Dynasym

Symbiosis as a biological interaction describes the physical association of individuals from different species. These associations can be positive, negative or neutral to either species such as in mutualism, parasitism or commensalism. The traditional view on symbioses is that the relationship between the species is a fixed property and only recently it was found that these relationships can be context-dependent, for example, a positive relationship can become negative to a symbiosis partner, when conditions (the context) change. An example for context-dependence is density dependence which alters the form of symbiosis when the density of one or both symbiosis partners changes. A specific form of symbiosis is protective symbiosis where one symbiosis partner (host) provides a shelter for the other partner. For the host, this form of symbiosis can be positive, negative or neutral. In the proposed project, context-dependence will be studied in a system of protective symbiosis in which other interactions (predation, competition and parasitism) occur. The study system comprises five planktonic freshwater species: the two herbivors Brachionus rubens (Rotifera) and Daphnia sp. (Cladocera) which compete for the same resource, the chlorophyte Monoraphidium minutum. Daphnia also provides shelter for B. rubens which attaches to Daphnia when the rotiferan predator Asplanchna brightwelli is present (protective symbiosis) which decreases the fitness of Daphnia. These interactions are further influenced by parasitism: when a microsporidian parasite (Microsporidium tvaerminnensis) infects Daphnia, it slows down the movement of Daphnia which in turn facilitates the attachment of B. rubens. The focus of this project lies on the Brachionus-Daphnia symbiosis and density-dependence will be studied by altering the density of Daphnia and the predator Asplanchna. By performing series of laboratory experiments with interacting symbiosis partners, the individual costs and benefits for each of them will be quantified. These experiments will allow for a detailed density-dependent cost-benefit analysis. Extending the complexity to four- and five-species communities and to longer-term experiments, the population dynamics of the symbiosis partners under different scenarios will be studied. Overall, the proposed projects aims to test the hypotheses that (i) the form of symbiosis is density-dependent as costs and benefits change with changing densities of the partners and (ii) the density-dependent form of symbiosis determines the population dynamics. Testing these hypotheses with a five-species interaction web goes beyond typical studies on symbioses, where rarely more than two interacting species are studied.