Plasticity of circadian and circatidal rhythms in activity and transcriptomic dynamics in a freshwater snail

Abstract

Organisms have diverse biological clocks synchronized with environmental cycles depending on their habitats. Anticipation of tidal changes has driven the evolution of circatidal rhythms in some marine species. In a freshwater snail, Semisulcospira reiniana, individuals in nontidal areas exhibit circadian rhythms, whereas those in tidal areas exhibit both circadian and circatidal rhythms. We investigated whether the circatidal rhythms are genetically determined or induced by environmental cycles. After exposure to a simulated tidal cycle, both tidal and nontidal populations exhibited circatidal rhythms in locomotive activity. Transcriptome analysis revealed that genes with circatidal oscillation increased due to entrainment to the tidal cycle in both populations and dominant rhythmicity was consistent with the environmental cycle. These results suggest plasticity in the endogenous rhythm in both populations. However, circatidal oscillating genes were more abundant in the tidal population than in the nontidal population, suggesting that a greater number of genes are associated with circatidal clocks in the tidal population compared to the nontidal population. Our findings suggest that the plasticity of biological rhythms and genetic changes in the molecular network between clocks and downstream genes may have contributed to the adaptation to the tidal environment in S. reiniana.