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A central issue in the evolutionary ecology of species interactions is coevolution, which involves the reciprocal selection between individuals of interacting species. Understanding the importance of coevolution in shaping species interactions requires the consideration of spatial variation in their strength. This is exactly what the, recently developed, geographic mosaic theory of coevolution addresses. Another major development in the study of population ecology is the introduction of the population genomics approach in this field of research. This approach addresses spatial processes through molecular methods. It is of particular interest that population genomics is especially applicable to natural populations of non-model species. We describe how population genomics can be used in the context of the geographic mosaic of coevolution, specifically to identify coevolutionary hot-spots, and to attribute genetic variation found at specific loci to processes of selection versus trait remixing. The proposed integration of the population genomics approach with the conceptual framework of the geographic mosaic of coevolution is illustrated with a few selected, particularly demonstrative, examples from the realm of insect--plant interactions.

There is growing evidence of the importance of microbial mutualistic symbioses in insect-plant interactions. Mutualists may affect host plant range and enable insects to manipulate plant physiology for their own benefit. The plant can also be a route for the horizontal transfer of mutualistic microorganisms among their host insects. Where this occurs, selection for improved transmission might cause the insect mutualist to damage the plant and become a plant pathogen. Insect microbial associates can influence ecological communities by changing the way the plant interacts with their hosts' competitors and natural enemies. We review recent research in this field and suggest that insect mutualists may be more important 'hidden players' in insect-plant interactions than is currently realised.