Drastic mortality in tetraploid induction results from the elevation of ploidy in masu salmon Oncorhynchus masou

Although tetraploidy is considered to be important and useful for the production of sterile triploids and fertile allo-tetraploids in aquaculture, in most cases the resultant embryos exhibit extremely low survival. In this study, we aimed to clarify the cause of the inviability of tetraploids in masu salmon. Firstly, we compared developmental rates of the first cell cycle among 9 single pairs produced by all possible matings between 3 females and 3 males. The results showed that the fertilized eggs developed almost synchronized among individuals from each pair but asynchronous among pairs and this asynchronous development was more apparent among pairs from different female than those from different male. Secondly, we induced both tetraploidy (4N) and gynogenetic diploidy (G2N) by the same hydrostatic pressure shock (PS) conditions (700 kg/cm2, 7 min duration) for the first cleavage inhibition using single-pair mating. This experiment was designed to verify whether the cause of mortality of induced tetraploidy was the elevated ploidy itself. Eggs from one female were divided into 2 groups and fertilized with normal or UV-irradiated sperm from one male, respectively. Then each group of eggs was subdivided into 6 groups: one was an intact control and the other 5 groups were treated with PS at every 30 min from 5 to 7 hpf at 10 °C. As a result, the treated eggs of 4N and G2N showed the highest survival (90.8% and 60.6%, respectively) and embryogenesis rate (both 100% relative to surviving eggs) at 33 dpf, only when PS treatment was performed at late prometaphase (6 hpf and 6.5 hpf, respectively). The other PS groups showed extremely low survival (1.5–52.8% and 3.1–14.8%), ceased morphogenetic development and developed into only undifferentiated cell masses. This experiment was repeated 2 more times using other single-pair mating and the results showed the same tendency. The embryonic bodies were confirmed by flow cytometry to have approximately objective ploidy, tetraploid, hypo- or hypertetraploid in 4N and diploid or hypodiploid in G2N. However, all tetraploid embryos, even those with normal morphology, began to die simultaneously around the hatching period (34 dpf), while G2N embryos showed normal morphology and survived beyond 50 dpf (55.6%). Most tetraploid embryos showed malformation and had very poor vascular systems even in normal-looking ones. These results suggest that the mortality of induced tetraploids depends not only on the side effects of the PS treatment but also strongly depends on the induced tetraploidy itself.