Characterization and expression analysis of the key genes for early development of swim bladder in Atlantic cod

Some genes have been proved critical for swim bladder inflation during early stages. Hence, our researches were focused on the investigation into the genetic features of the key genes to early development of Atlantic cod swim bladder. The elovl1, pbx1, psap, and sftpb genes were selected, and the expression modes during embryonic and larval stages were studied by qPCR quantification procedure. Genetic structures, multiple alignment, phylogeny of these genes were also investigated. Researches of Atlantic cod sftpb was further examined by studying relative expression levels in different organs and multiple genomic conserved synteny. Cod elovl1 and psap genes showed similar expression patterns during early development, with significant decline before early somitogenesis and stable expression thereafter. Cod pbx1a declined dramatically at late gastrula stage, while it was stable at relatively high level during larval period, while pbx1b seems to be stable throughout the early development. Cod sftpb dropped significantly before onset gastrula, thereafter, sftpb expression was stable before hatching, and abundant during larval stage. Cod sftpb was much enriched in male reproductive organ than in ovary, sftpb was also enriched in heart, spleen, head kidney, and gas gland. However, sftpb expression was weak in ovary, pancreas, brain, and liver, and gill. The phylogenetic tree showed Atlantic cod Pbx1 proteins seem to have closer evolutionary relation with coelacanth Pbx1 than ballan wrasse Pbx1, tilapia Pbx1b, or platyfish Pbx1b. Cod Psap seems to intimate with most of Actinopterygii Psap except for spotted gar Psap, and elephantfish Psap. Fish Sftpb and Psap proteins were closely clustered in the phylogenetic trees. Whale shark Sftpb and Psap may exhibit the differentiation process between these two genes in the long- term evolution history. Atlantic cod Sftpb seems to be intimate with the Sftpb existed in stickleback group XIII, tetraodon chromosome 12, Amazon molly scaffold KI519905.1, platyfish scaffold JH556662.1, and medaka chromosome 9, and their genomic environments were highly conserved as well. Highly conserved orthologous pairwise between Atlantic cod scaffold 2788 and stickleback group XIII was observed. Extensive pairwise between the sftpb paralogons in stickleback group XIII and XIV may indicate a large-scale duplication event in the evolutionary history.M-A