# Dynamic Transcriptional Regulation of the Hypothalamic–Pituitary–Testis Axis in the Seasonally Breeding Teleost Sebastes schlegelii

## TL;DR

This study explores how the brain and testes of a seasonally breeding fish coordinate to start sperm production, revealing molecular mechanisms that could help improve aquaculture breeding.

## Contribution

The study identifies gene regulatory networks in the brain and testis that drive testis initiation in a seasonally breeding teleost.

## Key findings

- Brain upregulated genes are involved in neuroactive ligand–receptor interactions during testis initiation.
- Testis differentially expressed genes are linked to cell cycle processes and chromatin remodeling.
- The HPT axis regulates testis initiation through dynamic transcriptional changes.

## Abstract

Spermatogenesis, the process of male germ cell development, is tightly regulated by the hypothalamic–pituitary–testis (HPT) axis in seasonally breeding teleosts. Despite its importance, our understanding of how the brain and male germ cells coordinate key transitions—such as testis initiation and maturation—remains limited, particularly in species with distinct seasonal reproductive cycles. Black rockfish (Sebastes schlegelii), a marine viviparous teleost, exhibits a prolonged testis quiescent phase lasting three-quarters of the year, with testis initiation occurring in September and maturation concluding in November and December. The mechanisms underlying these transitions are poorly characterized, leaving a critical gap in our knowledge of seasonal spermatogenesis and its regulation. Addressing this gap is crucial for advancing artificial breeding technologies, which could significantly benefit the aquaculture industry. RNA-seq was used to explore the gene regulatory networks involved in testis initiation in S. schlegelii. Transcriptomic analyses of brain and testis were conducted across key developmental phases. In the brain, upregulated genes were notably involved in neuroactive ligand–receptor interactions, whereas in the testis, differentially expressed genes were linked to cell cycle processes and ATP-dependent chromatin remodeling. Our findings reveal the molecular mechanisms underlying testis initiation in S. schlegelii, providing omics evidence for the role of the HPT axis in regulating this process. By elucidating the gene regulatory networks of the brain and testis during critical transitions, this study advances our understanding of spermatogenesis in seasonally breeding teleosts. These insights pave the way for developing year-round artificial breeding technologies, contributing to the sustainable management of commercially valuable fish species.