The biology of the germ line in echinoderms

Abstract

SUMMARY The formation of the germ line in an embryo marks a fresh round of reproductive potential. The developmental stage and location within the embryo where the primordial germ cells (PGCs) form, however, differs markedly among species. In many animals, the germ line is formed by an inherited mechanism, in which molecules made and selectively partitioned within the oocyte drive the early development of cells that acquire this material to a germ-line fate. In contrast, the germ line of other animals is fated by an inductive mechanism that involves signaling between cells that directs this specialized fate. In this review, we explore the mechanisms of germ-line determination in echinoderms, an early-branching sister group to the chordates. One member of the phylum, sea urchins, appears to use an inherited mechanism of germ-line formation, whereas their relatives, the sea stars, appear to use an inductive mechanism. We first integrate the experimental results currently available for germ line determination in the sea urchin, for which considerable new information is available, and then broaden the investigation to the lesser-known mechanisms in sea stars and other echinoderms. Even with this limited insight, it appears that sea stars, and perhaps the majority of the echinoderm taxon, rely on inductive mechanisms for germ-line fate determination. This enables a strongly contrasted picture for germ-line determination in this phylum, but one for which transitions between different modes of germ-line determination might now be experimentally addressed.

Differential Nanos 2 protein stability results in selective germ cell accumulation in the sea urchin

“…The GFP constructs, Sp Nanos 2 5′ and ΔGNARLE 3′UTRs, were amplified and cloned next to the GFP open reading frame, and the T7 promotor as described previously (Wessel et al, 2013). Nanos 2 ORF was amplified and inserted in frame in C-terminal of the GFP ORF (Supplemental figure S1B).…”

Mechanisms of the epithelial-to-mesenchymal transition in sea urchin embryos

“…However, unlike other blastomeres, the small micromeres are silent cells with regard to gene transcription activity, which makes similar GRN analysis difficult if not impossible for PGC differentiation. 149 Moreover, PGC dissociation from the tip of the archenteron is not well associated with the typical EMT process. Although they dissociate from adjacent epithelial cells, the inter-cellular adhesiveness among these cells is retained, which results in the production of quasi-mesenchymal cells.…”

Echinodermata

“…The origin of the germ cells in development has been determined for echinoids, where these cells have been identifi ed to express conserved specifi c germ line genes -e.g., nanos (Wessel et al 2013 ). The so-called genital rachis -associated with the hemal system -is thought to be the site where the gonads originate from.…”

“…While echinoids appear to use an inherited mechanism of germ line formation, the sea stars appear to use an inductive mechanism (which involves the interaction with neighboring cells; a mechanism most probably used by the majority of echinoderms; see Wessel et al 2013 ;Fresques et al 2014 ).…”