THE YIELD AND QUALITY OF FISH FILLETS ARE DETERMINED BY CELLULAR PROCESSES THAT GOVERN HOW MUSCLE TISSUE GROWS AND DEVELOPS. THE ACTIVIN/MYOSTATIN CELLULAR SIGNALING PATHWAY IS ONE OF THE MOST IMPORTANT MUSCLE GROWTH PATHWAYS IN VERTEBRATES AND WORKS BY DIRECTING MUSCLE FIBERS TO LIMIT THEIR OVERALL SIZE. DISRUPTION OF THE PATHWAY LEADS TO LARGE INCREASES IN MUSCLE MASS IN MAMMALS AND FISH. WHILE THE SIGNALING PROTEIN MYOSTATIN IS A WELL-ESTABLISHED ACTIVATOR OF THIS PATHWAY, IT DOES NOT ACT ALONE. IN MAMMALS, ACTIVIN A HAS AN EQUAL AND OVERLAPPING FUNCTION WITH MYOSTATIN TO SLOW MUSCLE GROWTH. OTHER THAN MYOSTATIN, THE FUNCTION OF THE BROADER ACTIVIN/MYOSTATIN SIGNALING PATHWAY IN FISH IS POORLY UNDERSTOOD. WE HYPOTHESIZE THAT ACTIVIN A HAS AN ESSENTIAL ROLE IN CONTROLLING THE SIZE OF MUSCLE TISSUE IN FISH AND THAT THERE ARE SPECIFIC MECHANISMS BY WHICH THE PATHWAY COORDINATES SIGNALING ACROSS MUSCLE AND NON-MUSCLE TISSUES. TO ADDRESS THESE HYPOTHESES, THE RESEARCH WILL FIRST ESTABLISH A COMPREHENSIVE ATLAS OF THE EXPRESSION OF ALL ACTIVIN/MYOSTATIN SIGNALING PATHWAY MEMBERS ACROSS 11 TISSUES AND 12 MUSCLE DEVELOPMENTAL STAGES, USING COMMERCIAL RAINBOW TROUT (RBT) AS A MODEL SYSTEM. CRISPR GENOME EDITING TECHNOLOGY WILL ALSO BE USED TO DISRUPT THE FUNCTION OF ALL FOUR ACTIVIN A GENES IN RBT AND INVESTIGATE WHETHER LOSS OF ACTIVIN A SIGNALING RESULTS IN AN INCREASE IN MUSCLE SIZE IN FISH. THESE GENE-EDITED LINES WILL BE PRODUCED SOLELY AS A BASIC RESEARCH MODEL SYSTEM AND ARE EXPECTED TO PROVIDE A WEALTH OF KNOWLEDGE ON THE PHYSIOLOGICAL FUNCTION OF ACTIVIN A IN FISH. BY DEVELOPING A DETAILED GENE EXPRESSION ATLAS FOR THE PATHWAY, THE RESEARCH WILL CREATE AN IMPORTANT RESOURCE TO THE SCIENTIFIC COMMUNITY, THE FIRST TO BE ESTABLISHED FOR ANY SPECIES TO DATE. THE IMPACT OF THE PROPOSED BASIC RESEARCH WILL FACILITATE A DEEPER UNDERSTANDING OF PERHAPS THE MOST IMPORTANT MUSCLE GROWTH PATHWAY IN FISH. THIS WILL PROVIDE THE FOUNDATION FOR EXPLOITING THE PATHWAY TO IMPROVE THE OVERALL YIELD AND QUALITY FISH FILLETS IN UNITED STATES FINFISH AQUACULTURE.