Symbiosis is an important factor in evolution as a source of evolutionary novelty and ecological diversity. Microbial symbionts in particular have had a large impact on the evolution of eukaryotic organisms. An intimate type of symbiosis is endosymbiosis, in which (typically) bacteria live within the host cells.
Mitochondria and chloroplasts are examples of ancient endosymbioses that dramatically extended the range of eukaryotic life. Besides on these organelles of bacterial origin, many organisms depend on endosymbiotic bacteria for survival and reproduction. If both partners benefit from the symbiosis, evolution towards mutualism can be straightforward.
The question is however, how two distinct organisms can become totally dependent on each other. Although this question is much debated, the lack of suitable model systems has made the evolution of dependence largely an unexplored area in biology. The association between the parasitoid wasp Asobara tabida and the endosymbiotic Wolbachia bacteria provides an excellent model system to study the evolution of dependence.
Wolbachia bacteria totally depend on their hosts and maintain themselves in host populations by manipulating their hosts reproductive system. Here, symbiosis is also obligatory for the host. Without Wolbachia, European A. tabida cannot complete oogenesis. North-American strains, however, can complete oogenesis in the absence of Wolbachia.
The aim of the proposed study is to investigate the mechanisms involved in dependence, as well as its evolutionary consequences for the host. We will use cytological observations and QTL mapping to determine the processes and host genetic factors involved in Wolbachia dependent oogenesis.
The consequences of dependence for the life-history evolution of A. tabida will be determined, as well as the relation between bacterial density and egg load. The results of these studies will be integrated in model simulations of the dynamics of dependence in host populations.
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