Temperature preference of Nile tilapia (Oreochromis niloticus) juveniles induces spontaneous sex reversal

Nile tilapia (Oreochromis niloticus) is an African freshwater fish that displays a genetic sex determination system (XX vertical bar XY) where high temperatures (above 32 degrees C to 36.5 degrees C) induce masculinization. In Nile tilapia, the thermosensitive period was reported from 10 to 30 days post fertilization. In their natural environment, juveniles may encounter high temperatures that are above the optimal temperature for growth (27-30 degrees C). The relevance of the thermal sex reversal mechanism in a natural context remains unclear. The main objective of our study is to determine whether sexually undifferentiated juveniles spontaneously prefer higher, unfavorable temperatures and whether this choice skews the sex ratio toward males. Five full-sib progenies (from 100% XX crosses) were subjected to (1) a horizontal three-compartment thermal step gradient (thermal continuum 28 degrees C-32 degrees C-36.5 degrees C) during the thermosensitive period, (2) a control continuum (28 degrees C-28 degrees C-28 degrees C) and (3) a thermal control tank (36.5 degrees C). During the first days of the treatment, up to an average of 20% of the population preferred the masculinizing compartment of the thermal continuum (36.5 degrees C) compared to the control continuum. During the second part of the treatment, juveniles preferred the lower, nonmasculinizing 32 degrees C temperature. This short exposure to higher temperatures was sufficient to significantly skew the sex ratio toward males, compared to congeners raised at 28 degrees C (from 5.0 +/- 6.7% to 15.6 +/- 16.5% of males). The proportion of males was significantly different in the thermal continuum, thermal control tank and control continuum, and it was positively correlated among populations. Our study shows for the first time that Nile tilapia juveniles can choose a masculinizing temperature during a short period of time. This preference is sufficient to induce sex reversal to males within a population. For the first time, behavior is reported as a potential player in the sex determination mechanism of this species.