In order t o evaluate utilization variation of, and growth response to, soybean meal diets in largemouth bass for breeding programs, test diets with 0%, 35%, and 50% fish meal having been replaced by soybean meal (SBM; S0, S35, and S50) were administered to three northern strains of largemouth bass (LMB; Jones, Marlow, and Remlinger ).
In the S35 and S50 groups, the Marlow and Remlinger strains had much better growth res ponse to soybean meal diet (SMD) than the Jones strain , indicating that these two strains can better utilize diets containing high level of SMD. The results suggest that sufficient genetic variation exists for growth and feed efficiency in LMB fed diets containing both SBM and fishmeal , and genetic improvement may be possible for the trait of feed efficiency. In all the three strains, t he growth performance of individuals in the S35 and S50 groups obviously declined when compared with the S0 group, and the specific growth rate and weight gain correlated negatively with dietary levels of SBM .
Genes that were differentially expressed between dietary SBM levels were identified . Transcripts of genes related to insulin- signaling pathway and fatty acid metabolism and biosynthesis were significantly downregulated in the S35 and S50 groups compared with the S0 group. Interestingly, several transcripts of genes (egfr , pla2g4 , and ugt) involved in GnRH-signaling pathway and steroid hormone biosynthesis were also downregulated in the S35 and S50 groups, inferring that feeding with SMD could impair the sexual development of juvenile LMB . Likewise, t he genes related to insulin signaling and fatty acid metabolism and biosynthesis were downregulated in the Jones strain compared to the Marlow and Remlinger strains in all three treatments. These results also suggest that insulin-signaling pathway and fatty acid metabolism and biosynthesis play important roles in adaptation to diets with high levels of SBM in LMB. Overall, the Marlow and Remlinger strains exhibited better adaptation to SBM diets, and could be considered as potential breeding candidates.
This study provides useful information for LMB breeding programs, and it contributes to our understanding of genetic mechanisms related to growth performance of LMB fed with high levels of SMB.