Plasticity of circadian and circatidal rhythms in the activity and transcriptomic dynamics in a freshwater snail
Biological clocks contribute to the adaptation to the periodic variation of environments. Organisms have diverse biological clocks synchronized with environmental cycles depending on their habitats. In some marine species, the advantages of anticipation of tidal changes have favored the evolution of circatidal rhythm. In a freshwater snail, Semisulcospira reiniana, individuals in a non-tidal area exhibit the circadian rhythm but those in a tidal area do both circadian and circatidal rhythms. Here, we investigated whether the circatidal rhythms of activity and transcriptome are determined genetically or induced by environmental cycles. After the exposure to the simulated tidal cycle, individuals in both tidal and non-tidal populations exhibited the circatidal rhythm in the locomotive activity, suggesting that the endogenous rhythm plastically changed depending on the environmental cycle. Transcriptome analysis revealed that genes with circatidal oscillation increased by the entrainment to tidal cycle in both tidal and non-tidal populations and the relative significance of circadian and circatidal rhythms are plastic depending on the environmental cycles. However, we found the circatidal oscillating genes more abundantly in the tidal population than in the non-tidal one. Our findings suggest that there is the plasticity of the endogenous rhythm in both populations but the number of genes associated with circatidal clocks in the tidal population was greater than that in the non-tidal population. The plasticity of biological rhythms and subsequent genetic changes in a molecular network between clocks and genes in downstream pathways may have contributed to the adaptation to the tidal environment in the freshwater snail.