Selection experiments to alter the sex ratio in rainbow trout (Oncorhynchus mykiss) by means of temperature treatment

A selection experiment in rainbow trout (Oncorhynchus mykiss) was initiated to verify if the proportion of females after temperature treatment could be significantly altered by selection. Rainbow trout spawners out of six populations were used to produce a diallel cross in which all populations were used as male as well as female parents. The 95 families of this breeding base were tested for their temperature sensitivity as follows. After an incubation period of 42 days, alevins of each family were subdivided into a treatment group and a full sib control group (12 °C), each consisting of 300 fish. Temperature treatments were carried out with 18 °C for 30 days. Each treatment and corresponding control group was raised separately until an age of 9 months post fertilization. Then a random sample of 10 fingerlings out of each control group was kept for later selection decisions. All other fish from the treatment and control groups were sexed by microscopic inspection of gonad squashes. The families were ranked according to the percentage of females, and families with the highest (n = 6) and lowest percentage of females (n = 6) in their sex ratios after temperature treatment were selected to produce two divergent lines (high and low line). After one generation of selection the temperature-treated groups in the high line showed a female percentage of 58%, whereas the low line had an average female proportion of 44%. The realized heritability was 0.63 in the high and 0.71 in the low line. This study provides the first evidence that the sex ratios in temperature-treated groups of rainbow trout can be selected as a quantitative trait. Additionally, in the meiotic gynogenetic offspring derived from a female spawner, which showed low percentages of females after temperature treatment in previous matings, 16% males were observed in the temperature-treated group. Thus, there seem to be two possible consumer- and environment-friendly ways to increase significantly the proportion of females in rainbow trout: directly via selection of families which show a high percentage of females in their sex ratios after temperature treatment to build up a corresponding temperature sensitive line or indirectly via neo-males derived from temperature treatments of gynogenetic offspring.