The Xenopus Maternal-to-Zygotic Transition from the Perspective of the Germline

Abstract

In Xenopus, the germline is specified by the inheritance of germ-plasm components synthesized at the beginning of oogenesis. Only the cells in the early embryo that receive germ plasm, the primordial germ cells (PGCs), are competent to give rise to the gametes. Thus, germ-plasm components continue the totipotent potential exhibited by the oocyte into the developing embryo at a time when most cells are preprogrammed for somatic differentiation as dictated by localized maternal determinants. When zygotic transcription begins at the mid-blastula transition, the maternally set program for somatic differentiation is realized. At this time, genetic control is ceded to the zygotic genome, and developmental potential gradually becomes more restricted within the primary germ layers. PGCs are a notable exception to this paradigm and remain transcriptionally silent until the late gastrula. How the germ-cell lineage retains full potential while somatic cells become fate restricted is a tale of translational repression, selective degradation of somatic maternal determinants, and delayed activation of zygotic transcription.

Transposable elements: Self‐seekers of the germline, team‐players of the soma

“…Xenopus embryos divide rapidly 12 times using stored maternal products before the somatic genome is activated at the ‘midblastula transition’ . Transcription is repressed in the germ‐stem until the germline separates from endoderm in late gastrulas . ZGA in zebrafish occurs after ten embryonic divisions in both somatic and germline cells .…”

Leapfrogging: primordial germ cell transplantation permits recovery of CRISPR/Cas9-induced mutations in essential genes

“…1B) may not represent future PGCs. It has been suggested that microRNAs contribute to the clearing of germ plasm transcripts from somatic cells that do not contribute to the PGCs (Koebernick et al, 2010;Yang et al, 2015). In the future it would be interesting to deplete PGCs in host embryos to determine whether this results in increased frequency of mutant alleles in the 'leapfrogged' germline.…”

Expanding the genetic toolkit in Xenopus: Approaches and opportunities for human disease modeling

“…Another method to produce germ cell-specific mutations is to transplant primordial germ cells from a mutant embryo into a wild-type host embryo. In Xenopus , this can be done by transplanting a portion of the vegetal hemisphere, which contains the germ plasm of the early developing embryo, at the blastula stage (Yang et al, 2015). Recent work from the Cho lab showed that this method works well in X. tropicalis to produce F0 adults that are wild-type in the soma, but contain bi-allelic mutations in the germ cells (Blitz and Cho personal communication); adults produced using this method can be mated to create F1 null progeny.…”