Effect of early temperature treatments on sex differentiation in Nile tilapia, Oreochromis niloticus lines selected for high and low thermo-sensitivity

The phenotypic sex in Nile tilapia is determined through major and minor genetic factors as well as environmental factors, such as temperature. Temperature treatments at 36 °C applied from 10 days post fertilisation for at least 10 days are known to influence the phenotypic sex. Recently a second window of temperature responsiveness was identified. Early rearing temperature (34–36 °C) applied from 12–52 h post fertilisation proved to exert an effect on the phenotypic sex in all-female progenies (XX). To study a possible correlation between the effect of early (12–52 h post fertilisation) and late [10–20 days post fertilisation (dpf)] temperature treatments on the phenotypic sex Nile tilapia, the present study investigates the influence of early rearing temperatures using mixed sex (XX/XY) progenies with predefined temperature responsiveness for the period from 10–20 dpf.The brood stock which produced the progenies for this experiment was derived from families of selected lines showing a strong or weak responsiveness of the male proportion towards a temperature treatment of 36 °C from 10–20 dpf. Sires and dams were chosen for the present experiment if the sex ratio in temperature treated groups showed male percentages of > 90% in the high-line and < 60% males in the low-line. Three progenies obtained from each line were subjected to an early temperature treatment at 34 °C, 35 °C or 36 °C starting at 12 h post fertilisation, whereas the corresponding full sib control groups were kept at 28 °C throughout the experiment. The experimental temperatures were reached within 5 h and were applied until hatching (52 ± 3 h). Temperatures above 34 °C led to total mortalities in all progenies tested (n = 12 batches), whereas temperatures of 34 °C allowed survival rates that were comparable to those of the controls after nine days, except for progenies from the high-line. Irrespective of the line the progenies belonged to, the sex ratios were not affected by the early temperature treatment starting at 12 h post fertilisation. In conclusion, the temperature responsiveness towards early and late temperature treatments seems not to be correlated and might be dependent on the genotypic sex of the progenies. More studies are needed including information on the temperature responsiveness during both stages (before hatching and during sex differentiation), the genetic sex of the offspring, timing of temperature treatments as well as pedigree information to improve our understanding of early larval temperature treatment effects on the phenotypic sex.