The formation and biological characterization of two allotriploid fish derived from interploid crosses

Interploid cross is regularly observed in plants and often results in phenotypic changes, including reduced fertility and heterosis. Yet few triploids have been observed in animals due to the rare coexistence of diploid and allotetraploid populations in species. However, heterosis in growth traits and innate immunity have been detected in the allotriploids obtained from interploid crossing of the male allotetraploids of red crucian carp and common carp (4nAT) with the females of their inbred parents. Here, we obtained two new types of allotriploids (3nR2C and 3nRC2) by the interploid crossing of female 4nAT with the inbred parents (red crucian carp and common carp, respectively). The embryos of the two allotriploids developed normally through the zygote, cleavage, blastula, gastrula, segmentation and hatching stages, and high fertilization rates (3nR2C: 91.31%, 3nRC2: 84.09%) and hatching rates (3nR2C: 72.96%, 3nRC2: 71.15%) were observed. The measured DNA content and chromosome numbers suggested that both 3nR2C and 3nRC2 were allotriploids with 150 chromosomes. Intermediate characteristics of some morphological traits, such as the body length to body height ratio (BL/BH), were observed in 3nR2C and 3nRC2. In contrast to the stage II oocytes in the ovaries of the inbred parents, cystic structures formed by small, underdeveloped oogonia were observed in the two allotriploids. In contrast to the observation of mature spermatids in the testes of the inbred parents, spermatids with irregular shapes that began to degrade and disintegrate were observed in the two allotriploids. These results revealed abnormal gonad development in the allotriploids. Taken together, our results indicate that the two new allotriploids will be significant in aquaculture.