Localization of 5S rDNA analysis of homologous pairing in tetraploid hybrids of red crucian carp (♀) x common carp (♂)

Interspecies crossing and allopolyploidization increase the opportunity for generating new breeding resources for fish farming. Allotetraploid hybrid fish (4nAT) were created by crossing red crucian carp (RCC, Carassius auratus red var., female) with common carp (CC, Cyprinus carpio L., male). However, the genetic pattern of the heterogeneous genomes during gametogenesis in 4nAT is still unclear, which hinders the understanding of the mechanism of generation of new phenotypes during allopolyploidization in fish breeding. In the present study, fluorescence in situ hybridization of 5S rDNA was employed to track homologous pairing, and the characteristics of bivalents were observed using air-drying preparation technology and electron microscopy. The results showed two strong signals of 5S rDNA localization from the RCC genome, paired in the pachytene stage and metaphase stage of meiosis I cells in 4nAT, suggesting pairing of homologous chromosomes in 4nAT occurred between identical chromosomes. Furthermore, all bivalent pairs possessed one monochiasmate and were neatly arranged in the equatorial plate during meiosis I, which ensured normal separation of homologous chromosomes in 4nAT. This study elucidated the genetic pattern of heterogeneous genomes in 4nAT, providing important evidence for the genetic stability of 4nAT. These findings are of great significance for aquatic breeding.