Transcriptomic Studies On The Growth Superiorities In A Grouper Hybrid (Epinephelus Fuscogutatus ♀)×(Epinephelus Lanceolatus ♂)

As a novel hybrid offspring crossed between Epinephelus fuscogutatus♀ and Epinephelus lanceolatus♂, Hulong grouper exhibits significant growth superiority comparing to Efu, which made it a promising cultivated species in grouper aquaculture industry in China. Although some research efforts have been made on studying its physiology and genetic characteristics, the molecular mechanism of its heterosis remains poorly understood.Based on RNA-Seq and gene expression profiling, we conducted comparative transcriptomic analysis among Hulong grouper, Epinephelus fuscoguttatus and Epinephelus lanceolatus.We identified a number of differentially expressed genes involved in the growth hormone/insulin-like growth factor(GH/IGF) axis and its downstream signaling pathways for protein and glycogen synthesis. The expressions of GnRH1 and GnRH3, as well as GH/IGF axis related genes, such as IGF-1, IGF-2b, IGFBP-1, IGFBP-2, IGFBP-4 and IGFBP-5a were significantly up-regulated in the hybrid grouper; and genes related to protein and glycogen synthesis, such as PI3 KC, PI3 KR, Raptor, EIF4E3, and PP1 were up-regulated, while PYG was down-regulated. These results might contribute to increased protein and glycogen synthesis in hybrid grouper. In brains, we also identified higher expression of EIF4E-BP1 and lower expression of POMC in hybrid grouper, which might contribute to enhance appetite of the hybrid grouper. All these results are in accordance with the growth superiority of hybrid grouper.Glycolysis is a downstream pathway for the GH/IGF axis in muscle. In our study, we first found that almost all glycolytic genes were up-regulated, which provided evidence for a possible role of the upstream GH/IGF pathway in muscle development by stimulating and activating the downstream glycolysis in the target tissue of muscle. Also, we found Ca2+ signaling is activated and troponins are up-regulated in the muscle of hybrid grouper, which suggests an active state of muscle contraction, which leads to fiber growth. Our data, at the first time, demonstrated that the up-regulated genes of glycolysis, Ca2+ signaling and troponins might account for the growth superiority in the hybrid grouper.In addition, we identified four main types of alternative splicing(AS) in the brain and liver of Hyb, Ela and Efu, and found more alternative splicing in the brain than in the liver. Meanwhile both the number of genes involved in alternative splicing and the number of alternative splicing events in the brain of hybrid grouper were higher than those in the other two groupers. In addition, many appetite associated signaling genes were found to be involved in althernative splicing, including GH, EIF4E-BP1, Agouti-related peptide(AgRP), leptin, peptide YY(PYY), mTOR, AMP-activated protein kinase(AMPK), Tachykinin and tuberous sclerosis proteins 1(TSC-1).All these differences, to some extent, shed a light on the growth superiority of hybrid grouper and provided insights into understating of molecular basis underlying the heterosis of hybrid grouper.