We examined the reproductive cycle and transcription-level changes of the major yolk protein (MYP) in gonads of both sexes during the different reproductive stages of wild northern sea urchin, Mesocentrotus nudus, collected in two areas of southern Hokkaido, Japan. Histological evaluation revealed mature gonads in both April and August, suggesting two annual spawning seasons at Otoshibe, whereas one annual spawning season was observed in autumn (September) at Usujiri. The transcription levels of MYP, which is synthesized and stored in the gonads, increased from stage 1 (recovering) to stage 2 (growth) and thereafter decreased, in both female and male sea urchins. The current results indicate that two annual spawning seasons may occur in wild populations of the northern sea urchin in southern Hokkaido
