Abstract
Nile tilapia (Oreochromis niloticus) is an African freshwater fish that displays a genetic sex determination system (XX|XY) where high temperatures (above 32°C to 36.5°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°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°C– 32°C– 36.5°C) during the thermosensitive period, (2) a control continuum (28°C– 28°C– 28°C) and (3) a thermal control tank (36.5°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°C) compared to the control continuum. During the second part of the treatment, juveniles preferred the lower, nonmasculinizing 32°C temperature. This short exposure to higher temperatures was sufficient to significantly skew the sex ratio toward males, compared to congeners raised at 28°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.
Highlights
Sexual determination (SD) is a process by which a vertebrate decides which direction to take when it initiates its sexual differentiation as male or female [1]
The general occupation pattern was different between control continuum (CC) and thermal continuum (TC) (Fig 2A)
During the high temperature (HT) treatment period, fish were mainly located in compartment 2 at 32 ̊C, while occupation of compartment 3 (36.5 ̊) decreased from 34.13 ±18.21% (15 dpf) to 7.13 ±11.13% (29 dpf)
Summary
Sexual determination (SD) is a process by which a vertebrate decides which direction to take when it initiates its sexual differentiation as male or female [1]. Since the first evidence of TSD in fish in Atlantic silverside, Menidia menidia, in 1981 [5], almost 60 species displaying TSD or genetic sex determination with a temperature effect (GSD+TE) have been described [6]. Nile tilapia (Oreochromis niloticus) is an African freshwater fish living in contrasting habitats This species exhibits a complex sex determination system including 1) a genetic male heterogametic system (XX|XY) without morphologically differentiated sex chromosomes [7] 2) GSD polygenic factors [8] and 3) a masculinizing effect of high temperature (above 32 ̊C) that can override the GSD [9,10,11]. Bezault et al (2007) reported naturally sex-reversed male specimens of Nile tilapia in Lake Volta, Ghana At this location, temperatures in open water vary monthly from 27 ̊C to 32 ̊C throughout the year
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