Abstract
The germline is the only cellular lineage capable of transferring genetic information from one generation to the next. Intergenerational transmission of epigenetic memory through the germline, in the form of DNA methylation, has been proposed; however, in mammals this is largely prevented by extensive epigenetic erasure during germline definition. Here we report that, unlike mammals, the continuously-defined ‘preformed’ germline of zebrafish does not undergo genome-wide erasure of DNA methylation during development. Our analysis also uncovers oocyte-specific germline amplification and demethylation of an 11.5-kb repeat region encoding 45S ribosomal RNA (fem-rDNA). The peak of fem-rDNA amplification coincides with the initial expansion of stage IB oocytes, the poly-nucleolar cell type responsible for zebrafish feminisation. Given that fem-rDNA overlaps with the only zebrafish locus identified thus far as sex-linked, we hypothesise fem-rDNA expansion could be intrinsic to sex determination in this species.
Highlights
The germline is the only cellular lineage capable of transferring genetic information from one generation to the
DNA demethylation in human primordial germ cells (PGCs) shows a similar dynamic—PGC specification occurs around E12–E16 and DNA methylation drops to approximately 4.5% by week 75
At 24 hpf, when PGC migration is finished, we found a compact cluster of cells between the yolk ball and yolk extension in the gonadal region (Fig. 1a–d)
Summary
The germline is the only cellular lineage capable of transferring genetic information from one generation to the next. Unlike mammals, the continuously-defined ‘preformed’ germline of zebrafish does not undergo genome-wide erasure of DNA methylation during development. The peak of fem-rDNA amplification coincides with the initial expansion of stage IB oocytes, the poly-nucleolar cell type responsible for zebrafish feminisation. The germline is established during early development of almost all metazoans with the founding of primordial germ cells (PGCs)[1]. Most other vertebrates including reptiles, fish, birds, and anuran amphibians have an ‘immortal’ or ‘preformed’ germline whereby PGCs are specified by cytoplasmic determinants called germplasm. One of the most astonishing features of PGC specification in mammals is simultaneous genome-wide erasure of DNA methylation marks. DNA demethylation in human PGCs shows a similar dynamic—PGC specification occurs around E12–E16 and DNA methylation drops to approximately 4.5% by week 75
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