Fine-scale population structure, inbreeding risk and avoidance in a wild insect population.

The ecological and evolutionary importance of fine-scale genetic structure within populations is increasingly appreciated. However, available data are largely restricted to wild vertebrates and eusocial insects. In addition, there is the expectation that most insects tend to have such large- and high-density populations and are so mobile that they are unlikely to face inbreeding risks through fine-scale population structuring. This has made the growing body of evidence for inbreeding avoidance in insects and its implication in mating systems evolution somewhat enigmatic. We present a 4-year study of a natural population of field crickets. Using detailed video monitoring combined with genotyping, we track the movement of all adults within the population and investigate genetic structure at a fine scale. We find some evidence for relatives being found in closer proximity, both across generations and within a single breeding season. Whilst incestuous matings are not avoided, population inbreeding is low, suggesting that mating is close to random and the limited fine-scale structure does not create significant inbreeding risk. Hence, there is little evidence for selective pressures associated with the evolution of inbreeding avoidance mechanisms in a closely related species.