Local adaptation in a phoretic mite-burying beetle symbiosis

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When organisms adapt to their environment, differences between habitats can lead to differences between populations in the adapted traits. While these local adaptations can be straightforward when evolved in response to relatively stable abiotic factors, such as temperature, local adaptation to other species that themselves also adapt will be more complex. This is particularly important in cases of coevolution, which leads to reciprocal and potentially undirected co-adaptation between symbionts and can cause a geographic mosaic of coevolution, where fitness is determined by a gene (focal species) by gene (symbiont) by environment interaction (GxGxE). I propose to study local adaptation in phoretic mites that live in symbiosis with burying beetles. Preliminary experiments show that mite and beetle fitnesses are negatively correlated, indicating parasitism by the mites. Each of the multiple mite species is specifically adapted to one of the many beetle species, and vice versa. Mite generation time, a trait crucial to the interaction, varies between mite populations and seems to be locally adapted to the host beetles and to the environment. All data we collected so far indicate a coevolutionary system that is affected by environmental parameters. The system is ideal to study local adaptation and coevolution because it is amenable to controlled laboratory fitness experiments and to transplantation experiments in the field. Combined with genetic analyses, we will use these experiments to test the hypothesis that mites and beetles are locally adapted to each other and to the remaining environment in a geographic mosaic of coevolution. Then, each organism's fitness should depend on the interaction of the organism's genotype with that of its symbiont, and how the interaction is affected by the environment (GxGxE). The alternative hypotheses are a pure GxE interaction with local adaptation to abiotic factors, a pure GxG interaction (the coevolutionary interaction is not affected by the environment), or no interaction. Unlike many studies of coevolutionary systems, which are typically using microbial symbionts and/or are characterised by strong fitness effects (e.g. lethal parasites) and exclusive symbioses (no alternative symbionts), the mite-beetle system represents a more diffuse case of coevolution. By partitioning environmental and genetic effects and all possible interactions in the mite-beetle association, we will contribute to the understanding of diffuse coevolution, which despite being the most common form of species interaction is grossly understudied because it is inconspicuous and often seems a difficult experimental system.