Combining Marker-Assisted Selection with Traditional Selection in Yellowtail (Seriola quinqueradiata)

In this study, we attempted to combine traditional selection and marker-assisted selection (MAS) in yellowtail (Seriola quinqueradiata), a popular cultured fish in Japan, to develop a new strain resistant to the parasite Benedenia seriolae, which attaches to the body surface, causing secondary infections and reduced growth. We obtained the first generation by traditional selection, examining 200 adult yellowtails from coastal waters for B. seriolae and selecting a few fish with low numbers of parasites. These fish were one-on-one crossed to produce F₁ families. In the F₁ families, we identified two QTLs for resistance to the parasitic infection on different linkage groups (Squ2 and Squ20).  In this study, MAS was performed using Squ2 and Squ20 QTL markers to produce F₂ families. F₁ siblings were placed in two groups according to whether or not they inherited the QTL alleles for resistance. Then, two types of F₂ families were established by one-on-one crossing. Six families were putative resistant families (R families) whose parents were selected for their inheritance of QTL alleles, and six families were putative susceptive families (S families) whose parents were selected for not having inherited the QTL alleles. The two families (R family: n=200 and S family: n=200) were reared together and challenged by exposure to B. seriolae. The R family fish had significantly fewer parasites than the S family fish. These results indicate that it is possible to improve resistance to B. seriolae by combination of traditional selection and MAS.