Decoding the RNA Regulatory Network in Medaka (Oryzias latipes) Spermatogenesis: Insights from a Germ Cell Reprogramming Model

TL;DR

This study explores how germ cells in medaka fish can produce sperm without a male environment, revealing key RNA networks that control the process.

Contribution

The study identifies a candidate ceRNA regulatory network in medaka germ cells that drives spermatogenesis independently of somatic signals.

Findings

A foxl3 mutant medaka model shows female germ cells can produce functional sperm in an ovarian environment.

A ceRNA network involving 58 circRNAs, 27 miRNAs, and 2965 mRNAs was identified as central to germ cell-driven spermatogenesis.

The findings suggest germ cells have intrinsic regulatory programs for sperm development, independent of external male signals.

Abstract

Spermatogenesis—the biological process of sperm production—is essential for vertebrate reproduction and is typically thought to require a male-specific environment. However, the extent to which germ cells (reproductive cells) can coordinate the complex stages of sperm development through their intrinsic regulatory programs remains poorly understood. In this study, we utilized a unique Japanese medaka fish model where, due to the loss of the foxl3 gene, female germ cells initiate and complete functional sperm production within an ovarian environment. This allowed us to enrich for the core genetic program of spermatogenesis largely independent of external male-specific somatic signals. By analyzing the complete RNA regulatory landscape, we identified a candidate network of molecules that may act as internal controllers for sperm development. We discovered that a specific group of RNAs likely functions as a central regulatory hub, associated with the pathways necessary for sperm maturation and structural formation. These findings provide a new understanding of the relative functional independence of germ cells during spermatogenesis. This research offers valuable candidate molecular targets for advancing reproductive medicine and improving breeding efficiency in aquaculture.

Spermatogenesis is a sophisticated process coordinated by germ cells and the somatic microenvironment. Circular RNAs (circRNAs), key components of competitive endogenous RNA (ceRNA) networks, form intricate post-transcriptional regulatory systems by sequestering microRNAs (miRNAs). However, the specific functions of these networks in spermatogenesis, particularly regarding the cell-intrinsic regulatory programs of germ cells, remain poorly understood. To address this, we utilized a unique foxl3 mutant model in medaka (Oryzias latipes), in which XX female germ cells spontaneously transdifferentiate into functional sperm within the ovarian somatic environment. This model enables the functional enrichment of core spermatogenic programs largely independent of male-specific somatic cues. Through whole-transcriptome sequencing and bioinformatic analysis, we identified 58 key circRNAs, 27 core miRNAs, and 2965 mRNAs, and constructed a candidate ceRNA regulatory network mediated by six circRNAs. Under genetically consistent conditions, this study elucidated a putative ceRNA network directly involved in the germ cell-dominant initiation of spermatogenesis, suggesting an essential role of these networks in germ cell fate determination. These findings provide new insights into the regulatory mechanisms of teleost spermatogenesis and offer valuable molecular targets for advancing reproductive medicine and improving breeding efficiency in aquaculture.

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