Reconstructing the population history of the sandy beach amphipod Haustorioides japonicus using the calibration of demographic transition (CDT) approach.

Calibration of the molecular rate is one of the major challenges in marine population genetics. Although the use of an appropriate evolutionary rate is crucial in exploring population histories, calibration of the rate is always difficult because fossil records and geological events are rarely applicable for rate calibration. The acceleration of the evolutionary rate for recent coalescent events (or more simply, the time dependency of the molecular clock) is also a problem that can lead to overestimation of population parameters. Calibration of demographic transition (CDT) is a rate calibration technique that assumes a post-glacial demographic expansion, representing one of the most promising approaches for dealing with these potential problems in the rate calibration. Here, we demonstrate the importance of using an appropriate evolutionary rate, and the power of CDT, by using populations of the sandy beach amphipod Haustorioides japonicus along the Japanese coast of the northwestern Pacific Ocean. Analysis of mitochondrial sequences found that the most peripheral population in the Pacific coast of northeastern Honshu Island (Tohoku region) is genetically distinct from the other northwestern Pacific populations. By using the two-epoch demographic model and rate of temperature change, the evolutionary rate was modeled as a log-normal distribution with a median rate of 2.2%/My. The split-time of the Tohoku population was subsequently estimated to be during the previous interglacial period by using the rate distribution, which enables us to infer potential causes of the divergence between local populations along the continuous Pacific coast of Japan.