Extensive nuclear gyration and pervasive non-genic transcription during primordial germ cell development in zebrafish.

Stefan Redl,Stefanie Möckel,Antonio M De Jesus Domingues,Edoardo Caspani,Willi Salvenmoser,Maria Mendez-Lago,René F Ketting

doi:10.1242/dev.193060

Abstract

ABSTRACTPrimordial germ cells (PGCs) are the precursors of germ cells, which migrate to the genital ridge during early development. Relatively little is known about PGCs after their migration. We studied this post-migratory stage using microscopy and sequencing techniques, and found that many PGC-specific genes, including genes known to induce PGC fate in the mouse, are only activated several days after migration. At this same time point, PGC nuclei become extremely gyrated, displaying general broad opening of chromatin and high levels of intergenic transcription. This is accompanied by changes in nuage morphology, expression of large loci (PGC-expressed non-coding RNA loci, PERLs) that are enriched for retro-transposons and piRNAs, and a rise in piRNA biogenesis signatures. Interestingly, no nuclear Piwi protein could be detected at any time point, indicating that the zebrafish piRNA pathway is fully cytoplasmic. Our data show that the post-migratory stage of zebrafish PGCs holds many cues to both germ cell fate establishment and piRNA pathway activation.

Highlights

The specification of the primordial germ cells (PGCs) in the zebrafish starts at the 32-cell stage by the uptake of germ plasm, or nuage, an electron-dense phase-separated structure containing RNA and protein, into four blastomeres (Raz, 2003)
Studies in Drosophila have revealed that maternal piRNAs are very important in establishing transposable elements (TEs) silencing (Brennecke et al, 2008; Marie et al, 2016), and this maternal effect is thought to act through the activation of piRNA clusters by nuclear Piwi protein activity (Akkouche et al, 2017; Le Thomas et al, 2014)
We show that zebrafish most likely lacks a nuclear piRNA mechanism, and the question arises of how maternal piRNAs can be involved in establishing the zygotic piRNA system in this animal

Summary

Introduction

The specification of the primordial germ cells (PGCs) in the zebrafish starts at the 32-cell stage by the uptake of germ plasm, or nuage, an electron-dense phase-separated structure containing RNA and protein, into four blastomeres (Raz, 2003). The PGCs divide and migrate towards the genital ridge, where they arrive around 16-24 h postfertilization (hpf ) (Raz, 2003; Weidinger et al, 1999; Yoon et al, 1997). By this time, the number of PGCs has increased to 25-50 (Braat et al, 1999; Knaut et al, 2000; Weidinger et al, 1999; Yoon et al, 1997). It was recently shown that global DNA methylation is maintained during this period, indicating that no de-methylated state occurs during the zebrafish germ cell development cycle (OrtegaRecalde et al, 2019)

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