Jacobus J. Boomsma

Variable sensitivity of fungi and bacteria to compounds produced by the metapleural glands of leaf-cutting ants

Summary: Ants are the only group of insects that have metapleural glands. Secretions of these exocrine glands are known to have antibiotic properties and have been hypothesised to function as a general defence against microbial and fungal infections. Such defences are likely to be particularly important in leaf-cutting ants that need to protect both themselves and their clonal mutualistic fungus against pathogens. The metapleural gland of the leaf-cutting ant Acromyrmex octospinosus produces an array of organic compounds (Ortius-Lechner et al., 2000), suggesting that different compounds may be effective against different kinds of infections. Here we provide a detailed analysis of the sensitivity of two species of bacteria and seven species of fungi (including the mutualistic fungus) to these metapleural gland compounds, grouped in seven classes: acetic acid, short chain acids, medium chain acids, long chain acids, indoleacetic acid, γ-lactones and γ-ketoacids. All classes of compounds inhibited the growth of at least some of the tested micro-organisms. Cluster analysis produced four groups of micro-organisms differing in their overall sensitivity. Among-cluster differences explained a major part of the total variation in sensitivity (MANOVA), although differences between micro-organisms within clusters were also significant. Fungal hyphae and fungal spores never clustered together, indicating that defence mechanisms against these fungal life stages are fundamentally different. The mutualistic fungus was sensitive to all classes of compounds, which suggests that defence via metapleural gland secretion is under constraint when the protection of the fungus garden is concerned.

Identifying the transition between single and multiple mating of queens in fungus-growing ants.

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 ‘attin’) 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 ‘leafcutte’ 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.

Colony-level and season-specific variation in cuticular hydrocarbon profiles of individual workers in the ant Formica truncorum

Summary: Cuticular hydrocarbon profiles of individual workers of the ant Formica truncorum were measured and found to contain relatively few hydrocarbons. Pentacosane, heptacosane, nonacosane and hentriacontane dominated the mixture, but small amounts of the corresponding alkenes were also present. Principal component analysis and nested analysis of variance showed that workers from different colonies varied significantly in quantitative aspects of their cuticular hydrocarbon profiles. Furthermore, differences between habitat-patches within populations and (to some extent) between populations were also detected. Finally, workers from the same colony, sampled only a few months apart, were found to be different in the quantitative composition of their cuticular hydrocarbon profiles, emphasising the importance of collecting samples from a colony at a single point in time.

Multiple mating increases the sperm stores of Atta colombica leafcutter ant queens

Abstract Multiple mating is likely to be costly for ant queens and yet it is common. Whether multiple mating brings benefits to queens that outweigh the costs has, therefore, received considerable theoretical attention. Empirical tests of hypotheses have been scarce and no clear evidence has been reported. We tested the “multiple-mating-for-more-sperm” hypothesis on individual young queens in a natural population of the leafcutter ant Atta colombica, a monogynous ant characterised by very large colonies and high colony longevity. We found that the number of sperm stored by queens was positively correlated with the number of mates per queen estimated through mother-offspring analysis with microsatellite DNA markers. Queen sperm stores increased on average by 30 million sperm for each additional mate. Life-history information for Atta indicate that the number of stored sperm observed is likely to constrain the reproductive lifespan of queens in nature. Multiple mating, despite costs, may therefore enhance the fitness of Atta queens because it enables them to store more sperm.

Multiple paternity, relatedness and genetic diversity in Acromyrmex leaf-cutter ants

Multiple queen–mating occurs in many social insects, but high degrees of multiple paternity have only been found in honeybees and some yellowjacket wasps. Here we report the first case of an ant species where multiple mating reduces relatedness among female offspring to values significantly lower than 0.5. Genetic analysis of a Panamanian population of the leaf–cutter ant Acromyrmex octospinosus showed that queens mate with at least 4 – 10 males. The detected (minimum) genetically effective paternity of nestmate females was 3.9 and estimates of mean relatedness among nestmate females were ca. 0.33. This implies that multiple queen–mating in Acromyrmex octospinosus reduces relatedness to 44% of the value in full–sib colonies (0.75), realizing 84% of the maximum reduction (to 0.25) that would be obtained with an infinite number of matings. Queens of Panamanian Acromyrmex octospinosus mate with more males than sympatric queens of Atta colombica, which is contrary to the positive relationship between queen–mating frequency and colony size found across more distantly related ant species. Possible selective forces that maintain high queen–mating frequencies in leaf–cutter ants are discussed.

Multiple paternity in the leafcutter ant Atta colombica — a microsatellite DNA study

An evolutionary understanding of the causes, correlates and consequences of multiple paternity in eusocial Hymenoptera (all ants, some bees and some wasps) relies on accurate estimation of this parameter at the species, population and colony level. We developed dinucleotide microsatellite DNA markers in order to study in detail the degree of multiple paternity in a population of the monogynous Panamanian leafcutter ant Atta colombica. These microsatellite markers were highly polymorphic such that nondetection caused by finite allelic diversity was very low (0.016). Hence, accurate information on the patterns of multiple paternity could be obtained. Genetic analysis revealed that in 33 out of 36 colonies two or more males had contributed genetically to the offspring. The mean number of fathers per colony was 2.6 and the mean effective paternity was 2.31. These are the highest values of effective paternity yet reported for any ant species. We examined the patterns of paternity frequency in young and old colonies to test if within-colony genetic diversity is related to colony survival, but found no support for any relationship. Our results confirm previous nongenetic studies showing high levels of multiple mating by queens of higher leafcutter ants. We discuss our findings in relation to known patterns of polyandry and paternity in other eusocial Hymenoptera.

Facultative Sex Allocation by Workers and the Evolution of Polyandry by Queens in Social Hymenoptera

Recent theoretical and empirical studies have shown that worker Hymenoptera may facultatively bias colony sex allocation according to the mating frequency of their queen, which results in more male-biased allocation in colonies with queens mated to an above-average number of males. Given the occurrence of this worker reproductive strategy, and because the queen-optimum sex-allocation ratio is more male biased than the population-wide worker-optimum ratio, individual queens may be able to enhance their inclusive fitness by mating with additional males. Using inclusive fitness models, we examine the fitness of single-, double-, and treble-mated queens in a population in which workers facultatively bias colony sex allocation with respect to queen mating frequency. We show that the relative inclusive fitness of double-versus single-mated queens can vary from 3:1 to 1:1. Low error rates in the assessment of queen mating frequency by workers, low worker production of males, low proportions of double-mated queens, and other factors enhance the relative fitness of the double-mated queens. Inclusive fitness gains to double-mated queens through more male-biased colony sex allocation must compensate for any costs arising from making additional matings. Double mating can be favored under some conditions but disfavored under others. Double- and single-mated queens may have equal total fitness at an intermediate proportion of double-mated queens in the population, which leads to a mixed mating strategy. We develop three nonexclusive scenarios under which treble mating can potentially be favored over double mating. The conditions favoring treble mating are more restrictive than those favoring double mating.

METAPLEURAL GLAND SECRETION OF THE LEAF-CUTTER ANT Acromyrmex octospinosus: NEW COMPOUNDS AND THEIR FUNCTIONAL SIGNIFICANCE

Ants of the myrmicine tribe Attini live in symbiosis with a fungus that provides them with food. In return the ants maintain optimal growth conditions for the fungus, weed out competing microorganisms, and inhibit the growth conditions of these competitors by chemical means. We present a comprehensive analysis of metapleural gland secretions of Acromyrmex octospinosus, using a recently developed method for the analysis of polar compounds by gas chromatography and mass spectrometry. We show that the chemical identity and quantitative recovery of different compounds in the metapleural gland secretion depends upon the method used and the type of colony from which the samples are taken. In addition to the two compounds previously recorded in the metapleural gland secretion of Acromyrmex ants (indolacetic acid and myrmicacin), 20 new compounds were detected in the secretion of a random sample of workers from two laboratory colonies and two field colonies. These compounds span the whole range of carboxylic acids from acetic acid to the long-chain fatty acids but comprise also some alcohols, lactones, and keto acids. The possible function of this highly complex secretion mixture is discussed.

Task partitioning in insect societies: bucket brigades

Summary: There are many ways in which social insect foragers may organise the collection of resources and their transportation back to the nest. One way is to partition the task into a number of sequential stages in which material is passed from one worker to another in a relay fashion. This relatively new concept is known as task partitioning. In this study, we focus on a particular form of task partitioning, bucket brigades, which we define as a multistage (i.e., three or more stages) partitioned transport scheme that uses only direct transfer between individual workers and without any predetermined transfer locations, other than the first or last stages. We first consider the potential costs and benefits of bucket brigades compared to other transportation schemes. We then use theory and computer simulation to analyse some of these aspects in detail. In one empirical study of a bucket brigade, foragers were generally found to be sequenced from smallest (near the food source) to fastest (nearest the nest). This exactly matches what dynamical systems theory would predict as an ergonomically efficient solution. However, we also demonstrate that a single and simple local rule - larger ants win fights over material - will generate this sequencing as an epiphenomenon that is not necessarily optimal. We use the behaviour of bucket brigades to reveal some general points about the optimality of task partitioning in more detail.

SEX-INVESTMENT RATIOS AND RELATEDNESS IN THE MONOGYNOUS ANT LASIUS NIGER (L.)

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).