Type I Iodothyronine Deiodinase Functions in Thyroid Hormone Synthesis and Growth Regulation in Bivalve

Bivalve breeding programs account for over 50% of global marine aquaculture, and growth is one of the most important traits of them. With the purpose of investigating the key genes related to growth performance of Yesso scallop (Patinopecten yessoensis), an economically important bivalve, we conducted transcriptome comparison between fast- and slow-growing individuals, and identified 1,531 differentially expressed genes (DEGs), with 656 up-regulated in fast-growing scallops, categorizing into six molecular function terms, of which the thyroxine 5'-deiodinase activity term exhibited the lowest p values. This term contained only two genes, and they were both type I iodothyronine deiodinase (D1), the enzyme catalyzing triiodothyronine (T₃) production in vertebrates. Meanwhile, T₃ concentration was significantly higher in fast-growing scallops. To further understand D1 functions in bivalve growth, RNA interference (RNAi) was performed in the dwarf surf clam (Mulinia lateralis), a suitable bivalve for verifying the function of growth-related genes. Suppression of MlD1-1 and MlD1-2 expression was detected after RNAi, and in MlD1-1 RNAi group, dwarf surf clams exhibited significant reductions in growth, with shell length, shell width, shell height, and body weight decreasing by 9%, 9%, 9%, and 25%, respectively. Furthermore, the triiodothyronine (T₃) and thyroxine (T₄) levels of clams in MlD1-1 RNAi group were significantly decreased by 11% and increased by 5%, respectively, while no changes were observed in the MlD1-2 RNAi group. Our data imply that bivalve D1 could be a candidate gene for the breeding of fast-growing bivalve.