Hormone-enzyme dynamics leads to serial sex change in bidirectional ways: an approach from the proximate cause

      Marine gobies (4 species in genus Trimma) show bidirectional sex change, and have a specific feature for their gonads. The gonad has an inactive ovarian tissue within functional testis when an individual is male, and vice versa. The cue of the switching whether an ovarian tissue is active or inactive is the presence of a dominant male in a mating group. The largest female changes sex to male in the absence of the dominant male. When two males coexist accidentally, a smaller male changes sex back to female. Therefore the change of the social status may promote gonad development (active or inactive). I focus on one enzyme and two sex hormones from physiological experimental data of Sunobe et al. (2005): P450arom (an enzyme which synthesizes from testosterone to estradiol), testosterone (male hormone), and estradiol (female hormone). P450arom induces the active production of estradiol in female phase, on contrast the decrease of P450arom brings the low concentration level of estradiol in male phase. The alteration in concentration of estradiol is important for sex change, and the specific activation-deactivation of P450arom pathway controls serial sex change in both directions. I propose a simple hormone-enzyme dynamics the production of P450arom depending on the social condition controls that of estradiol to explain experimental data about the concentration levels of two sex hormones and P450arom in various sexual phase, and discuss the asymmetry of sex-change time in both directions (7 days from female to male, 11days from male to female in Trimma okinawae).