Stochastic specification of primordial germ cells from mesoderm precursors in axolotl embryos

Abstract

A common feature of development in most vertebrate models is the early segregation of the germ line from the soma. For example, in Xenopus and zebrafish embryos primordial germ cells (PGCs) are specified by germ plasm that is inherited from the egg; in mice, Blimp1 expression in the epiblast mediates the commitment of cells to the germ line. How these disparate mechanisms of PGC specification evolved is unknown. Here, in order to identify the ancestral mechanism of PGC specification in vertebrates, we studied PGC specification in embryos from the axolotl (Mexican salamander), a model for the tetrapod ancestor. In the axolotl, PGCs develop within mesoderm, and classic studies have reported their induction from primitive ectoderm (animal cap). We used an axolotl animal cap system to demonstrate that signalling through FGF and BMP4 induces PGCs. The role of FGF was then confirmed in vivo. We also showed PGC induction by Brachyury, in the presence of BMP4. These conditions induced pluripotent mesodermal precursors that give rise to a variety of somatic cell types, in addition to PGCs. Irreversible restriction of the germ line did not occur until the mid-tailbud stage, days after the somatic germ layers are established. Before this, germline potential was maintained by MAP kinase signalling. We propose that this stochastic mechanism of PGC specification, from mesodermal precursors, is conserved in vertebrates.

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

“…PGC transplantation has also been demonstrated in several of these species. For example, transplantation of ventral marginal zone between early gastrula stage embryos in caudate amphibians (salamanders and newts) such as the Mexican axolotl Amblystoma mexicanum results in transfer of the germline between animals (Nieuwkoop, 1947;Smith, 1964;Chatfield et al, 2014). Leapfrogging might also be viable in birds.…”

The transcriptional repressor Blimp-1 acts downstream of BMP signaling to generate primordial germ cells in the cricket Gryllus bimaculatus

“…Blimp-1 orthologs have been identified in many bilaterian animals, including D. melanogaster (Ng et al, 2006;Agawa et al, 2007;Akagi and Ueda, 2011), rabbit (Hopf et al, 2011), D. rerio (Baxendale et al, 2004;Wilm and Solnica-Krezel, 2005), X. laevis (de Souza et al, 1999), salamander (Chatfield et al, 2014), Gallus gallus (Wan et al, 2014), lamprey (Nikitina et al, 2011), sea star (Hinman and Davidson, 2003;Fresques et al, 2014) and sea urchin (Wang et al, 1996;Livi and Davidson, 2006;Fresques et al, 2014). Consistent with evidence from mice, Blimp-1 is widely expressed in a variety of endodermal and/or mesodermal tissues in these animals.…”

Linking a Biological Mechanism to Evolvability

“…The other mechanism is termed "induced" and here the PGCs are determined later in development in response to a cascade of inducing signals and gene expression [25]. Previous work by Johnson et al [26]; Crother et al [6 , 21] has shown that groups that exhibit the determinative mode are significantly more species rich in general and are always more species rich than their sister clade with the induced mode.…”

“…The explicit molecular developmental pathways that are thought to function as constraint or release have been characterized and reviewed by Chatfield et al [25] and Johnson and Alberio [16]. As a constraint release, it has been argued that this increases the opportunities for the development of evolutionary novelties, some of which become key innovations that enhance speciation rates.…”