A new type of hybrid bream derived from a hybrid of Megalobrama amblycephala (♀) × Xenocypris davidi (♂)

Abstract

Blunt snout bream ( Megalobrama amblycephala , BSB, 2 n = 48) is an endemic and economically important freshwater fish in China, characterized by its ease of culture, rapid growth, and high meat quality. However, the industry faces challenges such as germplasm degradation and a lack of new genetic resources. Distant hybridization, which integrates genetic material from different species and alters offspring genotypes and phenotypes, represents a powerful tool for genetic breeding. In this study, we successfully generated backcross progeny by crossing female BSB with F 1 hybrids (BY) derived from the distant hybridization of BSB (♀) × Yellowtail ( Xenocypris davidi , YT, 2 n = 48, ♂). Two types of offspring were obtained: diploid (2nBBY, 2 n = 48) and triploid (3nBBY, 3 n = 72). DNA content, chromosome number, morphological traits, and gonad development of these backcross progeny were comprehensively characterized. Morphologically, 2nBBY resembled the maternal BSB but exhibited a significantly higher body depth and smaller head than the maternal BSB ( P < 0.05). The 3nBBY displayed an intermediate morphology between BSB and BY, with a body depth greater than that of the paternal BY hybrids ( P < 0.05). Growth-related results further showed that both 2nBBY and 3nBBY had significantly higher body weights and lower coefficients of variation than the parental and F 1 hybrid groups. These morphological traits suggest that the backcross progeny have promising potential for BSB genetic improvement. Gonadal histological analysis revealed that 2nBBY included both females and males with normal gonad development, whereas only males with retarded gonadal development were found among 3nBBY. Preliminary studies on the genetics and variation of the backcross progeny using three molecular markers (5S rDNA, Sox-HMG box, and the complete mitochondrial genome) indicated that both backcross types inherited genetic material from both BSB and YT. Notably, sequence alignments of 5S rDNA and Sox9 revealed various insertions, substitutions, and deletions, suggesting novel genetic recombination and variation in the backcross progeny. These newly developed backcross progeny not only provide valuable germplasm resources for BSB genetic improvement but also offer essential insights into the genetic mechanisms underlying distant hybridization in fish.