Comparative study on the induction of triploidy in tilapias, using cold- and heat-shock techniques

Optimal conditions for the induction of triploidy in two different species of tilapia, Oreochromis aureus and Oreachromis niloticus , were determined using either heat-shock or cold-shock techniques. These treatments were applied at conditions that were separately determined for each species. Embryos of these two species were found to differ in their response to heat-shock, but not to cold-shock treatments. The yield of triploid O. niloticus embryos induced at 40.5°C at the zygotic age of 3-5 min did not exceed 13%, in contrast to 60% obtained in O. aureus (heat-shocked at 39.5°C at zygotic age of 3 min). The possibility that the different responses of the two species to heat shock is maternally inherited was supported by the fact that the two-way F₁ hybrids also responded in a similar maternally dominant manner. Although the induction of triploidy using heat shock was possible only within a narrow range of zygotic ages (2.5–3.5 and 3.5–4.5 min for O. aureus and O. niloticus , respectively), the induction of triploidy using cold-shock treatment (11°C for 60 min) could be achieved within a wider range of zygotic ages (0–15 min). A satisfactory yield of troploids in the two species (50 and 60% in O. niloticus and O. aureus , respectively) was obtained using this technique. The possible interpretation of these results, that triploidy could be induced by interfering at two distinct meiotic cell-division stages, is discussed. It is concluded that the cold-shock technique is more advantageous for inducing triploidy in O. niloticus , whereas both cold-shock and heat-shock techniques are equally efficient in O. aureus.