Attempt to produce a Nile tilapia tetraploid line by heat shock induction

Abstract

Nile tilapia can achieve sexual maturity before reaching market size, which can be a great problem for production farms. To circumvent this problem the standard solution is to use androgen treatment to produce only male fish. However, the use of hormones has undesirable environmental issues due to residues in the water. One alternative would be by producing sterile triploid fish, which is conceptually possible by mating tetraploid x diploid. Therefore, our objective was to develop a tetraploid Nile tilapia broodstock. We performed three experiments, where females were hormonally induced spawning, oocytes were collected, fertilized with a semen pool, and eggs were incubated at 27 °C. In experiment I (E-I), we determined the first cleavage interval (FCI) to establish a suitable moment for heat shock. Using the obtained FCI of 90 min, we designed the Experiment II (E-II) with nine treatments: control (no shock); one shock (40 °C) at 60 min post-fertilization of 1, 2, 3 or 4 min duration; and two shocks (40 °C) at 50 and 70 min post-fertilization of 1, 2, 3 or 4 min. The ploidy status of juveniles was evaluated by flow cytometry, however, no tetraploid was found, thus, using the results from E-II, a third experiment was devised. In Experiment III (E-III), we applied nine treatments: control (no shock); one shock (40 °C) at 75, 80, 85 or 90 min post-fertilization; or two shocks (40 °C) at 75 and 85 min, 80 and 90 min, 85 and 95 min or 90 and 100 min post-fertilization, all shocks were of 4 min. Triploid, mosaic, and aneuploid tilapias were identified in the treated groups. Even though we analyzed a large number of animals (>2800) that were submitted to different heat shocks applied at suitable windows of tetraploid induction, no 100% 4 N juvenile was found, indicating that the generation of tetraploid Nile tilapia broodstock using the protocols described is impractical.