to occur (an Allee effect). Although spore production was positively density-dependent, parasite fitness decreased with increasing parasite density, indicating within-host scramble competition. The dynamics differed little between the four strains tested. In mixed infections of three strains the infection-growth dynamics were unaffected by parasite heterogeneity., Conclusions: The strength of within-host competition makes dispersal the best strategy for the parasite. Parasite heterogeneity may not have effected virulence or the infection dynamics either because the most virulent strain outcompeted the others, or because the interaction involved scramble competition that was impervious to parasite heterogeneity. The dynamics observed may be common for virulent parasites, such as Metarhizium, that produce aggregated transmission stages. Such parasites make useful models for investigating infection dynamics and the impact of parasite competition.

Army ants have long been suspected to represent an independent origin of multiple queen-mating in the social Hymenoptera. Using microsatellite markers, we show that queens of the African army ant Dorylus (Anomma) molestus have the highest absolute (17.3) and effective (17.5) queen-mating frequencies reported so far for ants. This confirms that obligate multiple queen-mating in social insects is associated with large colony size and advanced social organization, but also raises several novel questions. First, these high estimates place army ants in the range of mating frequencies of honeybees, which have so far been regarded as odd exceptions within the social Hymenoptera. Army ants and honeybees are fundamentally different in morphology and life history, but are the only social insects known that combine obligate multiple mating with reproduction by colony fission and extremely male-biased sex ratios. This implies that the very high numbers of matings in both groups may be due partly to the relatively low costs of additional matings. Second, we were able to trace recent events of colony fission in four of the investigated colonies, where the genotypes of the two queens were only compatible with a mother-daughter relationship. A direct comparison of male production between colonies with offspring from one and two queens, respectively, suggested strongly that new queens do not produce a sexual brood until all workers of the old queen have died, which is consistent with kin selection theory., (Copyright 2004 Blackwell Publishing Ltd)

Multiple mating by females (polyandry) remains hard to explain because, while it has substantial costs, clear benefits have remained elusive. The problem is acute in the social insects because polyandry is probably particularly costly for females and most material benefits of the behavior are unlikely to apply. It has been suggested that a fitness benefit may arise from the more genetically diverse worker force that a polyandrous queen will produce. One leading hypothesis is that the increased genetic diversity of workers will improve a colony's resistance to disease. We investigated this hypothesis using a polyandrous leaf-cutting ant and a virulent fungal parasite as our model system. At high doses of the parasite most patrilines within colonies were similarly susceptible, but a few showed greater resistance. At a low dose of the parasite there was more variation between patrilines in their resistance to the parasite. Such genetic variation is a key prerequisite for polyandry to result in increased disease resistance of colonies. The relatedness of two hosts did not appear to affect the transmission of the parasite between them, but this was most likely because the parasite tested was a virulent generalist that is adapted to transmit between distantly related hosts. The resistance to the parasite was compared between small groups of ants of either high or low genetic diversity. No difference was found at high doses of the parasite, but a significant improvement in resistance in high genetic diversity groups was found at a low dose of the parasite. That there is genetic variation for disease resistance means that there is the potential for polyandry to produce more disease-resistant colonies. That this genetic variation can improve the resistance of groups even under the limited conditions tested suggests that polyandry may indeed produce colonies with improved resistance to disease.

Multiple mating (polyandry) is widespread among animal groups, particularly insects. But the factors that maintain it and underlie its evolution are hard to verify because benefits and costs are not easily quantified and they tend to be similar in related species. Here we compare the mating strategies of the leaf-cutting ant Acromyrmex echinatior and its recently derived social parasite Acromyrmex insinuator, which is also its closest relative. We find that although the host queens mate with up to a dozen different males, the social parasite mates only singly. This rapid and surprising reversion to single mating in a socially parasitic ant indicates that the costs of polyandry are probably specific to a free-living lifestyle.

Within-host competition is an important factor in host-parasite relationships, yet most studies consider interactions involving only single parasite species. We investigated the interaction between a virulent obligate entomopathogenic fungus, Metarhizium anisopliae var. anisopliae, and a normally avirulent, opportunistic fungal pathogen, Aspergillus flavus, in their leaf-cutting ant host, Acromyrmex echinatior. Surprisingly, the latter normally out-competed the former in mixed infections and had enhanced fitness relative to when infecting in isolation. The result is most probably due to Metarhizium inhibiting the host's immune defences, which would otherwise normally prevent infections by Aspergillus. With the host defences negated by the virulent parasite, the avirulent parasite was then able to out-compete its competitor. This result is strikingly similar to that seen in immunocompromised vertebrate hosts and indicates that avirulent parasites may play a more important role in host life histories than is generally realized.

Leaf-cutting ants finely fragment the leaf material that they bring to the nest and place faecal droplets on this substrate before incorporating it in the upper part of the fungus-garden. The faecal droplets contain enzymes of which some have been shown to be of fungal origin. Here we explicitly address the enzymatic activity of faecal droplets in the leaf-cutting ants Acromyrmex echinatior and Atta colombica. We used isoelectric focusing and specific staining to show that faecal droplets of both species contain carboxymethylcellulases, laccases, proteases, and pectinases (both pectin esterases and pectin lysases) and we demonstrate that these enzymes originate from the symbiotic fungus and not from the ants themselves. The level of activity of fungal pectin lyase in faecal droplets indicates that fungal enzymes may be protected and possibly concentrated during their passage through the ant gut. This would imply that enzymes that are transferred by ants from mature parts of the fungus gardens may play an important role for the colonization of new substrate by new mycelium of the same fungal clone.

We investigated the prevalence of entomopathogenic fungi associated with leaf-cutting ant colonies in a small area of tropical forest in Panama. There was a high abundance of Metarhizium anisopliae var. anisopliae near the colonies. Beauveria bassiana was also detected in the soil, Aspergillus flavus in dump material, and six Camponotus atriceps ants were found infected with Cordyceps sp. Based on a partial sequence of the IGS region, almost all of the M. anisopliae var. anisopliae isolates fell within one of the three main clades of M. anisopliae var. anisopliae, but with there still being considerable diversity within this clade. The vast majority of leaf-cutting ants collected were not infected by any entomopathogenic fungi. While leaf-cutting ants at this site must, therefore, regularly come into contact with a diversity of entomopathogenic fungi, they do not appear to be normally infected by them.

The fungus gardens of fungus-growing ants are a potentially valuable resource for exploitation by natural enemies, but few of these antagonistic interactions have been studied. Here we describe key aspects of the behavioral ecology of Gnamptogenys hartmani (Ponerinae: Ectatommini), a specialized "agro-predator" of Trachymyrmex and Sericomyrmex fungus-growing ants in Panama. Raiding columns of G. hartmani attack and usurp nests with remarkably little effort: a few intruding workers are sufficient to cause panic among the attine ants and make them abscond from the nest. Both G. hartmani larvae and adults consume the fungus and the host brood, after which the colony migrates to a new fungus-growing ant nest discovered by scouting workers. The morphology of the G. hartmani larval mouthparts is similar to that of Gnamptogenys species with a non-fungal diet. However, we suggest that the presence of long spinules on the larval mandibles in the genus Gnamptogenys, comparable to those found in attine larvae, may have pre-adapted G. hartmani to fungus eating. G. hartmani workers do not actively maintain or modify fungus gardens, which makes them less efficient exploiters than Megalomyrmex, the only other agro-predatory ant species known so far.

In Acromyrmex octospinosus leaf-cutting ants the metapleural glands produce an array of antibiotic compounds that serve as a general defence against unwanted microbes on the cuticle. Leaf-cutting ants also grow mutualistic Pseudonocardiaceae bacteria on their cuticle that produce antibiotics controlling a microfungal parasite of their fungus gardens. Interaction between this bacterium and gland secretion therefore seems unavoidable. We document the typical development of bacterial growth on the cuticle of young major workers, show that growth starts a few days after eclosion, and that the maximal cover is reached after 2-3 weeks and gradually declines when workers mature. Experimental closure of the metapleural glands had no effect on the initial exponential growth phase of the bacterium, but significantly reduced the cover during the decline phase. The age-dependent abundance of the bacterium and its partial dependence on metapleural gland secretion support the hypothesis that the abundance of this mutualist is actively regulated.

Division of labor is fundamental to the success of all societies. The most striking examples are the physically polymorphic worker castes in social insects with clear morphological adaptations to different roles. These polymorphic worker castes have previously been thought to be a classic example of nongentically controlled polymorphism, being mediated entirely by environmental cues. Here we show that worker caste development in the leaf-cutting ant Acromyrmex echinatior has a significant genetic component. Individuals of different patrilines within the same colony differ in their propensities to develop into minor or major workers. The mechanism appears to be plastic, with caste destiny resulting from interplay between nurture and nature. Unlike the few other recently discovered examples of a genetic influence on caste determination, the present result does not relate to any rare or exceptional circumstances, such as interspecific hybridization. The results suggest that a significant role of genetics may have been overlooked in our understanding of other complex polymorphisms of social insects.

Workers of the ant Formica truncorum specialize in rearing females or males depending on the number of fathers of a colony. These split sex ratios increase inclusive fitness, but it has remained unknown how workers assess the number of patrilines in their colonies and to what extent their reproductive decisions are constrained by lack of information. By analysis of the quantitative variation in cuticular hydrocarbon profiles of workers of multiply mated queens, we show that the heritable component of recognition cues is low and that the extent of sex ratio biasing toward males is correlated with patriline differences in hydrocarbon profiles. Workers are thus able to capitalize on colony-level relatedness asymmetry, but their inclusive fitness is constrained by uninformative recognition cues. These results are consistent with the hypothesis that the occasional expression of nepotistic phenotypes favoring full-sisters over half-sisters maintains selection against informative recognition cues. We evaluate how inclusive fitness theory may be used to predict the number and kind of recognition cues in insect societies of a specific relatedness structure.

Social parasites exploit the socially managed resources of their host's society. Inquiline social parasites are dependent on their host throughout their life cycle, and so many of the traits inherited from their free-living ancestor are removed by natural selection. One trait that is commonly lost is the worker caste, the functions of which are adequately fulfilled by host workers. The few inquiline parasites that have retained a worker caste are thought to be at a transitional stage in the evolution of social parasitism, and their worker castes are considered vestigial and non-adaptive. However, this idea has not been tested. Furthermore, whether inquiline workers have an adaptive role outside the usual worker repertoire of foraging, brood care and colony maintenance has not been examined. In this paper, we present data that suggest that workers of the inquiline ant Acromyrmex insinuator play a vital role in ensuring the parasite's fitness. We show that the presence of these parasite workers has a positive effect on the production of parasite sexuals and a negative effect on the production of host sexuals. This suggests that inquiline workers play a vital role in suppressing host queen reproduction, thus promoting the rearing of parasite sexuals. To our knowledge, these are the first experiments on inquiline workers and the first to provide evidence that inquiline workers have an adaptive role.

By sequencing part of the wsp gene of a series of clones, we detected an unusually high diversity of nine Wolbachia strains in queens of three species of leafcutter ants. Up to four strains co-occurred in a single ant. Most strains occurred in two clusters (InvA and InvB), but the social parasite Acromyrmex insinuator hosted two additional infections. The multiple Wolbachia strains may influence the expression of reproductive conflicts in leafcutter ants, but the expected turnover of infections may make the cumulative effects on host ant reproduction complex. The additional Wolbachia infections of the social parasite A. insinuator were almost certainly acquired by horizontal transmission, but may have facilitated reproductive isolation from its closely related host.

We have estimated phylogenies of fungus-growing termites and their associated mutualistic fungi of the genus Termitomyces using Bayesian analyses of DNA sequences. Our study shows that the symbiosis has a single African origin and that secondary domestication of other fungi or reversal of mutualistic fungi to a free-living state has not occurred. Host switching has been frequent, especially at the lower taxonomic levels, and nests of single termite species can have different symbionts. Data are consistent with horizontal transmission of fungal symbionts in both the ancestral state of the mutualism and most of the extant taxa. Clonal vertical transmission of fungi, previously shown to be common in the genus Microtermes (via females) and in the species Macrotermes bellicosus (via males) [Johnson, R. A., Thomas, R. J., Wood, T. G. & Swift, M. J. (1981) J. Nat. Hist. 15, 751-756], is derived with two independent origins. Despite repeated host switching, statistical tests taking phylogenetic uncertainty into account show a significant congruence between the termite and fungal phylogenies, because mutualistic interactions at higher taxonomic levels show considerable specificity. We identify common characteristics of fungus-farming evolution in termites and ants, which apply despite the major differences between these two insect agricultural systems. We hypothesize that biparental colony founding may have constrained the evolution of vertical symbiont transmission in termites but not in ants where males die after mating.

Some Tetraponera ants (Formicidae, Pseudomyrmecinae) subsist almost entirely on amino acid deficient honeydew secretions of pseudococcids and harbour a dense aggregation of bacterial symbionts in a unique pouch-shaped organ at the junction of the midgut and the intestine. The organ is surrounded by a network of intruding tracheae and Malpighian tubules, suggesting that these bacteria are involved in the oxidative recycling of nitrogen-rich metabolic waste. We have examined the ultrastructure of these bacteria and have amplified, cloned and sequenced ribosomal RNA-encoding genes, showing that the ant pouch contains a series of close relatives of Flavobacteria and Rhizobium, Methylobacterium, Burkholderia and Pseudomonas nitrogen-fixing root-nodule bacteria. We argue that pouch bacteria have been repeatedly 'domesticated' by the ants as nitrogen-recycling endosymbionts. This ant-associated community of mutualists is, to our knowledge, the first finding of symbionts related to root-nodule bacteria in animals.

Sociality can be associated with significant costs due to the increased risk of disease transmission. However, in some organisms the costs may be offset by benefits due to improvements in defences against parasites. To examine this possible trade-off between infection risk and disease resistance, we used Acromyrmex leaf-cutting ants and the entomopathogenic fungus Metarhizium anisopliae as the model system. Ants exposed to the parasite were found to have substantially improved survival when they were kept with nest-mates, while the cost of being in a group in terms of increased disease transmission was very low. The efficiency of transmission is described by the transmission parameter, which decreased with increasing host density showing that transmission rates are inversely density dependent. Both grooming and antibiotic secretions appeared to be important in resistance against the parasite, with the defences of small workers being particularly effective. The results indicate that leaf-cutting ant colonies may have much greater resistance to disease than would be predicted from the high densities of host individuals within them. Unlike most organisms, group living in these ants may actually be associated with a net benefit in terms of disease dynamics.

Obligate mating of females (queens) with multiple males has evolved only rarely in social Hymenoptera (ants, social bees, social wasps) and for reasons that are fundamentally different from those underlying multiple mating in other animals. The monophyletic tribe of ('attine') fungus-growing ants is known to include evolutionarily derived genera with obligate multiple mating (the Acromyrmex and Atta leafcutter ants) as well as phylogenetically basal genera with exclusively single mating (e.g. Apterostigma, Cyphomyrmex, Myrmicocrypta). All attine genera share the unique characteristic of obligate dependence on symbiotic fungus gardens for food, but the sophistication of this symbiosis differs considerably across genera. The lower attine genera generally have small, short-lived colonies and relatively non-specialized fungal symbionts (capable of living independently of their ant hosts), whereas the four evolutionarily derived higher attine genera have highly specialized, long-term clonal symbionts. In this paper, we investigate whether the transition from single to multiple mating occurred relatively recently in the evolution of the attine ants, in conjunction with the novel herbivorous 'leafcutter' niche acquired by the common ancestor of Acromyrmex and Atta, or earlier, at the transition to rearing specialized long-term clonal fungi in the common ancestor of the larger group of higher attines that also includes the genera Trachymyrmex and Sericomyrmex. We use DNA microsatellite analysis to provide unambiguous evidence for a single, late and abrupt evolutionary transition from exclusively single to obligatory multiple mating. This transition is historically correlated with other evolutionary innovations, including the extensive use of fresh vegetation as substrate for the fungus garden, a massive increase in mature colony size and morphological differentiation of the worker caste.

Many ant species have independently evolved colony structures with multiple queens and very low relatedness among nestmate workers, but it has remained unclear whether low-relatedness kin structures can repeatedly arise in populations of the same species. Here we report a study of Danish island populations of the red ant Myrmica sulcinodis and show that it is likely that such repeated developments occur. Two microsatellite loci were used to estimate genetic differentiation (F(ST)) among three populations and nestmate relatedness within these populations. The F(ST) values were highly significant due to very different allele frequencies among the three populations with relatively few common alleles and relatively many rare alleles, possibly caused by single queen foundation and rare subsequent immigration. Given the isolation of the islands and the low investment in reproduction, we infer that each of the populations was most likely established by a single queen, even though all three extant populations now have within-colony relatedness 95%), and the genetic differentiation of nests showed a significantly positive correlation with the distance between them. Both male-biased sex-ratio and genetic viscosity are expected characteristics of populations where queens have very local dispersal and where new colonies are initiated through nest-budding. Based on a comparison with other M. sulcinodis populations we hypothesise a distinct succession of population types and suggest that this may be a possible pathway to unicoloniality, ie, development towards a complete lack of colony kin structure and unrelated nestmate workers.

Background: Molecular biological techniques are dramatically changing our view of microbial diversity in almost any environment that has so far been investigated. This study presents a systematic survey of the microbial diversity associated with a population of Acromyrmex leafcutter ants. In contrast to previous studies on social insects, which targeted specific groups of symbionts occurring in the gut (termites, Tetraponera ants) or in specialised cells (Camponotus ants) the objective of our present study was to do a total screening of all possible micro-organisms that can be found inside the bodies of these leafcutter ants., Results: We amplified, cloned and sequenced SSU rRNA encoding gene fragments from 9 microbial groups known to have insect-associated representatives, and show that: (1) representatives of 5 out of 9 tested groups are present, (2) mostly several strains per group are present, adding up to a total of 33 different taxa. We present the microbial taxa associated with Acromymex ants in a phylogenetic context (using sequences from GenBank) to assess and illustrate to which known microorganisms they are closely related. The observed microbial diversity is discussed in the light of present knowledge on the evolutionary history of Acromyrmex leafcutter ants and their known mutualistic and parasitic symbionts., Conclusions: The major merits of the screening approach documented here is its high sensitivity and specificity, which allowed us to identify several microorganisms that are promising candidates for further study of their interactions with Acromyrmex leafcutter ants or their gardens.

We investigate the nature and duration of incompatibility between certain combinations of Acromyrmex leaf-cutting ants and symbiotic fungi, taken from sympatric colonies of the same or a related species. Ant-fungus incompatibility appeared to be largely independent of the ant species involved, but could be explained partly by genetic differences among the fungus cultivars. Following current theoretical considerations, we develop a hypothesis, originally proposed by S. A. Frank, that the observed incompatibilities are ultimately due to competitive interactions between genetically different fungal lineages, and we predict that the ants should have evolved mechanisms to prevent such competition between cultivars within a single garden. This requires that the ants are able to recognize unfamiliar fungi, and we show that this is indeed the case. Amplified fragment length polymorphism genotyping further shows that the two sympatric Acromyrmex species share each other's major lineages of cultivar, confirming that horizontal transfer does occasionally take place. We argue and provide some evidence that chemical substances produced by the fungus garden may mediate recognition of alien fungi by the ants. We show that incompatibility between ants and transplanted, genetically different cultivars is indeed due to active killing of the novel cultivar by the ants. This incompatibility disappears when ants are force-fed the novel cultivar for about a week, a result that is consistent with our hypothesis of recognition induced by the resident fungus and eventual replacement of incompatibility compounds during force-feeding.

Hamilton's principle of inclusive fitness implies that reproductive altruism can evolve, because individuals can pass on genes not only through their own offspring, but also through the offspring of their relatives. Social insects are spectacular examples of how some individuals may be selected to forgo reproduction and instead help others reproduce. Social Hymenoptera are also special because relatedness patterns within families can be asymmetrical, so that optimal sex-ratios, preferred male parentage or preferred mating frequencies become objects of reproductive conflict. The now extensive inclusive fitness theory provides precise qualitative predictions with respect to the emergence of such conflicts. Recent advances in the power of genetic markers applied to resolve family structure in insect societies have brought about a series of studies that have tested these predictions. In support of kin selection as a major evolutionary force, the results suggest that workers frequently control sex allocation. However, the very establishment of such worker control has made new conflicts come to light, between mothers and fathers and between adult individuals and brood. Evidence for these conflicts is only just beginning to be gathered. Recent studies tend to include issues such as 'information' and 'power' (i.e. the ability to perceive signals and the opportunity to act upon this information), and to address selection for selfishness at the individual level with costs of social disruption at the colony level.

Ants provide excellent opportunities for studying the evolutionary aspects of reproductive conflict. Relatedness asymmetries owing to the haplodiploid sex determination of Hymenoptera create substantial fitness incentives for gaining control over sex allocation, often at the expense of the fitness interests of nest-mates. Under worker-controlled split sex ratios either the reproductive interests of the mother queen (when workers male bias the sex ratio) or the father (when workers female bias the sex ratio), but never that of both parents simultaneously, are fulfilled. When workers bias sex ratios according to the frequency of queen mating, males which co-sire a colony have a joint interest in manipulating their daughter workers into rearing a more female-biased sex ratio. Here we show that males of the ant Formica truncorum achieve such manipulation by partial sperm clumping, so that the cohort-specific relatedness asymmetry of the workers in colonies with multiple fathers is higher than the cumulative relatedness asymmetry across worker cohorts. This occurs because a single male fathers the majority of the offspring within a cohort. Colonies with higher average cohort-specific relatedness asymmetry produce more female-biased sex ratios. Posthumously expressed male genes are thus able to oppose the reproductive interests of the genes expressed in queens and the latter apparently lack mechanisms for enforcing full control over sperm mixing and sperm allocation.

Queens of leafcutter ants exhibit the highest known levels of multiple mating (up to 10 mates per queen) among ants. Multiple mating may have been selected to increase genetic diversity among nestmate workers, which is hypothesized to be critical in social systems with large, long-lived colonies under severe pressure of pathogens. Advanced fungus-growing (leafcutter) ants have large numbers (104-106 workers) and long-lived colonies, whereas basal genera in the attine tribe have small (< 200 workers) colonies with probably substantially shorter lifespans. Basal attines are therefore expected to have lower queen mating frequencies, similar to those found in most other ants. We tested this prediction by analysing queen mating frequency and colony kin structure in three basal attine species: Myrmicocrypta ednaella, Apterostigma collare and Cyphomyrmex longiscapus. Microsatellite marker analyses revealed that queens in all three species were single mated, and that worker-to-worker relatedness in these basal attine species is very close to 0.75, the value expected under exclusively single mating. Fungus growing per se has therefore not selected for multiple queen mating. Instead, the advanced and highly productive social structure of the higher attine ants, which is fully dependent on the rearing of an ancient clonal fungus, may have necessitated high genetic diversity among nestmate workers. This is not the case in the lower attines, which rear fungi that were more recently derived from free-living fungal populations.

Muskoxen are large herbivores living in Arctic environments. Lack of genetic variation in allozymes has made it difficult to study the social and genetic structure of this species. In this study, we have tried to find polymorphic microsatellite loci using both cattle-derived microsatellite primers and primers developed from a genomic plasmid library of muskoxen. Only limited variation was found for both sets of microsatellite loci. We conclude that this consistent low genetic variation is probably due to demographic features of the muskoxen populations rather than to methodological constraints caused by the transfer of microsatellites between species.

Various aspects of optimal foraging and seasonal diet composition of bulls (bachelor and dominant), cows, subadults, and yearlings of muskoxen Ovibos moschatus were investigated in West Greenland during the following seasons: calving, post-calving, summer, rut and mid-winter. The following hypotheses were tested: (1) muskoxen maximize daily energy intake during spring and summer, (2) dominant bulls monopolizing cows during the rutting season shift from an energy maximizing to a time minimizing foraging strategy in order to maximize the time available for reproductive activities, and (3) muskoxen employ a time minimizing foraging strategy during winter to conserve energy. As forage quality changed throughout the short Arctic growing season, muskoxen responded by changing the proportions of daily time spent feeding on graminoids (Cyperaceae, Poaceae) and dicots (Salix, Betula), respectively. This seasonal variation in the relative proportion of daily feeding time spent ingesting graminoids followed approximately the energy maximization prediction over the periods calving to rut. Neither time minimizing nor random foraging could explain the observed diets in this period, thus confirming hypothesis 1. Dominant bulls did not shift to the time minimizing strategy as predicted by hypothesis 2. However, during the pre-rutting and rutting seasons bulls deviated from the other sex/age classes by failing to obtain the daily maximum energy predicted by the model, as a result of a higher proportion of time allocated to agonistic and sexual behaviour. During winter, none of the sex/age classes employed a time minimizing strategy, so rejecting hypothesis 3. Instead, muskoxen were found to maximize Na intake, indicating that Na is of major importance for winter survival. The results emerging from a linear programming model with constraint settings varying over seasons confirm that the constraint parameters applied are indeed important limiting factors for muskoxen in natural populations.

Colony-level predictions about sex-ratio optimization in eusocial Hymenoptera are different from the ones that apply to the population level. A recent empirical study on the sweat bee Halictus rubicundus has revealed a distinct pattern in the colony sex ratio of the summer brood: eusocial colonies produced more female-biased sex ratios and non-eusocial colonies produce more male-biased sex ratios. These data are consistent with theoretical hypotheses as put forward by Trivers and Hare and several later authors. When interpreted in the light of these theoretical contentions, the sex-ratio variation in Halictus rubicundus appears to be adaptive for workers, replacement queens and - under reasonable additional assumptions - also for foundresses., (Copyright © 1991. Published by Elsevier Ltd.)

We consider worker-controlled sex investments in eusocial Hymenoptera (ants in particular) and assume that relatedness asymmetry is variable among colonies and that workers are able to assess the relatedness asymmetry in their own colony. We predict that such "assessing" workers should maximize their inclusive fitness by specializing in the production of the sex to which they are relatively most related, i.e., colonies whose workers have a relatedness asymmetry below the population average should specialize in males, whereas colonies whose workers have a higher than average relatedness asymmetry should specialize in making females. Our argument yields the expectation that colony sex ratios will be bimodally distributed in ant populations where relatedness asymmetry is variable owing to multiple mating, worker reproduction, and/or polygyny. No such bimodality is expected, however, in ant species where relatedness asymmetry is known to be constant, or in cases where relatedness asymmetry is supposed to be irrelevant due to allospecific brood rearing under queen control, as in the slave-making ants. Comparative data on colony sex ratios in ants are reviewed to test the predictions. The data partly support our contentions, but are as yet insufficient to be considered as decisive evidence., (© 1990 The Society for the Study of Evolution.)

In 1977, 1978, and 1979 nuptial flights of Lasius niger L., Lasius flavus F., Myrmica rubra L., and Myrmica scabrinodis Nyl. were observed on the island of Schiermonnikoog and in the area around Amsterdam. Weather conditions during these flights were determined using data from meteorological stations at Schiermonnikoog and Schiphol Airport. Significant differences were found concerning daytime, global radiation and relative humidity at the beginning of flights of Lasius niger, Lasius flavus, and Myrmica rubra; Myrmica scabrinodis had no defined preferences for these parameters. Wind velocity at 2 m of height was less than 1.7 m.s -1 during all flights before 20 August. After that date all species tended to fly at higher wind velocities as well.The calculated ranges for daytime, temperature, global radiation, relative humidity, and wind velocity appeared to be sufficient to characterize all nuptial flight occasions at Schiermonnikoog.Micrometeorological measurements in typical habitats of different ant species revealed that during flights the air temperature at 20 cm above ground and the soil temperature at 5 to 7 cm below ground were about equal in the habitat of the flying species, but unequal in the neighbouring habitats of coexisting ants.

The genetic variation at two marker loci in three populations of the monogynous ant Lasius niger was used to analyze the importance of relatedness structure to sex-investment ratios in sexuals produced by colonies living in different resource conditions. From a resource-rich monoculture to a population in a resource-limited environment, dry weight investment in queens decreased from female-biased (0.76) to equality with male investment (Boomsma et al., 1982). The investment ratios in sexuals expected from the estimated relatedness ratios, resulting from kin-selection theory (Trivers and Hare, 1976), were in good agreement with the observed ratios in all populations. This indicated that the workers can capitalize on the asymmetry in relatedness, opposing the queen's interest, despite the contrast in environmental conditions in the different populations. This asymmetry, however, almost disappeared in the marginal population, due to a high frequency of double mating and worker production of males, resulting in a much reduced queen-worker conflict. We suggest that different levels of polyandry might be favored at different points of the resource gradient, with the sex ratio secondarily depending on these polyandry levels. As there was no population subdivision or spatial structure within the populations, group-selection and local-mate-competition models cannot account for the observed female-biased ratios, whereas they were rather accurately predicted by kin-selection theory., (© 1988 The Society for the Study of Evolution.)