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Ants of the species Camponotus floridanus live in huge colonies composed of genetically identical or closely related animals, which should predispose them to an increased vulnerability towards infection by pathogens (Cremer et al. 2007). Therefore the question is how ants (or social insects in general) can nevertheless efficiently combat infections. In order to investigate the immune response of the ant C. floridanus, the present study initially focused on the identification of possible immune factors, encoded by the ant´s genome. By using the method “suppression subtractive hybridization” as well as by Illumnia sequencing technology, several immune-related genes could be identified. Among these were genes encoding proteins involved in pathogen recognition, signal transduction, antimicrobial activity, or general stress response. In accordance with the ant´s genome sequence (Bonasio et al. 2010), only three antimicrobial peptide (AMP) genes could be identified in C. floridanus. The gene and cDNA sequences of these AMPs were established and their expression was shown to be induced by microbial challenge. Two different defensin genes (type 1 and 2) were characterized. A detailed characterization of the mRNA and gene sequence of the other AMP, a hymenoptaecin, revealed a special repeat structure. The C. floridanus hymenoptaecin has a signal and a pro-sequence followed by a hymenoptaecin-like domain and six directly repeated hymenoptaecin domains (HDs). Since each HD is flanked by two known processing sites, proteolytic processing of the precursor protein may generate several mature AMPs. Bioinformatical analyses revealed the presence of hymenoptaecin genes with similar multipeptide precursor structure in genomes of other ant species suggesting an evolutionary conserved important role of this gene in ant immunity. C. floridanus ants harbor the obligate intracellular bacterium, Blochmannia floridanus, in specialized cells (so-called bacteriocytes), which are intercalated between midgut cells as well as in ovaries of females (Blochmann 1882; Sauer et al. 2002; Schröder et al. 1996). Ant hosts face the problem that on the one hand they have to maintain the beneficial symbiotic bacteria and on the other hand they need to raise an immune response against harmful pathogenic bacteria during an infection. It was investigated, if endosymbionts are actually detected by the host immune system. Injection of B. floridanus induced an immune response of its host C. floridanus, which was comparable to the one towards pathogens. This means that, despite the evolutionary established cooperation of the endosymbionts and their hosts, these bacteria are still recognized as „non-self“ by the host immune system. This finding led to the question, if the ant immune system might be involved in regulation of the endosymbiont number in the midgut tissue in order to avoid their uncontrolled replication. During the holometabolous life cycle of the ant hosts the distribution of bacteriocytes and of Blochmannia endosymbionts is remarkably dynamic and peaks in late pupal stages, in which the entire midgut is transformed into a symbiotic organ (Stoll et al. 2010). It was hypothesized that hosts could regulate the number of endosymbionts present in their tissues via the innate immune system. A quantitative gene expression analysis of assumed symbiosis-relevant candidate genes revealed distinct expression patterns of some genes according to developmental stage and tissue. Moreover, the immune gene expression in response to bacterial challenge was investigated in the pupal stage. By an artificial immune-challenge of pupae it was confirmed that in fact the immune response of the endosymbiont-bearing midgut tissue differs from that of other body parts. The data support a key role for amidase peptidoglycan recognition proteins (PGRPs), especially PGRP-LB, in endosymbiont tolerance and suggest an involvement of the lysosomal system in control of Blochmannia endosymbionts. In sum, this thesis provides a first description of the immune response of the ant C. floridanus. A comprehensive set of immune-relevant genes was determined. Especially, the identification and molecular characterization of the hymenoptaecin gene delivered new insights into the immune competence of ants in general. Moreover, first indications could be gathered for the involvement of the immune system in controlling the endosymbiont B. floridanus.
Bees are subject to permanent threat from predators such as ants. Their nests with large quantities of brood, pollen and honey represent lucrative targets for attacks whereas foragers have to face rivalry at food sources. This thesis focused on the role of stingless bees as third party interactor on ant-aphid-associations as well as on the predatory potential represented by ants and defense mechanisms against this threat. Regular observations of an aphid infested Podocarpus for approaching stingless bees yielded no results. Another aim of this thesis was the observation of foraging habits of four native and one introduced ant species for assessment of their predatory potential to stingless bees. All species turned out to be dietary balanced generalists with one mostly carnivorous species and four species predominantly collecting nectar roughly according to optimal foraging theory. Two of the species monitored, Rhytidoponera metallica and Iridomyrmex rufoniger were considered potential nest robbers. As the name implies, stingless bees lack the powerful weapon of their distant relatives; hence they specialized on other defense strategies. Resin is an important, multipurpose resource for stingless bees that is used as material for nest construction, antibiotic and for defensive means. For the latter purpose highly viscous resin is either directly used to stick down aggressors or its terpenic compounds are included in the bees cuticular surface. In a feeding choice experiment, three ant species were confronted with the choice between two native bee species - Tetragonula carbonaria and Austroplebeia australis - with different cuticular profiles and resin collection habits. Two of the ant species, especially the introduced Tetramorium bicarinatum did not show any preferences. The carnivorous R. metallica predominantly took the less resinous A. australis as prey. The reluctance towards T. carbonaria disappeared when the resinous compounds on its cuticle had been washed off with hexane. To test whether the repulsive reactions were related to the stickiness of the resinous surface or to chemical substances, hexane extracts of bees’ cuticles, propolis and three natural tree resins were prepared. In the following assay responses of ants towards extract treated surfaces were observed. Except for one of the resin extracts, all tested substances had repellent effects to the ants. Efficacy varied with the type of extract and species. Especially to the introduced T. bicarinatum the cuticular extract had no effect. GCMS-analyses showed that some of the resinous compounds were also found in the cuticular profile of T. carbonaria which featured reasonable analogies to the resin of Corymbia torelliana that is highly attractive for stingless bees. The results showed that repellent effects were only partially related to the sticky quality of resin but were rather caused by chemical substances, presumably sesqui- and diterpenes. Despite its efficacy this defense strategy only provides short time repellent effects sufficient for escape and warning of nest mates to initiate further preventive measures.
A hitherto unresolved problem is how workers are prevented from reproducing in large insect societies. The queen informs about her fertility and health which ensures sufficient indirect fitness benefits for workers. In the ant Camponotus floridanus, I found such a signal located on eggs of highly fertile queens. Groups of workers were regularly provided with different sets of brood. Only in groups with queen eggs workers refrain from reproducing. Thus, the eggs seem to inform the workers about queen presence. The signal on queen eggs is presumably the same that enables workers to distinguish between queen and worker-laid eggs, latter are destroyed by workers. Queen and worker-laid eggs differ in their surface hydrocarbons in a similar way as fertile queens differ from workers in the composition of their cuticular hydrocarbons. When I transferred hydrocarbons from the queen cuticle to worker eggs the eggs were no longer destroyed, indicating that they now carry the signal. These hydrocarbons thus represent a queen signal that regulates worker reproduction in this species. But the signal is not present in all fertile queens. Founding queens with low egg-laying rates differ in the composition of cuticular hydrocarbons from queens with high productivity. Similar differences in the composition of surface hydrocarbons were present on their eggs. The queen signal develops along with an increasing fertility and age of the queen, and this is perceived by the workers. Eggs from founding queens were destroyed like worker eggs. This result shows that founding queens lack the appropriate signal. In these little colony foundations chemical communication of queen status may not be necessary to prevent workers from reproducing, since workers may benefit more from investing in colony growth and increased productivity of large colonies rather than from producing male eggs in incipient colonies. If the queen is missing or the productivity of the queen decreases, workers start laying eggs. There is some evidence from correlative studies that, under queenless conditions, worker police each other because of differences in individual odors as a sign of social status. It can be expressed as either aggressive inhibition of ovarian activity, workers with developed ovaries are attacked by nest-mates, or destruction by worker-laid eggs. I found that in C. floridanus workers, in contrast to known studies, police only by egg eating since they are able to discriminate queen- and worker-laid eggs. Workers with developed ovaries will never attacked by nest-mates. This is further supported by qualitative and quantitative differences in the cuticular hydrocarbon profile of queens and workers, whereas profiles of workers with and without developed ovaries show a high similarity. I conclude that workers discriminate worker eggs on the basis of their hydrocarbon profile, but they are not able to recognize egg-laying nest-mates. Improving our knowledge of the proximate mechanisms of the reproductive division of labor in evolutionary derived species like C. floridanus will help to understand the evolution of extreme reproductive altruism involving sterility as a characteristic feature of advanced eusocial systems.
This study investigated patterns of arthropod community organisation and the processes structuring these communities on a range of different tree species in a natural West African savannah (Comoé National Park, Côte d'Ivoire). It described and analysed patterns of arthropod distribution on the level of whole communities, on the level of multiple-species interactions, and on the level of individual insect species. Community samples were obtained by applying (i) canopy fogging for mature individuals of three tree species (Anogeissus leiocarpa, Burkea africana, Crossopteryx febrifuga) and (ii) a modified beating technique allowing to sample the complete arthropod communities of the respective study plants for medium-sized (up to 3 m) individuals of two other species (Combretum fragrans, Pseudocedrela kotschyi). General information on ant-plant interactions was retrieved from ant community comparisons of the mature savannah trees. In addition, ant-ant, ant-plant and ant-herbivore interactions were studied in more detail considering the ant assemblages on the myrmecophilic tree Pseudocedrela kotschyi. Herbivore-plant interactions were investigated on a multiple-species level (interrelationships between herbivores and Pseudocedrela trees) and on a species level (detailed studies of interrelationships between herbivorous beetles and caterpillars and the host tree Combretum fragrans). The studies on individual herbivore species were complemented by a study on an abundant ant species, clarifying not only the relationship between host plant and associated animal but allowing also to look at interactive (competitive) aspects of community organisation. The study demonstrated for the first time that (i) the structure of beetle communities on tropical trees can be strongly dependent on the host tree species, (ii) individual trees can host specific arthropod communities whose characteristic structure is stable over years and is strongly determined by the individual tree's attributes, (iii) ants can express a pronounced fidelity to single leaves as foraging area and can thereby determine distribution patterns of other ants, (iv) intraspecifically variable palatability of plants for insect herbivores can be stable over years and can influence the distribution of herbivores that can distinguish between individual hosts according to palatability and (v) intraspecific host plant change can positively affect fitness of herbivores if host plant quality is variable. In general, the present study contributes to our knowledge of anthropogenically unaltered processes affecting community assembly in a natural environment. The fundamental understanding of these processes is crucial for the identification of anthropogenic alterations and the establishment of sustainable management measures. The study points out the important role local factors can play for the distribution of organisms and thereby for community organisation. It emphasises the relevance of small scale heterogeneity of the abiotic and biotic environment to biodiversity and the need to consider these factors for development of effective conservation and restoration strategies.
The Mouthparts of Ants
(2001)
Ant mandible movements cover a wide range of forces, velocities and precision. The key to the versatility of mandible functions is the mandible closer muscle. In ants, this muscle is generally composed of distinct muscle fiber types that differ in morphology and contractile properties. Volume proportions of the fiber types are species-specific and correlate with feeding habits. Two biomechanical models explain how the attachment angles are optimized with respect to force and velocity output and how filament-attached fibers help to generate the largest force output from the available head capsule volume. In general, the entire mandible closer muscle is controlled by 10-12 motor neurons, some of which exclusively supply specific muscle fiber groups. Simultaneous recordings of muscle activity and mandible movement reveal that fast movements require rapid contractions of fast muscle fibers. Slow and accurate movements result from the activation of slow muscle fibers. Forceful movements are generated by simultaneous co-activation of all muscle fiber types. For fine control, distinct fiber bundles can be activated independently of each other. Retrograde tracing shows that most dendritic arborizations of the different sets of motor neurons share the same neuropil in the suboesophageal ganglion. In addition, some motor neurons invade specific parts of the neuropil. The labiomaxillary complex of ants is essential for food intake. I investigated the anatomical design of the labiomaxillary complex in various ant species focusing on movement mechanisms. The protraction of the glossa is a non muscular movement. Upon relaxation of the glossa retractor muscles, the glossa protracts elastically. I compared the design of the labiomaxillary complex of ants with that of the honey bee, and suggest an elastic mechanism for glossa protraction in honey bees as well. Ants employ two different techniques for liquid food intake, in which the glossa works either as a passive duct (sucking), or as an up- and downwards moving shovel (licking). For collecting fluids at ad libitum food sources, workers of a given species always use only one of both techniques. The species-specific feeding technique depends on the existence of a well developed crop and on the resulting mode of transporting the fluid food. In order to evaluate the performance of collecting liquids during foraging, I measured fluid intake rates of four ant species adapted to different ecological niches. Fluid intake rate depends on sugar concentration and the associated fluid viscosity, on the species-specific feeding technique, and on the extent of specialization on collecting liquid food. Furthermore, I compared the four ant species in terms of glossa surface characteristics and relative volumes of the muscles that control licking and sucking. Both probably reflect adaptations to the species-specific ecological niche and determine the physiological performance of liquid feeding. Despite species-specific differences, single components of the whole system are closely adjusted to each other according to a general rule.