Among the various mutualistic plant-insect relationships, one of the most interdependent and fascinating interactions is known as “obligate pollination mutualism”. In this symbiotic relationship, certain insects feed on host plant seeds during their larval stage and later act as pollinators for the host's flowers. This not only provides valuable insights into the mechanisms driving floral diversity, but also serves as an important model for studying the dynamics of biological interactions and coevolution. This study focuses on two species of sawflies belonging to the genus Nipponorhynchus: N. bimaculatus and N. mirabilis. These two herbivorous sawflies have been documented as specialized seed feeders on two plant species, Chrysosplenium macrostemon var. shiobarense and C. echinus, both of which belong to the Chrysosplenium sect. Chrysosplenium series Macrostemon (hereafter just Macrostemon). In addition, visits of adult N. bimaculatus to C. macrostemon var. shiobarense have been observed. If Nipponorhynchus adhere to pollen and act as effective pollinators of Macrostemon flowers, their relationship is expected to be symbiotic rather than parasitic, although Nipponorhynchus larvae feed exclusively on Macrostemon seeds, implying an obligate pollination mutualism. It is also noteworthy that adult Nipponorhynchus have unusually elongated proboscises adapted for nectar feeding. This specialized proboscis is thought to be an adaptation for accessing hidden nectar deep within flowers, developed through interactions with Macrostemon flowers. In this study, I hypothesized two things: (1) Macrostemon plants and Nipponorhynchus sawflies are in an obligate pollination mutualism, and (2) Nipponorhynchus has a proboscis adapted to the diverse flowers found in each region. To test these hypotheses, I observed pollinators of Macrostemon flowers to determine whether Nipponorhynchus sawflies are the only efficient pollinators. I then compared pollination-related traits in both Nipponorhynchus sawflies and Macrostemon flowers across multiple populations to examine whether coevolutionary processes are driving morphological adaptations specific to each region.
As a result, two species of Nipponorhynchus sawflies were identified as the primary and effective pollinators of Macrostemon flowers, confirming that these plants and insects are in an obligate pollination mutualism. In addition, proboscis length varied among different populations of Macrostemon species, and proboscis length of Nipponorhynchus sawflies and stamen and pistil length of Macrostemon flowers were associated in two different populations, demonstrating the possibility of coevolution of pollination-related traits in highly interdependent plants and pollinators.