Natural selection on PHYE by latitude in the Japanese archipelago: insight from locus specific phylogeographic structure in Arcterica nana (Ericaceae)

Phytochromes play a key role in allowing plants to monitor their surrounding environment and, conversely, adaptation to local environments has driven the evolutionary history of phytochromes. As a result of natural selection, polymorphisms in phytochrome genes would thus be expected to exhibit locus-specific phylogeographic structure. To evaluate this hypothesis, we conducted a phylogeographic investigation based on four nuclear genes, including two phytochrome genes (PHYB and PHYE) using 155 samples of Arcterica nana from the entire range of the Japanese archipelago. Bayesian clustering revealed geographic differentiation between northern and southern Japan when all four genes were included. However, this geographic differentiation is inconsistent with previously reported genetic structure of genome-wide polymorphisms based on amplified fragment length polymorphisms, as these did not show geographic differentiation throughout the Japanese archipelago. In contrast, the north-south differentiation was not apparent when PHYE was excluded. This indicates that PHYE alone could be responsible for the north-south differentiation (F-CT = 0.15, P < 0.001). Furthermore, a single nonsynonymous polymorphism (C360T) strongly contributed to geographic differentiation (F-CT = 0.57, P < 0.001) and its corresponding amino acid replacement (P120L) was significantly under positive selection based on maximum likelihood analysis (P = 0.98). Consequently, the locus-specific geographic differentiation in PHYE could be caused by natural selection, suggesting the involvement of PHYE in local adaptation between populations of A. nana in northern and southern Japan. This finding is consistent with a previous study on Cardamine nipponica, indicating the importance of PHYE for local adaptation in Japanese alpine plants.