In temperate regions, an external environment changes dramatically according to seasons. Many of organisms sense the seasonal change from photoperiod and adjust their physiological status and behaviors appropriately. It is suggested that the circadian clock that generates an internal rhythm of about 24 hours plays an important role in the photoperiodic time measurement. On the other hand, brain neural signals that transmit the circadian clock-based photoperiodic information and cellular photoperiodic responses caused by these signals were unclear. The authors’ research group uses field-collected insects that show clear photoperiodic responses, and analyzes the brain neural mechanism of circadian clock-based photoperiodism by a combination of physiological and RNA interference-mediated gene knockdown methods. In this paper, I first introduce the history of research on photoperiodic time measurement systems based on the circadian clock. Next, I introduce our recent researches of the neural signaling that transmits photoperiodic information and the cellular photoperiodic responses of reproductive control cells in the bean bug Riptortus pedestris that shows clear photoperiodism in reproduction.

The task of self-other discrimination is ubiquitous across animal species. Corollary discharge is an internal predictive motor signaling, which modulates sensory processing and mediates self-other discrimination. Studies using weakly electric fish of the family Mormyridae, which communicate and electrically localize by emitting electric pulses, have made significant contributions to our understanding of the neural mechanisms of corollary discharge. Here I review (1) the discovery of the concept of corollary discharge, (2) the roles and mechanisms of corollary discharge in electro-communication, passive electrolocation, and active electrolocation, and (3) the evolution of corollary discharge, including relatively recent findings.

Sexual dimorphism, the phenotypic differences between males and females, is prevalent but rapidly diversified within and among species. It has been known that sexual selection, an evolutionary process driven by competition for mates with the opposite sex, plays important roles in the evolution of sexually dimorphic traits. However, few studies have identified the genes responsible for the diversification of sexual dimorphism, and thus the detailed genetic mechanisms underlying the sex-biased expression of the responsible genes and the detailed processes by which the novel alleles can be spread within populations remain unknown. In this review, we focus on medaka fishes (the family Adrianichthyidae) endemic to Sulawesi, Indonesia, which exhibits a remarkable diversity in sexual dimorphic traits, as an experimental model system for studying the evolution of sexual dimorphism. We present our recent study on the red nuptial coloration found in the male pectoral fins of Oryzias woworae as a practical case study. Furthermore, we highlight the potential advantages of the Sulawesian medaka fishes as a model system for comparative biology and other research fields.