LAUSR

Abstract Growth is an important trait with significant impact on economic value of cultured organisms. Identification of critical genes and nucleotide variations associated with growth is an important step toward molecular breeding of growth-related traits. In this study, we showed that tryptophan hydroxylase (TPH), the rate-limiting enzyme of serotonin (5-HT) biosynthesis, played an indispensable role in growth regulation in the Pacific oyster, Crassostrea gigas. We identified the C. gigas TPH (CgTPH) and performed comparative analyses of its expression and nucleotide variations between selectively bred fast-growing Pacific oysters (“Haida No. 1”) and wild oysters. Expression profiling and RNA localization showed that CgTPH was specifically expressed in the nerve-rich tissues. Notably, expression level of CgTPH was significantly higher in fast-growing “Haida No.1” than wild oysters. Association and haplotype analysis of single nucleotide polymorphisms (SNPs) in upstream regulatory region of CgTPH revealed four SNPs (located at −904, −522, −272, −262 bp) and two haplotypes (CCCC and TCTC) that were significantly associated with growth rate of C. gigas. Luciferase reporter assay identified the core transcription regulatory element located at −403 ~ −179 bp, in which the SNP -272C/T was critical for transcription activity of CgTPH. Further analysis revealed that the different transcription activity was due to the distinct binding efficiency with two transcription factors, FoxO and CUX. Together, we showed that a critical SNP in the regulatory region of CgTPH leading to differential transcription regulation by FoxO and CUX, which would affect 5-HT synthesis and growth in C. gigas. This work is of great significance for elucidating the role of 5-HT system in regulation of growth in mollusks and provides a marker for molecular breeding of growth with other production and performance traits in C. gigas.