Evolution history of duplicated smad3 genes in teleost: insights from Japanese flounder, Paralichthys olivaceus.

Following the two rounds of whole-genome duplication (WGD) during deuterosome evolution, a third genome duplication occurred in the ray-fined fish lineage and is considered to be responsible for the teleost-specific lineage diversification and regulation mechanisms. As a receptor-regulated SMAD (R-SMAD), the function of SMAD3 was widely studied in mammals. However, limited information of its role or putative paralogs is available in ray-finned fishes. In this study, two SMAD3 paralogs were first identified in the transcriptome and genome of Japanese flounder (Paralichthys olivaceus). We also explored SMAD3 duplication in other selected species. Following identification, genomic structure, phylogenetic reconstruction, and synteny analyses performed by MrBayes and online bioinformatic tools confirmed that smad3a/3b most likely originated from the teleost-specific WGD. Additionally, selection pressure analysis and expression pattern of the two genes performed by PAML and quantitative real-time PCR (qRT-PCR) revealed evidence of subfunctionalization of the two SMAD3 paralogs in teleost. Our results indicate that two SMAD3 genes originate from teleost-specific WGD, remain transcriptionally active, and may have likely undergone subfunctionalization. This study provides novel insights to the evolution fates of smad3a/3b and draws attentions to future function analysis of SMAD3 gene family.