Delayed effect of low rearing temperature on gonadal DNA methylation in juvenile barramundi (Lates calcarifer)

Sex differentiation in many gonochoristic fish is highly responsive to temperature treatment. In some species, exposure to high or low temperatures during primary gonadal development induces phenotypic reversal of an otherwise genetically encoded sex. These changes in sexual phenotype occur in the absence of changes in genotype and are often accompanied by epigenetic modifications, such as DNA methylation. It is unknown if sequentially hermaphroditic fish exhibit similar temperature sensitivity to what has been shown in gonochoristic species; however, sex-specific differences in DNA methylation have been observed. The barramundi (Lates calcarifer) is a protandrous (male-first) sequential hermaphrodite that exhibits sex-specific DNA methylation patterns. To examine the effect of temperature on DNA methylation and primary gonadal differentiation in a sequential hermaphrodite, sexually immature barramundi were exposed to high (34 °C), control (29 °C) low (24 °C) and fluctuating (25–33 °C) temperatures from 10 to 90 days post hatch, and then on-reared at 29 °C for 12 months. At 6 months and 12 months, individuals were sampled, and their gonads examined for evidence of changes in DNA methylation and/or gonadal differentiation. Male-specific DNA methylation patterns and advanced testis development were observed at 12 months but were absent at 6 months, suggesting that low temperature treatment had a delayed effect on the gonads of juvenile barramundi. Additionally, there was an initial, but not lasting, negative effect of low temperature treatment on fish length and weight, indicating that compensatory growth may have occurred. This research provides the first evidence for an effect of temperature on gonadal DNA methylation and primary sexual development in a sequential hermaphrodite. A refinement of treatment timing and intensity could lead to the development of a hormone-free method of sex control for this widely cultured species.