Japanese Flounder Sex Determination

Perhaps you'll get a clever title next time...

SO in humans, the sex of an individual basically is determined by whether or not they have XX or XY. Females are XX and males XY. This is not the case in all animals, as we saw in this chapter. As this article explains, Japanese Flounder also use the XX/XY system to determine the sex of their species. However, unlike humans the fish can be induced to become phenotypic males or females. This is done by gender rearing them in either 18 or 27 degrees Celsius.

 

Because of this, Japanese flounder are great to use to study the molecular mechanisms that cause temperature-dependent sex determination. Researchers previously showed that cortisol causes female-to-male sex reversal by directly suppressing mRNA expression of ovary-type aromatase (cyp19a1).  cyp19a1 is a steroidogenic enzyme that converts androgens to estrogens in the gonads.

 An inhibitor of cortisol prevented XX flounder from becoming masculine at 27 degrees. This suggests that masculinization by high temperature is because of the suppression of cyp19a1 mRNA expression caused by elevated cortisol levels during differentiation in the gonads. In this current study, researchers found that exposure to high temperatures during gonadal sex differentiation upregulates mRNA expression of retinoid-degrading enzyme (cyp26b1). This happens concurrently with the masculinization of XX gonads and also delays meiotic initiation of germ cells. Cortisol induces cyp26b1 mRNA expression thereby suppressing specific meiotic marker synaptonemal complex protein 3 (sycp3) mRNA expressions in gonads during the sexual differentiation.
This suggests that exposure to high temperature causes cyp26b1 mRNA expression and delays meiotic initiation of germ cells. It does do by elevating cortisol levels during gonadal sex differentiation in Japanese flounder.