Two new types of triploid hybrids derived from Cyprinus carpio (♀) × Megalobrama amblycephala (♂)

Abstract

Based on the formation of an autotetraploid fish lineage (4nNC, 4n = 200, F1-F4) derived from common carp (Cyprinus carpio, 2nCOC, 2n = 100) (♀) × blunt snout bream (Megalobrama amblycephala, 2nBSB, 2n = 48) (♂), we produced two new types of triploid hybrids. One type of triploid hybrid (3nNCRC) was produced by crossing females of a crucian carp-like homodiploid fish (2nNCRC, 2n = 100) with males of 4nNC; the other type of triploid hybrid (3nMCC) was generated by crossing females of an improved diploid scattered mirror carp (2nIDMC) with males of 4nNC. The 2nNCRC and 2nIDMC were derived from the first generation of 2nCOC (♀) × 2nBSB (♂). In this study, we comparatively investigated the morphological traits, chromosomal numbers, DNA contents, erythrocyte traits, and DNA variations in 5S rDNA and Hox genes in 3nNCRC and 3nMCC. The results at the chromosome and DNA content levels indicated that 3nNCRC and 3nMCC had 150 chromosomes originating from common carp and were triploid hybrids. The proportion of erythrocytes with a dumbbell-shaped nucleus in 3nNCRC (1.74 ± 0.44%) was not significantly different (P > .05) from that in 3nMCC (2.86 ± 0.59%). Regarding the cluster organization of six Hox genes, 3nNCRC and 3nMCC had greater genetic variations; they not only inherited some Hox gene cluster types (including partial recombinant clusters) from each of the parents but also presented some new gene cluster types (including a richer variety of recombinant clusters) or lost some gene cluster types, suggesting a clear source of hybridization for 3nNCRC and 3nMCC and instability in the newly established triploid hybrid genomes. Moreover, 3nNCRC and 3nMCC had derived the same type of HoxB5b gene cluster, or some derived gene clusters from one fish were identical to those of the other fish with the same gene clusters inherited from their parents. Meanwhile, compared with 3nMCC, 3nNCRC exhibited greater variation in 5S rDNA and Hox gene structures, which revealed that the genome composition of 3nNCRC was more complex and less stable. The successful development of these two new types of triploid hybrids is of great significance for fish genetic breeding.