Abstract
Exposure to stress induces a series of responses and influences a wide range of biological processes including sex differentiation in fish. The present work investigated the molecular and physiological response to thermal stress throughout the early development stage covering the whole period of sex differentiation of bluegill, Lepomis macrochirus. Larvae were treated using three temperatures, 17, 24, and 32°C from 6 to 90 days posthatching (dph) in 30-L round tanks. There is no significant difference of the sex ratio and survival among the three temperature groups in the geographic population used in this study. Two ovarian differentiation-related genes foxl2 and cyp19a1a were detected at 7 dph suggesting that these genes have already played a role prior to sex differentiation. The expression of foxl2 reached the peak and was thermosensitive just prior to the onset of ovarian differentiation at 27 dph. Histological examination displayed that the proliferation of germ cells and ovarian differentiation were delayed at the low-temperature treatment (17°C) at 97 dph compared with higher temperatures. In conclusion, the water temperature regulates the sex differentiation of bluegill through modulation of the expression of foxl2 and cyp19a1a. A comparative study of the expression profile of sex differentiation-related genes in species will shed light on the evolution of sex-determination mechanisms and the impact of stress on sex differentiation.
Highlights
Sex could be initiated by genetic or environmental signals, and the sexdetermining gene(s) vary among fishes (Kikuchi and Hamaguchi, 2013; Bachtrog et al, 2014; Shen and Wang, 2014, 2018a; Mei and Gui, 2015; Shen et al, 2016; Wang and Shen, 2018)
A general schematic diagram represents the molecular players that are involved in sex differentiation of teleosts, which employ genetic sex determination (GSD) (Figure 1)
Sexual dimorphic expression of foxl2 during sex differentiation has been found in all species investigated except American Alligator Alligator mississippiensis, including species with either temperature-dependent sex determination (TSD) or GSD (Table 1)
Summary
Sex could be initiated by genetic or environmental signals (e.g., temperature), and the sexdetermining gene(s) vary among fishes (Kikuchi and Hamaguchi, 2013; Bachtrog et al, 2014; Shen and Wang, 2014, 2018a; Mei and Gui, 2015; Shen et al, 2016; Wang and Shen, 2018). Several studies explored the molecular aspects of sex differentiation, such as transcription factor genes (foxl, dmrt, and sox9) and steroidogenic-related genes (cyp19a1, amh, and sf-1). Bluegill sunfish (Lepomis macrochirus) belongs to Centrarchidae (Near and Koppelman, 2009) and receives much attention because of the extraordinary biological characteristics, such as sex-determining mechanism (Wang et al, 2014; Shen et al, 2016), hybridization (Shen and Wang, 2018b), and alternative mating tactics (Gross and Charnov, 1980; Dominey, 1981; Garner and Neff, 2013) These biological features influence population sex ratio solely or interactively and make bluegill to be an excellent organism for investigation of interactions, population dynamics, speciation, and sexual selection. The present study aimed to investigate the molecular players that can regulate the sex differentiation of bluegill
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