| dc.description.abstracteng | Fungi are considered as main decomposers in terrestrial ecosystems and, due to their diverse lifestyles, engaged in a plethora of interactions with soil invertebrates. Determining the underlying mechanisms that regulate interactions with invertebrates is of major importance for understanding the dynamics of soil fungal and animal communities and assessing the impact of these interactions on ecosystem processes and functions. The outcome of invertebrate-fungus interactions has been assumed to depend strongly on fungal chemical properties, viz. the production of non-volatile and volatile secondary metabolites. Soil invertebrates are known to use fungal-derived volatiles as cues for the location of suitable habitats, feeding, and oviposition sites. However, especially with regard to springtails (Collembola) and woodlice (Isopoda), a direct link between the dynamics in fungal volatile emissions and the behaviours underlying food location and selection of these important decomposers is still missing. The aim of the present PhD project was to investigate this aspect in more detail by combining analyses of fungal volatile profiles and behavioural responses of animals by continuous video observation and by focussing on different behavioural components of the food selection process separately.
The first main intent was to find out whether a certain group of volatiles, namely oxylipins, are used as cues by isopods and Collembola during foraging. Derived from the fact that the constitutive and wound-activated emission is a conserved mechanism in higher fungi I assume that oxylipin volatiles are ubiquitous in soil habitats. Since oxylipin volatiles are well known to play significant roles as infochemicals in plant-insect interactions I hypothesised that oxylipin volatiles are of similar importance in mediating interactions between fungi and soil invertebrates and function as food-finding cues for Collembola (F. candida, S. curviseta, H. nitidus) and isopods (O. asellus). Unexpectedly, wound-activated increases in oxylipin emissions did not increase the food-finding efficiency of these animals, however, independent of the wounding treatment, isopods were found to be attracted to C. globosum fungal colonies. Moreover, the common fungal oxylipin volatile 3-octanone arrested isopods in close proximity to the volatile source. This provides very first evidence of a role of fungal volatiles in affecting the foraging behaviour of isopods and indicates that isopods use volatiles as information to locate fungal food from a distance. Furthermore, upon direct contact with fungal colonies, fungal tissue wounding increased the acceptance of C. globosum colonies as food source by Collembola and, most interestingly, the mere presence of
3-octanone elicited test-biting behaviour in these animals, a here newly-observed component of the Collembola foraging behaviour. These findings strongly suggest that oxylipin volatiles, at least 3-octanone, in fact influence the foraging behaviour of both Collembola and isopods by acting as attractants, arrestants, and phagostimulants.
The second main intent was to investigate whether Collembola (F. candida) are able to differentiate between fungi of varying suitability by means of volatiles. Therefore, I observed behavioural responses of Collembola to different yeasts and filamentous fungi, determined fitness consequences of the respective fungal diets, and analysed fungal volatile profiles. In line with expectations, volatile-mediated responses of Collembola were largely reflected in the acceptance of fungi as food source and finally in the fitness of the animals (growth and reproduction), indicating that the use of volatile cues is important for Collembola to optimise their fitness. The observed higher attractivity and acceptance of yeasts and the filamentous fungus A. nidulans can most likely be attributed to the presence of 3-methyl-1-butanol within an otherwise volatile-poor background. Interestingly, Collembola were deterred by the volatile bouquet of the filamentous fungus P. expansum, did not accept this fungus as food source and had the lowest fitness increase. Responses to the Penicillium characteristic terpenoid geosmin indicate that this compound may contribute to the repellent effect. Possibly, some fungi produce such repellent compounds to fend off predators. These results clearly show that Collembola discriminately use fungal-derived volatile cues to make adaptive foraging decisions. Besides chemical properties, differences in physical properties (unicellular or hyphal growth) may influence the accessibility and consequently the acceptance of fungal food sources by Collembola, however, this needs to be tested in future studies.
Overall, the results provide further evidence of a significant role of fungal chemical properties, volatiles in particular, in influencing the outcome of fungus-invertebrate interactions. Future studies should focus on investigating whether the here observed behavioural responses of isopods and Collembola actually reflect responses exhibited in their natural habitat. | de |