Germ cell differentiation requires Tdrd7-dependent chromatin and transcriptome reprogramming marked by germ plasm relocalization

Abstract

Highlights d No evidence for transcriptional activation delay in zebrafish PGCs d Germ-plasm-associated post-transcriptional divergence during ZGA d Epigenetic reprogramming marks onset of PGC migration d Epigenetic reprogramming in PGCs relies on Tdrd7, coupled to germ plasm relocalization

Conserved germ plasm characteristics across theDanioandDevariolineages

“…In each species, the putative germ plasm aggregates appear as intact, phase‐separated cytoplasmic masses that occupy a substantial portion of host cells, often near the nucleus. At the dome stage in each species, which occurs post‐zygotic genome activation in zebrafish (Aanes et al, 2011; Harvey et al, 2013; Heyn et al, 2014; Kane & Kimmel, 1993; Lee et al, 2014; Mathavan et al, 2005) and can be easily distinguished by the bulging appearance of the yolk cell under a concave blastoderm, the germ plasm undergoes a dramatic morphological transition from intact mass to “leaky” appearance, with the RNA particles dispersing to fill the cytoplasm (Figure 3) (D'Orazio et al, 2021). Another hallmark of this transition in zebrafish is the subsequent appearance of perinuclear Vasa protein‐containing germ granules, which are retained throughout PGC migration to the genital ridge during somitogenesis (Blaser et al, 2005; Knaut et al, 2000; Strasser et al, 2008).…”

“…These first four germ plasm aggregates are maintained largely intact throughout the late cleavage, blastula, and early gastrula stages, resulting in embryos with a small subset of cells containing germ plasm (approximately ~4–12 out of ~4,000–8,000 total embryonic cells during the late blastula period) (Braat et al, 1999; Eno et al, 2019; Knaut et al, 2000; Yoon et al, 1997). Beginning at dome stage (4.3 hr post‐fertilization at 28.5 °C) in zebrafish embryos, RNAs within the intact germ plasm aggregates initiate dispersal, a process during which they appear to be released from the phase‐separated mass into the host cells' cytoplasm (Braat et al, 1999; D'Orazio et al, 2021; Knaut et al, 2000; Yoon et al, 1997). The occurrence of germ plasm RNP cytoplasmic dispersal, which roughly coincides with the bulk of zygotic genome activation in the embryo, is closely followed by the appearance of perinuclear Vasa protein‐containing germinal granules (Hartwig et al, 2014; Houwing et al, 2007; Knaut et al, 2000; Strasser et al, 2008).…”

Primordial Germ Cell Specification in Vertebrate Embryos: Phylogenetic Distribution and Conserved Molecular Features of Preformation and Induction

“…This perinuclear accumulation of germline determinants, long recognized as associated with germ cells and initially termed “nuage,” is shared between preformative and inductive species ( Gao and Arkov, 2013 ; Kulkarni and Extavour, 2017 ). Such conservation suggests that the nuclear envelope localization of germ cell-specific particles is a key feature required for the specification of the PGC state, possibly through local changes that can influence nuclear gene expression ( Updike et al, 2011 ; D’Orazio et al, 2021 ). Presumably maternally inherited germ plasm facilitates PGC specification by providing ready-to-go factors to initiate this process, which constitutes a selective advantage.…”

“…In support of this hypothesis, in preformative species nuclear envelope-associated germ cell granules appear soon after the dissociation of maternally derived germ plasm and are thus likely formed through the redistribution of products from the germ plasm mass itself ( Yoon et al, 1997 ; Braat et al, 1999 ; Knaut et al, 2000 ; Eno et al, 2019 ). Under this scenario maternally inherited germ plasm components may facilitate establishing a PGC state that is subsequently maintained through zygotic gene function ( D’Orazio et al, 2021 ). The germ plasm as a structure could thus be regarded as an “innovation” to jump-start PGC specification, in the same way that other evolutionary innovations have been proposed to facilitate early somatic development.…”

Integrated annotation and analysis of genomic features reveal new types of functional elements and large-scale epigenetic phenomena in the developing zebrafish

“…Samples were prepared as previously described 81,82 . Briefly, Tg(Buc-GFP) heterozygous embryos were dissociated in 500 ml of HBSS supplemented with 0.25% BSA and 10mM Hepes by pipetting for 2 minutes with a glass pipette.…”