Temperature dependent sex differentiation in rainbow trout (Oncorhynchus mykiss)

The aim of the project was the evaluation of an alternative to the usage of hormonal treatment in order to produce rainbow trout neo-males for an all female production by mating these neo-males to normal rainbow trout females. The pursued approach to influence the sex differentiation in rainbow trout is based on the modification of the rearing temperature post hatching, during the early alevin stage. This has been scientifically proven to be effective in other fish species, e.g. Nile tilapia (Oreochromis niloticus). After the technical optimization of the treatment procedure a temperature treatment of 18 °C for 30 days was applied to ongoing experiments starting two weeks post hatching. The survival rates of the fry after temperature treatment did not differ significantly from the corresponding control groups. Depending on the population and family, the applied temperature treatments led to a significant increase in the percentage of females (up to 83%) or to a significant decrease in female percentages (down to 30%) in comparison to the untreated full sib control groups. The results obtained from repeated matings of identical mating partners differed only by 1 to 6% females in their offspring. Both, female and male spawners, contributed to the different sex ratios in temperature treated groups.Taking the initially obtained results as a base, further investigations were conducted concerning the heritability of thermo-sensitivity during sex differentiation in the form of a selection experiment. In order to measure the thermo-sensitivity on sex differentiation, the progenies were divided into a treatment and a control group. Both groups were reared separately two weeks post hatching, in temperatures of 12 °C and respectively 18 °C for 30 days. They were subjected to sexing after a rearing time of approximately 8 months in order to determine macroscopically or microscopically the sex ratio of the group. The selection criterion was the female percentage of the temperature treated full sib group. The base population consisted of 95 tested families, of which 6 families each were selected showing an increased female proportion (high line: selection differential of 9%) in the offspring and respectively a decreased female proportion (low line: selection differential of -10%) in order to create two divergent lines (a high and a low line). The results of the first generation showed that selection using thermo-sensitivity on sex differentiation is possible. In the high line the first generation of selection showed an increased mean female proportion of 57.6% ± 4.7 after temperature treatment, however, the corresponding value for the controls was 49.5% ± 1.8. The first generation of selection of the low line showed a decreased female proportion of 44.5% ± 4.3 after temperature treatment with the corresponding value of the control groups at 49.6% ± 2.1. The results indicated that the rearing temperature during the early alevin stage in rainbow trout can influence the sex ratio. Rainbow trout are known to have a genetic background of thermo-sensitivity depending to the population and the mating partners shown by an averaged heritability of both lines of h2R = 0.67.A potential alternative to produce all female progenies could be achieved by a meiotic gynogenesis and a following temperature treatment of gynogenetic progenies to obtain functional males. The functional males deliver all female progenies when they are mated to normal females. This procedure provides an alternative technique for the production of large rainbow trout with body weights above 1.2 kg, without using the common method of hormones that produce neo-males to attain all-female progenies when these neo-males are mated to normal females afterwards.