Background/Question/Methods   Ecologists and evolutionary biologists have recently acknowledged that coevolutionary interactions are geographically heterogeneous. However, factors generating variation in coevolutionary processes across broad latitudinal range have not been fully understood yet. An ideal system investigating large-scale “geographic mosaic” of coevolution is found in the interaction involving a seed-predatory weevil, the camellia weevil (Curculio camelliae; Curculionidae), and its host plant, the Japanese camellia (Camellia japonica; Theaceae). Females of the weevil drill into camellia fruit with their extremely long mouthparts and deposit eggs into the seeds. To defend seeds against weevils, the Japanese camellia has evolved defensive fruit coats (pericarps) and thus weevil mouthpart length and camellia pericarp thickness have been exaggerated through a coevolutionary arms race.

Results/Conclusions   In this system, we found that the extent of the arms race, which was evaluated by weevil mouthpart length and camellia pericarp thickness, varied remarkably among geographic populations throughout the Japanese archipelago and it increased clearly at lower latitudes. The quantitative evaluation of natural selection exerting on the weevil and camellia traits showed that the direction and strength of coevolutionary selection were differentiated geographically. In elucidating the factors responsible for the latitudinal gradient of the arms race processes, we focused on the dependence of camellia's photosynthetic activity on local climate. The daily net carbon gain estimated based on local climatic data (e.g., temperature) and the physiological parameters of the Japanese camellia significantly increased at lower latitudes, suggesting that camellias could gain access to sufficient resources for defense in southern populations. Thus, the relaxed costs of producing pericarps at lower latitudes have potentially resulted in the evolution of thick camellia pericarps and the counter-evolution of long weevil mouthparts. As expected, the extent of the weevil-camellia arms race was significantly correlated with the photosynthetic activity of camellias across populations. Since the coevolutionary relationship exhibited appreciable variation across a relatively small range of annual mean temperatures, we propose that ongoing global climatic change can dramatically alter coevolutionary interactions between organisms, thereby restructuring ecological interactions in natural communities.