Mechanical mismatch of pollination-related traits causes severe pollen limitation in the Roscoea purpurea-Philoliche longirostris mutualism.

The geographical variation in the strength and direction of selection in highly specialized plant-pollinator mutualisms can lead to different interaction outcomes among populations. In this study, we investigated the mutualistic interaction between an alpine ginger Roscoea purpurea and a long-tongued tabanid fly Philoliche longirostris across eight populations in central Nepal. We measured the flower and fly traits, assessed the visitation behaviour and frequencies of P. longirostris, and quantified the reproductive success of R. purpurea. Our results revealed that in three of the eight populations both nectar distance and corolla-tube length were longer than the fly proboscis length, indicating a clear mechanical mismatch between the interacting traits of plants and the fly. The visitation frequencies of the fly to R. purpurea flowers were substantially lower in the three populations compared to the other five populations, yet the fly did not visit any co-flowering species. In the three populations, the pollen limitation indices for both fruit set and seed set were over 0.95 indicating that plant reproduction was severely pollen limited. We suggest that if the mechanical mismatch is a persistent feature of the populations, it may eventually lead to a mutualism breakdown between R. purpurea and P. longirostris, which would have severe consequences for the existence of both organisms, particularly in the populations encountering mechanical mismatch between the interacting traits.