Genetic variation between a hybrid abalone and its parents (Haliotis discus hannai and H. fulgens ) based on 5S rDNA gene and genomic resequencing

Abalones are a group of economically important species of mollusks that represent a source of high-quality protein. China accounts for 90% of the world's abalone production, and make a significant contribution to the aquaculture industry's economic value. With the increasing scale of abalone farming, problems such as the degradation of germplasm resources and a reduced ability to resist environmental changes and stresses have become increasingly prominent. The hybrid breeding technique has significantly ameliorated this situation. The distant hybrid species H. discus hannai female x H. fulgens male (the Lvpan abalone) shows significant heterosis in terms of growth and stress resistance. However, our knowledge of the cytogenetics and molecular mechanisms of het-erosis in the hybrid abalone remains limited. In this study, we found significant non-transcribed spacer (NTS) variation of 5S rDNA gene among H. discus hannai, H. fulgens, and the Lvpan abalone, with five, two, and four types of NTSs identified, respectively. These NTSs exhibited genomic polymorphisms within and between spe-cies, making them reliable markers for species identification. Furthermore, the sequence alignment of NTSs and genomic resequencing analysis indicated that the hybrid abalone was biased toward H. discus hannai, suggesting a maternal effect. Moreover, numerous mutation sites that were identified only in the hybrid abalone potentially contributed to heterosis. This study provides a valuable molecular data reference for future research on distant hybridization and contributes to a better understanding of the molecular mechanisms underlying heterosis in mollusks.