A hybrid lineage derived from hybridization of Carassius cuvieri and Carassius auratus red var. and a new type of improved fish obtained by back-crossing

Hybridization is an important technique and is widely used in fish genetic breeding; this technique prevents variety degeneration and improves variety. In this research, we obtained a hybrid lineage (WR-F1-F4) by interspecific hybridization derived from female white crucian carp (Carassius cuvieri, WCC) and male red crucian carp (Carassius auratus red var., RCC), and a new type of improved fish (WR-II) was obtained by back-crossing between the female hybrid (WR-F1) and the male WCC. We investigated the morphological traits, chromosomal numbers, DNA content, gonadal development, sperm shape and 5S rDNA in the hybrid lineage and WR-II. The results showed that all the assessed morphological traits were not significantly different between the progenies of the hybrid lineage and their original parents, but the WR-II showed smaller head and higher body compared with its original parents; the hybrid lineage and WR-II were diploid with 100 chromosomes; all the tested males and females of the hybrid lineage and WR-II had normal gonadal development and produced mature sperm and egg, respectively; the 5S rDNA of the hybrid lineage and WR-II inherited the characteristics of their parents and had some mutant bases or chimeric sequences, which revealed heredity, variability and heterozygosity in the hybrid progenies; finally, the 5S rDNA of the hybrid lineage could be highly inherited from the second generation (WR-F2) to the next generations. This study will be beneficial for the establishment and application of hybrid lineages in genetic breeding.