Abstract

Female gamete production relies on coordinated molecular and cellular processes that occur in the ovary throughout oogenesis. In fish, as in other vertebrates, these processes have been extensively studied both in terms of endocrine/paracrine regulation and protein expression and activity. The role of small non-coding RNAs in the regulation of animal reproduction remains however largely unknown and poorly investigated, despite a growing interest for the importance of miRNAs in a wide variety of biological processes. Here, we analyzed the role of miR-202, a miRNA predominantly expressed in male and female gonads in several vertebrate species. We studied its expression in the medaka ovary and generated a mutant line (using CRISPR/Cas9 genome editing) to determine its importance for reproductive success with special interest for egg production. Our results show that miR-202-5p is the most abundant mature form of the miRNA and that it is expressed in granulosa cells and in the unfertilized egg. The knock out (KO) of mir-202 gene resulted in a strong phenotype both in terms of number and quality of eggs produced. Mutant females exhibited either no egg production or produced a dramatically reduced number of eggs that could not be fertilized, ultimately leading to no reproductive success. We quantified the size distribution of the oocytes in the ovary of KO females and performed a large-scale transcriptomic analysis approach to identified dysregulated molecular pathways. Together, cellular and molecular analyses indicate that the lack of miR-202 impairs the early steps of oogenesis/folliculogenesis and decreases the number of large (i.e. vitellogenic) follicles, ultimately leading to dramatically reduced female fecundity. This study sheds new light on the regulatory mechanisms that control the early steps of follicular development, including possible targets of miR-202-5p, and provides the first in vivo functional evidence that a gonad-predominant microRNA may have a major role in female reproduction.

Highlights

  • In fish, female fecundity is tightly linked to the proper completion of oogenesis in the ovary, whereby undifferentiated germinal stem cells undergo meiosis, dramatically increase their size and form the eggs [1]

  • We studied its expression in the medaka ovary and knocked out the mir-202 gene to study its importance for reproductive success

  • We previously performed a large-scale differential microarray analysis and identified a set of miRNAs that are predominantly expressed in medaka ovary compared to other tissues, which strongly supports a potential role for these miRNAs in fish reproduction

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Summary

Introduction

Female fecundity is tightly linked to the proper completion of oogenesis in the ovary, whereby undifferentiated germinal stem cells undergo meiosis, dramatically increase their size and form the eggs [1]. Such an important differentiation process requires interactions between the oocyte and the surrounding somatic cells (granulosa and theca cells), which together form the ovarian follicles [2]. We previously performed a large-scale differential microarray analysis and identified a set of miRNAs that are predominantly expressed in medaka ovary compared to other tissues, which strongly supports a potential role for these miRNAs in fish reproduction

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