Effect of temperature on the early sexual development of tambaqui Colossoma macropomum

Sex development is a multi-step process involving determination, differentiation, and maintenance of the gonads, which culminates in producing fertile sperm and eggs. In teleosts, candidate genes mastering this process can be thermosensitive before or just at the beginning of differentiation, the sensitive period in fish. The knowledge of these mechanisms may be useful for the production of monosex populations in aquaculture. Here we investigated the influence of temperature on survival, growth, expression profiles of genes involved in early sexual development, and sex ratio. 5-day-old (undifferentiated) larvae were reared at 26, 28 and 31 °C for 90 days, and expression of genes involved in testis- ( wt1b and ar ) and ovary-determining ( amh and foxl2 ) pathways, and vasa , were analysed four times from 20 to 90 days post-hatching, and the resulting sex ratios were estimated. Only foxl2 showed a thermosensitive transcription at 20 dph, decreasing at the highest temperature. None of the male-bias genes were regulated by temperature. Still, both wt1b and ar were upregulated in some individuals long after sex differentiation, suggesting the importance of testis-skewed genes not only at testis establishment but also at later stages. The sex ratio was not affected by temperature. Our study also revealed an unusual vasa expression profile before and after gonadal differentiation; in some fish increasing expression values were recorded while others presented low vasa expression even after differentiation. Our results support a strict genetic control in the sex determination of tambaqui, despite thermosensitivity of some genes involved in its sexual development.