June 11, 2020
The University of Tokyo
Kwansei Gakuin University
Keio University
Graduate School of Science and Faculty of Science, The University of Tokyo
The primary characteristic of eusocial insects is a reproductive division of labor, where only some individuals are responsible for reproduction, while others work as a sterile worker caste (workers). In termites, it has been thought that male and female reproductives (the queen and king) coexist within the colony and maintain an appropriate number and ratio of reproductives by influencing the caste differentiation of other individuals. However, the details of the mechanism determining caste differentiation through the interaction between male and female reproductives and other individuals remained unknown.
A research group led by Professor Toru Miura of the Misaki Marine Biological Station, Graduate School of Science, The University of Tokyo, and Senior Assistant Professor Yoshinobu Hayashi of the Faculty of Law (Department of Biology, Keio University), Keio University, focused on "supplementary reproductive differentiation" (hereafter, "reproductive differentiation"), a process where workers, which normally only engage in labor, molt into reproductive individuals when reproductives are lost from the termite nest. In this differentiation process, it has been hypothesized that a mechanism exists whereby the physiological state of workers changes in response to the presence of reproductives, leading to their differentiation into reproductives. In termites, it is known that fluctuations in the concentration of juvenile hormone (JH) in the body determine caste differentiation fate, but the function of JH in reproductive differentiation was unknown. Therefore, in this study, we used the termite *Hodotermopsis sjostedti* to conduct experiments and analyses on how the physiological state of workers housed with male and female reproductives changes to determine their caste differentiation fate. The results of the rearing experiments revealed that the promotional effect of male reproductives on female reproductive differentiation was clearly stronger than the reverse (the effect of female reproductives on male reproductive differentiation), and that rapid female reproductive differentiation was achieved by shortening the period until molting. We also clarified that this promotional effect is achieved by male reproductives lowering the internal JH concentration in female workers.
This was also supported by the fact that when a JH analog was administered to workers in the presence of a male reproductive, which should have induced female reproductive differentiation, the workers' molting period was extended and reproductive differentiation was inhibited. Thus, it was strongly suggested that behind termite reproductive differentiation, there is a mechanism where male reproductives promote female reproductive differentiation by manipulating the JH concentration in female workers, resulting in a female-biased sex ratio of reproductives.
The results of this research were published in "Scientific Reports" on June 10, 2020.
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