Previously, we demonstrated that medaka females recognize familiar males following prior visual exposure, and social familiarity influences female mating receptivity. Medaka females exhibit a positive response (high receptivity) to familiar males, and a negative response (low receptivity) to unfamiliar males. Further, we demonstrated the essential role of a subpopulation of gonadotropin-releasing hormone-producing neurons (GnRH3 neurons) in switching from low to high female receptivity (Science, 2014). Next, we found that medaka use faces for individual recognition. Females can discriminate between two male faces and two objects, but upside-down of the faces made it more difficult to discriminate them. When discriminating between two non-face objects, upside-down did not affect it. Thus, faces may be special for fish, just as humans (eLife, 2017). This is the first study that shows the face inversion effect in animals other than mammals. We further established various behavior paradigms to assess social interactions such as schooling behavior (PLoS One, 2010), conspecific recognition（Zool. Sci., 2016）, mate-guarding（PLOS Genetics, 2015; Front. Zool., 2016）, and social learning （PLoS One, 2013）. We established a new methodology for heat-inducible Cre/LoxP recombination in the medaka brain (PLoS One , 2013). Using the IR-LEGO system, heat shock induced in a very small area of the developing brains leads to spatially controlled recombination of progeny cells in adult medaka fish, which allows for genetic modulation and/or visualization of neuronal populations of interest. Now CRISPR/Cas9 system is available in medaka fish, which allows us to generate efficiently knock-out and knock-in medaka. Using the medaka systems we are trying to systematically identify internal factors (genes, neural networks, and brain regions) essential for vertebrate social interactions. On the other hand, to estimate behavior rules underlying social interactions, we developed a hypothesis-independent data mining, which could explain actual fish movement (PLoS One, 2013). Our eventual purpose is to how the internal factors (genes, neural networks, and brain regions) influences animal behaviors, which can serve as the basis for the emergence of sociality.
