Abstract
Temperature‐dependent sex determination (TSD) can be observed in multiple reptile and fish species. It is adaptive when varying environmental conditions advantage either males or females. A good knowledge of the thermosensitive period is key to understand how environmental changes may lead to changes in population sex ratio. Here, by manipulating temperature during development, we confirm that cold temperature (16°C) increases the proportion of fish that develop as females in European sea bass (Dicentrarchus labrax) until 56 days posthatching, but show that it has an opposite effect at later stages, with the proportion of males reaching ~90% after 230 days at 16°C. This is the first observation of opposite effects of temperature at different time periods on the sex ratio of a vertebrate. Our results highlight the potential complexity of environmental effects on sex determination.
Highlights
Temperature-dependent sex determination (TSD) exists in many vertebrate species, including reptiles and teleost fishes (OspinaAlvarez & Piferrer, 2008; Pieau, 1996)
There is a multitude of temperature sensitivity patterns, especially in reptiles (Pieau, 1996), but high temperatures mostly bias sex ratios toward males in fishes with TSD (Geffroy & Wedekind, 2020; OspinaAlvarez & Piferrer, 2008)
We examined the sex ratio of each of the 16 treatment groups and investigated the underlying molecular mechanisms involved with the expression of two genes with sex-biased expression at key periods of the development
Summary
Temperature-dependent sex determination (TSD) exists in many vertebrate species, including reptiles and teleost fishes (OspinaAlvarez & Piferrer, 2008; Pieau, 1996). Females are larger than males (Saillant et al, 2001), and it is likely that larger size has more fitness benefits to females, as generally observed in mass-spawning species with random pairing (Geffroy & Bardonnet, 2016; Kraak & de Looze, 1993) This leads to an apparent contradiction, as since high temperatures generally promote growth, we would logically expect a bias toward female sex determination when fish are reared under warm conditions. We hypothesized that there is a first temperature-sensitive period, in which low temperature orients sex determination toward females, which may extend up to 95 or even 120 dph (up to a body length of 50–55 mm—see Figure 1) With another mechanism, a second thermal sensitive period may exist in sea bass, with cold temperatures favoring males during the early juvenile stage, conforming with a direct negative effect of cold temperature on growth. We examined the sex ratio of each of the 16 treatment groups and investigated the underlying molecular mechanisms involved with the expression of two genes with sex-biased expression (cyp19a1a as an early ovarian marker and gsdf as an early testis marker) at key periods of the development
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