To deepen understanding the evolutionary process of lucanid–yeast association, the lateral transmission process of yeast symbionts among stag beetle genera Platycerus and Prismognathus around the border between Japan and South Korea was estimated based on molecular analyses and species distribution modelings. Phylogenetic analyses were based on yeast ITS and IGS sequences and beetle COI sequences using Prismognathus dauricus from the Tsushima Islands and Pr. angularis from Kyushu, Japan, as well as other sequence data from our previous studies. The range overlap based on the species distribution model (SDM) and differentiation in ecological space were analyzed. Based on the IGS sequences, Clade II yeast symbionts were shared by Platycerus hongwonpyoi and Pr. dauricus in South Korea and the Tsushima Islands, and Platycerus viridicuprus in Japan. Clade III yeasts were shared by Pr. dauricus from the Tsushima Islands and Pr. angularis in Japan. During the Last Interglacial period when the land bridge between Japan and the Korean Peninsula existed, range overlap was predicted to occur between Pl. viridicuprus and Pr. dauricus in Kyushu and between Pr. dauricus and Pr. angularis in Kyushu and the Tsushima Islands. The ecological space of Pl. hongwonpyoi was differentiated from that of Pl. viridicuprus and Pr. angularis. We demonstrated the paleogeographical lateral transmission process of Scheffersomyces yeast symbionts among lucanid genera and species: putative transmission of yeasts from Pr. dauricus to Pl. viridicuprus in Kyushu and from Pr. angularis to Pr. dauricus in Kyushu or the Tsushima Islands. We also found that the yeast symbionts are likely being replaced in Pr. dauricus on the Tsushima Islands. We present novel estimates of the lateral transmission process of microbial symbionts based on phylogenetic, SDM and environmental analyses among lucanid beetles.

Dorcus montivagus is a Japanese endemic stag beetle that feeds exclusively on dead beech wood, and its distribution is almost completely coincidental with that of beech forests. Japanese beech, Fagus crenata, is the dominant tree species of the cool-temperate deciduous broad-leaved forests in Japan. Historical changes in the distribution of beech forests due to Pleistocene climate change are expected to have also affected the phylogeographic structures of dependent beech-feeding phytophagous insects. In this study, we elucidated the present phylogeographic structures of D. montivagus using molecular markers, and also compared them with the present geographic genetic structures of beech tress and the post-glacial distribution as inferred by pollen fossil analyses. It was found that D. montivagus is largely differentiated into two phylogenetic clades: Clade I consisted of populations from Hokkaido, Honshu and Shikoku, and Clade II consisted only of Kyushu populations. Furthermore, the genetic diversity of the stag beetle showed a geographically declining gradient from south-west to north-east, consistent with the genetic variation observable in Japanese beech. Genetic differentiation between the Sea of Japan side and the Pacific Ocean side populations was also observed in both clades of the stag beetle. These results indicate a similar phylogeographic structure between the stag beetle and Japanese beech. Therefore, the distributional changes in Japanese beech correspondingly restricted the migratory dispersal of D. montivagus, and strongly influenced its phylogeographic structure.

Floods affect the population structure of organisms that inhabit streams. In recent decades, the scale of floods has become larger due to climate change. Under these circumstances, on 12 October 2019, the largest typhoon in the history of observation in Japan struck the Japanese Archipelago. This typhoon caused heavy rainfall in various places, and the Chikuma–Shinano River System (Japan's largest) suffered great damage. Eight years before the large-scale disturbance in the river system, the population structure of the mayfly Isonychia japonica was studied in detail using quantitative sampling (population numbers and biomass) and by sequencing the mtDNA cytochrome c oxidase subunit I. To understand the impact of the flood on the population and genetic structures, we repeated the same research approximately 1 year after the flood. Direct comparison of sites before and after the flood revealed no significant changes between pre- and post-flood population genetic structure. This indicates high in situ resistance and/or resilience recovery of the populations to the disturbance. We hypothesize that this high resistance/resilience to flood disturbance is a result of strong selection for such traits in the rivers of the Japanese Archipelago, which are short, steep, flow rapidly and violently, and are strongly affected by floods.