A Transcriptomic Examination of Sexual Differentiation in Zebrafish

Many aspects of zebrafish (Danio rerio) developmental biology have been studied in detail but understanding of the molecular pathways underpinning zebrafish sexual development is limited. Sex chromosomes and sex markers have not been identified in zebrafish. Nonetheless, genes likely do play important roles in zebrafish sex determination. Environmental factors can also influence sex ratios. While little is known of the molecular genetics of sexual development in zebrafish, the importance of sex hormones, such as estradiol and testosterone, in gonadal differentiation and the maintenance of sexual phenotype in the differentiated gonad of zebrafish is well established. In fact, sex hormone mediated manipulation of fish sexual phenotype is a routine procedure in aquaculture. Despite the abundance of evidence that exogenous sex hormones can alter sexual differentiation in zebrafish, our knowledge of the molecular mechanisms responsible for the altered gonadal phenotypes is inadequate. In this study, we attempted to unravel the unknown genetic pathways that underlie androgen-induced masculinisation in zebrafish.  Developing zebrafish were exposed to 17α-methyltestosterone (MT) during gonadal differentiation (20 to 60 days post fertilisation). We performed Illumina HiSeq2000 RNA-sequencing of MT-treated juvenile zebrafish brain and gonad transcriptomes at two developmental stages to elucidate the temporal and spatial gene expression responses to MT exposure. Among the controls, we identified sex and stage-specific transcriptional programs in the gonads and confirmed that there is little transcriptional difference between female and male brains. Our RNA-Seq data indicated partial masculinisation of the gonad transcriptome and minor sex-independent alteration of brain expression patterns in zebrafish exposed to MT.