Differential expression patterns of two sox9 genes in the teleost fish Dicentrarchus labrax

Abstract

[Introduction] The Sox (SRY-related genes containing a HMG box) genes code for a family of transcription factors that are involved in certain developmental processes. Several Sox genes are present in the genomes of vertebrates and particularly, Sox9 has been demonstrated to be responsible for testis development in mice. Previous analysis of the sox gene family in the European sea bass (Dicentrarchus labrax L.) showed that this teleost species has two distinct sox9 genes, namely sox9a and sox9b. Here, we report the isolation and characterization of the full length cDNAs of both genes, together with the identification of these two genes in non-teleost genomes. Their tissue expression patterns and the role of gene duplication for the evolution of developmental gene function is investigated.

[Methods] Medaka and zebrafish sox9a and sox9b amino acid sequences were used as queries in TBLASTN searches against the sea bass genome database. Two sequences were identifed and the complete coding sequences of both genes were amplified from sea bass cDNAs using specific primers. The amplified products were cloned into the pGEM-T Easy vector and plasmids were verified by sequencing. The 3 ¿untranslated regions (UTRs) of sox9a and sox9b were amplified to analyze the expression of both genes by qPCR of the cDNA samples. The expression of the elongation factor alpha (ef- ¿ ) and the ribosomal protein L13a (rpl13a) genes were used as reference for ovary and testis samples, respectively. Tissue distribution and female and male gonad samples throughout their first reproductive annual cycle were analysed. Phylogeny and synteny analyses were performed to understand the evolutionary relationship between sox9a and sox9b in vertebrates.

[Results and Discussion] Both sox9a and sox9b mRNAs are present in the brain and gonads of sea bass, similarly to those observations found in other teleosts. Both genes were also found to be expressed in other tissues. Gonad expression patterns of sox9 genes in the sea bass showed a strong sexual dimorphism, with high mRNA levels of sox9a in ovary and sox9b in testis of adult fish. The mRNA levels of both genes in gonads also significantly varied during the reproductive cycle of female and male sea bass. The sox9 gene sequences are highly conserved among vertebrate species, including the sea bass. Phylogenetic and synteny analyses supports that these two sox9 genes might originate from the teleost-specific whole genome duplication.

[Conclusion] Our observations provide evidence that the two sox9 genes are expressed in both male and female adult gonads. Further studies are focused in their expression during the first year of life and their functionality as transcription factors.