Rapid evolution of photoperiodic response in a recently introduced insect Ophraella communa along geographic gradients

The introduced beetle Ophraella communa was first found in 1996 in Japan and has rapidly expanded its distribution to include regions that encompass a wide range of latitude and altitude and are dominated by different host-plants. In this study, we investigated geographic variation in its photoperiodic response for the induction of reproductive diapause, with which the beetle adjusts its life cycle to local climate and host-plant phenology. The beetle lines were collected from 18 sites in Japan. The diapause incidence under a photoperiodic condition of 13h light:11h dark (LD13:11) and the critical day length differed among the beetle lines. Analysis with the generalized linear model showed that latitude, altitude and host-plant species (Ambrosia artemisiifoliavs. Ambrosia trifida) had significant effects on diapause incidence under LD13:11. These results suggest that the O.communa populations have rapidly adapted to local environmental conditions after their colonization. However, the photoperiodic response of the O.communa population in Tomakomai, the northernmost part of its distribution range in Japan, deviated significantly from the general trend. We suggest that this deviation is attributed to either: (i) that this beetle has colonized Tomakomai more recently compared to the other sites; or (ii) that the Tomakomai population has adapted to local environments in a different way from other populations.