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My main interested in biology is to gain an understanding of the factors that have led to the fascinating organismal diversity we observe today, and here especially the role of biotic interactions.
A key feature of the evolutionary success of social insects is their ability to form complex societies with division of labour and sophisticated communication systems. In addition to these complex social interactions of ants, they have shown an extraordinary capacity to form close associations with other organisms such as insects, plants, fungi and bacteria. A drawback of group-living is that individuals in groups or colonies may be more prone to diseases than organisms with a solitary life-style since a pathogen may be transmitted easily to the other individuals in a group, e.g. by social contact. On the other hand may group interactions enhance parasite resistance due to “social immunity”, i.e. behavioural or physiological mechanisms on the colony level.
- Diversity and function of bacterial endosymbionts and gut microflora in ants
- Ant-plant symbiosis of ants of the genus Crematogaster with their host-plants Macaranga
- Beyond the immune system: Behavioural adaptations towards pathogens in ant colonies
- Evolution of tolerance: Biology of the invasive ant Anoplolepis gracilipes
1. Diversity and function of endosymbionts and the intestinal microflora of ants
For a long time the nutritional basis for the high abundance and diversity of ants especially in canopies of tropical rain forests was a mystery. Only recently studies on the trophic level of ants using stable isotopes revealed that ants of certain genera mainly utilize nitrogen-poor plant sap or honeydew from trophobiosis with aphids and other insects instead of being mainly predators as had been postulated before. An alternative source of nitrogen for those ants could derive from symbiotic bacteria in the ants´ gut that recycle waste nitrogen or fix atmospheric dinitrogen similar to what is known from termites and their gut microflora or endosymbionts.
We therefore study and compare the diversity of bacteria found in the gut of different genera of arboreal ants as well as ants with other feeding habits. The aim is characterization of the gut microbiota as well as uncovering the functional role of such bacteria either for nutritional upgrading or other aspects of the ant host’s physiology like its immune system. We concentrate on the function of the intracellular endosymbiont Blochmannia of carpenter ants of the genus Camponotus as well as its sister genus Polyrhachis that together comprise almost 15% of all described ant species. Blochmannia upgrades the nutrition of the ant hosts by providing essential amino acids which should enable the ants to sustain on imbalanced food resources or food that contains only few essential amino acids. We are currently studying whether the bacteria are only important during larval development (direct benefits to the individual ant) or whether bacteria still present in workers feeding the brood contribute to the growth of larvae being fed (indirect benefits on colony level).
This project is conducted in close cooperation with the group of Prof. Roy Gross from the Department of Microbiology at the University of Würzburg, Germany.
2. Mechanisms of speciation in obligate plant-ants of the genus Crematogaster
Ecological interactions including biotic interactions could be crucial factors driving speciation. In one of the most species-rich ant-plant symbiotic systems worldwide pioneer trees of the genus Macaranga (Euphorbiaceae) are inhabited by specific partner ants, mostly of the genus Crematogaster (subgenus Decacrema). The various types of interaction in this symbiotic complex and its considerable degree of radiation concerning both partners offer an exceptional model system for studying the processes of speciation in mutualistic systems in general. Our current results indicate that multiple speciation mechanisms work within the Decacrema ants (geographic and ecological). Therefore, this system allows us to shed light simultaneously on different speciation mechanisms. A comparison of the phylogenies of the ants with that of their Macaranga host-plants supports the view that we have no strict co-speciation but host-expansion and host-shifting is common. Nonetheless, non-random association patterns are observed and we have first indications that at least some of the Decacrema-morphospecies develop ecotypes which are specifically adapted to "their" host-plant species. On the other hand the symbiosis with their host-plants may enhance allopatric speciation since these obligate -plant-ants are restricted to their hosts for nesting sites, which seems to leads to a strong fragmentation of the ant populations.
This project was part of the DFG Priority Programme 1127 “Radiationen – Genese Biologischer Vielfalt” (2002 – 2008)
3. Beyond the immune system: Behavioural adaptations towards pathogens in social insects
Pathogens are a major challenge to social insects since the high density, interaction rate and relatedness of individuals within colonies should render them particularly vulnerable to infections. On the other hand, sociality facilitates a collective immunity based on behavioral mechanisms. Thus, allogrooming and hygienic behavior employed by social insects may lead to a decrease in the susceptibility towards pathogens. The aim of this project is to examine the behavioral mechanisms employed by workers to reduce the risk of pathogen transmission within colonies and to uncover the mechanisms underlying the recognition of pathogen loaded nestmates in the carpenter ant Camponotus floridanus. The specificity of behavioral mechanisms towards different pathogens will be compared. The empirical data on changes in interaction rate and mortality are a prerequisite for an understanding of the consequences of pathogen infection on colony efficiency. This work will shed light on the evolutionary role of pathogens in shaping colony organization of social insects.
4. Evolution of tolerance: The invasive ant Anoplolepis gracilipes
Biological invasions are a leading threat to biodiversity worldwide, and invasive ants are among the most damaging of invasive species. Invasions by non-native ants can cause considerable economic damage, disrupt local arthropod communities, and once established, are extremely difficult to control. One of the factors thought to contribute to the ecological success of invasive ants is their colony structure. In their introduced ranges, invasive ants often form unicolonial populations or supercolonies, a trait which has also been assigned to populations of A. gracilipes that are thought to be introduced. Characteristics of supercolonies include low relatedness amongst nestmates, the presence of multiple queens, and a lack of clear behavioural boundaries or aggression between workers from distinct nests. Since this type of colony structure reduces costs associated with territoriality, it enables high worker densities and therefore numerical dominance within the invaded habitat. With the exception of nestmates showing high rather than low relatedness amongst nestmates, supercolonies of A. gracilipes display all these characteristics. Currently we are examining the reproductive system of Anoplolepis gracilipes (in cooperation with Prof. Laurent Keller, University of Lausanne, Switzerland and Prof. Serge Aron, University of Brussels, Belgium), the ecological impact of this ant as well as its nestmate recognition system.
This project is part of the SFB 554 “Arthropod behaviour” at the University of Würzburg, Germany. Collaborators within the SFB 554 are Dr. Nico Blüthgen (University of Würzburg, Germany) and Dr. Thomas Schmitt (University of Freiburg, Germany).
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