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

Abstract

The differentiation of primordial germ cells (PGCs) occurs during early embryonic development and is critical for the survival and fitness of sexually reproducing species. Here, we review the two main mechanisms of PGC specification, induction, and preformation, in the context of four model vertebrate species: mouse, axolotl, Xenopus frogs, and zebrafish. We additionally discuss some notable molecular characteristics shared across PGC specification pathways, including the shared expression of products from three conserved germline gene families, DAZ (Deleted in Azoospermia) genes, nanos-related genes, and DEAD-box RNA helicases. Then, we summarize the current state of knowledge of the distribution of germ cell determination systems across kingdom Animalia, with particular attention to vertebrate species, but include several categories of invertebrates - ranging from the "proto-vertebrate" cephalochordates to arthropods, cnidarians, and ctenophores. We also briefly highlight ongoing investigations and potential lines of inquiry that aim to understand the evolutionary relationships between these modes of specification.

Keywords: embryonic development; germ cells; germline specification; induction; preformation; primordial germ cells (PGCs).

FIGURE 1

Schematic depicting the two major mechanisms of germline differentiation, preformation and induction. In preformative species, as shown on left, maternally derived germline determinants, also known as germ plasm ribonucleoparticles (GP-RNPs), are already present in the zygote and subsequently localize in a subset of cells to confer germline fate. In inductive species, as shown on right, no maternally derived germline determinants are present in the zygote, and primordial germ cells gain their identity later in embryonic development through cell–cell signaling. The lower quotes represent opposing 19th century-ideologies, from which the terms preformation and epigenesis (induction) were historically associated.

FIGURE 2

Examples of germ plasm morphology in several invertebrate and vertebrate species used for research about the preformative (maternal inheritance) mechanism of primordial germ cell specification. (A) PGL-1 protein-labeled P-granules in 2-cell C. elegans embryo. Adapted with permission from Spike et al. (2008). (B) Vasa protein-labeled pole plasm in a Drosophila embryo less than 1 h after egg laying, also magnified (B’) to show individual Vasa-positive germ granules that comprise the pole plasm. Adapted from Trcek et al. (2015) and available via CC-BY license. (C) Nasonia vitripennis wasp embryos in division cycle 2–3 and (C’) pre-syncytial blastoderm formation, labeled for the oosome component oskar RNA. Adapted from Lynch et al. (2011) and available via CC-BY license. (D) Vegetal view of germ plasm in albino Xenopus 2-cell and 4-cell (D’) embryos labeled for dazl RNA. Adapted with permission from Houston et al. (1998). (E) Zebrafish 4-cell embryo labeled for nanos RNA to show rod-like germ plasm masses at the cleavage furrow ends, also magnified (E’) to show structure of aggregated nanos germ plasm ribonucleoparticles.

FIGURE 3

Illustration of germline determination cycle in the preformative model species Danio rerio (zebrafish), from maternally derived germ plasm dynamics during early embryogenesis to gamete maturation and reproduction during adulthood.

FIGURE 4

Taxonomic tree generated using the NCBI Common Tree Taxonomy Browser and Interactive Tree of Life (Letunic and Bork, 2021) to depict each species or clade referenced in this manuscript. Primordial germ cell (PGC) specification mechanism of each species is indicated according to key.

FIGURE 5

Illustration of germline determination timing in the inductive model species Mus musculus (mouse). Note the lack of maternally derived germ plasm in embryos and relatively late onset of primordial germ cell specification at approximately ∼E6.5. Image adapted from Richardson and Faulkner (2017) and available via license: Creative Commons Attribution-Non-Commercial 4.0 International.

FIGURE 6

Expression summary for the DAZ family genes Boule, Daz, and Dazl across stages of primordial germ cell specification and gametogenesis in selected model organisms. Image adapted from Xu et al. (2009) and available via CC-BY license.

FIGURE 7

Cladogram depicting the general evolutionary relationships between major vertebrate classes, invertebrate chordates (tunicates), and non-chordate metazoans. Animal graphics adapted from the Noun Project (https://thenounproject.com/) and available via CC-BY license.