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Mutualisms are ubiquitous in nature and shape whole ecosystems. Although species benefit by interacting with each other, they permanently act selfishly. As a consequence, the involved partners must balance gaining the maximal benefit while accepting a certain amount of costs. Changes in the environment, however, may alter selection pressures and lead to a shift in the relative costs and benefits for both involved species. Due to this complexity, many mutualisms and their underlying processes, such as the dependence of the involved species on each other, are only poorly understood. Moreover, in several so-called mutualistic interactions it is unclear if they are in fact beneficial for all partners because detailed cost-benefit analyses are missing. The aim of my thesis was to contribute to a better understanding of the basic principles of mammal-plant mutualisms with special emphasis on the interdependence of the involved species. Using the interaction between an insectivorous bat species (Kerivoula hardwickii) and carnivorous pitcher plants (genus Nepenthes) as a model system, I conducted a detailed cost-benefit analysis to test if the partners interact mutualistically and are strongly dependent on one another. I hypothesised that pitchers of these plants serve as high quality roosts for the bats while the bats in turn fertilise the plants via their nutritious faeces. For the involved species the costs of the interaction should be lower than the gained benefits, but general costs should increase in the absence of the partner. Over the course of my field research, I found the bats roosting in three Nepenthes species, but the bats occupied intact pitchers of only one species, Nepenthes hemsleyana. In Nepenthes bicalcarata and Nepenthes ampullaria, the bats used senescing or damaged pitchers whose high amount of digestive fluid had drained off. Thus, only N. hemsleyana was potentially able to digest bat faecal matter, and thereby benefit from the bats. My cost-benefit analysis showed that N. hemsleyana plants strongly benefited from their bat interaction partner: In feeding experiments the plants gained between 34% and 95% of their nitrogen from bat faeces, which significantly improved their growth, photosynthesis and survival. In contrast, plants without access to faeces could not fully compensate the induced lack of nutrients by using arthropod prey. Field observations revealed no obvious costs for the pitcher plant. N. hemsleyana pitchers occupied by bats did not differ in their lifespan from unoccupied ones as bats did not injure the plants’ tissue. The interaction was also advantageous for K. hardwickii because N. hemsleyana offered high quality roosts with a favourable microclimate and low parasite infestation risk. Consequently, bats roosting in N. hemsleyana pitchers were in better condition than those roosting in dead N. bicalcarata pitchers. Although N. hemsleyana pitchers are rare in the natural habitat, bats could easily find and identify them due to an echo reflector, which reduces time and energy costs for roost detection. Most N. hemsleyana plants continuously provided at least one intact pitcher meaning bats could return to the same plants over a period of several months or even years. The interaction between K. hardwickii and N. hemsleyana can be classified as an asymmetric facultative mutualism with stronger dependence of the plant partner. N. hemsleyana has outsourced arthropod capture and digestion to its mutualistic bat partner while arthropod attraction is strongly reduced. Contrastingly, several populations of K. hardwickii frequently use alternative roosts. Strong selective pressure on the plants could be the consequence to attract bats with a potential stabilising effect on the interaction: N. hemsleyana has to outcompete the involuntarily offered roosts of the other Nepenthes species in terms of quality and accessibility. My thesis revealed complex interdependencies in an animal-plant mutualism. This study exemplifies that rigorous cost-benefit analyses are crucial for the classification of interspecific interactions and the characterisation of how the involved species affect and depend on each other.
Bats are ancient mammals that evolved more than 50 million years ago. There are 1,240 different species (> 20% of mammalian species) described so far, which represent one of the most abundant, diverse and widely distributed mammalian groups. Bats are the only mammals which actively fly and therefore can migrate to different areas of the world. It has been increasingly recognized that bats are reservoirs for more than 100 virus species, and several are associated with animal and human epidemics. As natural hosts of rabies virus (RABV) and related lyssaviruses, bats have become a focus of research not only in South America and Africa, but also in Europe and North America. Bats are also considered to be unique in their potential to host emerging and re-emerging zoonotic viruses. To evaluate and reduce the potential risk of rabies transmission to humans or carnivore hosts (like fox, raccoon and dog etc.), active and passive surveillance studies of bat have been performed. Using these approaches diverse lyssaviruses have been detected in bats. However, these studies did not explain the rarely discovered epidemics and the underlying resistance or immune mechanisms in bats as natural hosts for lyssaviruses. Probably, bats are more resistant to lyssavirus infections than other animals. This hypothesis is introducing the research questions of the present thesis: (1) How do the innate immune responses protect bats from fatal outcome of lyssavirus infections? Interferon (IFN) responses which can be induced by the recognition of viruses by pattern recognition receptors act as the first line of defense against lyssavirus infections. Therefore, type I and type III IFNs from European bats were cloned and functionally characterized in this thesis (Chapter 2 and 4). (2) How do the lyssaviruses adapt to escape the host defenses by counteracting the IFN-mediated immune responses? And how do the bats control the viral replication via the IFN responses? To explore the complicated interactions and understand how European bats (Eptesicus serotinus, Myotis myotis and Nyctalus noctula) interact with European bat lyssaviruses (EBLV-1 and 2), a natural host related model for investigations of the bat´s immune system and the virus-host interactions has to be established. Since all of 52 identified European bats species are endangered and strictly protected, stable cell lines from different tissues of M. myotis for in vitro studies were developed and used for molecular and functional studies (Chapter 3 and 4). The data obtained from this thesis revealed that: (a) European bat IFNs do have similar but also distinct molecular characteristics compared with other mammalian IFNs (Chapter 2 and 4); (b) Both investigated bat type I IFNs, IFN-Kappa; and IFN-Omega; present potent anti-lyssaviral activities and display a pathogen associated pattern in the tested cell line (Chapter 2); (c) The established immortalized M. myotis cell lines are differently susceptible to lyssaviruses and contain a functional IFN-mediated signaling cascade (Chapter 3); (d) Bat type III IFN-Lambda;s display cell-type specific functions due to the distinct expression of the IFN-Lambda; receptor (Chapter 4); (e) In bat cell lines a possible evasive strategy of lyssavirus is based on the counteraction of IFN production and/or IFN-mediated defensive pathways (Chapter 3); (f) The higher resistance of brain derived cell line MmBr compared to other cell lines to lyssavirus infection indicates the natural ability of bat´s central nervous system to control the growth of neurotropic virus, which might be an essential reason for the nonclinical outcome (Chapter 3). Overall, the present thesis provides first insights into IFN-mediated innate immune responses against RABV and EBLVs infection in their natural reservoir hosts and an useful toolbox for comparative analysis of virus-host interactions.