Identification of differentially expressed miRNAs and their potential targets during fish ovarian development.

Oogenesis is a complex process requiring the coordinated sequential expression of specific genes and ultimately leading to the release of the female gamete from the ovary. In the present study we aimed to investigate the contribution of miRNAs to the regulation of this key biological process in teleosts using a model in which growing oocytes develop simultaneously. Taking advantage of the strong sequence conservation of miRNAs among phylogenetically distant species, we designed a generic microarray displaying most known chordate miRNAs. It allowed us to provide an overview of the ovarian miRNome during oogenesis for the first time in any vertebrate species. We identified 13 differentially expressed miRNAs, and a differential expression of at least one miRNA was observed at each step of oogenesis. A surprisingly high differential expression of several miRNAs was observed at several stages of oogenesis and subsequently confirmed using quantitative PCR. By refining in silico prediction of target genes with gene expression data obtained within the same sample set, we provide strong evidence that miRNAs target major players of oogenesis, including genes involved in rate-limiting steps of steroidogenesis and those involved in gonadotropic control of oocyte development, as well as genes involved in ovulation, oocyte hydration, and acquisition of the ability of the oocyte to support further development once fertilized (i.e., oocyte developmental competence). Together, these observations stress the importance of miRNAs in the regulation and success of female gamete formation during oogenesis.