The growth and development of metazoan tissues is regulated by the activin receptor signaling pathway, a member of the larger transforming growth factor-beta superfamily. The activin receptor signaling pathway is especially important for embryonic development, reproductive development, and muscle growth. Myostatin, perhaps the most well-known activin receptor signaling pathway is ligand, is a major negative regulator of muscle growth in vertebrates. However, in mammals myostatin functions in coordination with other ligands such as activin A to regulate muscle size, representing less than half of the total activin receptor signaling pathway muscle inhibitory functions. While myostatin has shown similar function in fish the role of the broader activin receptor signaling pathway in regulating muscle mass is poorly understood. Defining the function of this p athway is complicated in fish since they retain many duplicated gene ohnologs, which are the result of one or more whole genome duplication events.
To gain insight into the potential function and diversification of the activin receptor signaling pathway in fish we used rainbow trout (RBT , Oncorhynchus mykiss ) as a model system. Salmonids like RBT have genomes characterized by two whole genome duplication events, one ancestral teleost specific (Ts3R) event that is found in all teleost fish species, and a second more recent salmonid specific (Ss4R) whole genome duplication event that occurred in the salmonid lineage. We investigated the evolution of the activin receptor signaling pathway across these two whole genome duplication events by analyzing the molecular phylogeny and expression profile of 53 activin receptor signaling pathway genes across 23 adult tissues to develop a detailed expression atlas for the pathway. Our gene expression atlas includes all known duplicated gene ohnologs for ligands, receptors, inhibitors and signal transducers of the pathway . The gene expression atlas revealed unique insights into the evolution of duplicated genes in rainbow trout suggesting evidence of both subfunctionalization and neofunctionization of the genes after duplication.
The functional consequence of gene duplication was further examined by selec tively targeting the four ohnologs of activin A using CRISPR genome editing technology and examining changes in signaling pathway dynamics in skeletal muscle. Our results identified key interactions between activin receptor signaling pathway members in response to perturbation of activin A signaling, which provides insights into the diverse functions adapted by activin A after gene duplication. The findings highlight how teleost whole genome duplication e vents facilitate diversification and redundancy of gene function and how the complicated genome structure of fish is an important consideration when designing genetic enhancement efforts.