Mechanisms of germ cell specification across the metazoans: epigenesis and preformation

Abstract

Germ cells play a unique role in gamete production, heredity and evolution. Therefore, to understand the mechanisms that specify germ cells is a central challenge in developmental and evolutionary biology. Data from model organisms show that germ cells can be specified either by maternally inherited determinants (preformation) or by inductive signals (epigenesis). Here we review existing data on 28 metazoan phyla, which indicate that although preformation is seen in most model organisms, it is actually the less prevalent mode of germ cell specification, and that epigenetic germ cell specification may be ancestral to the Metazoa.

Genetics of germ cell development

“…5 Among the three organisms discussed here, worms and flies exhibit the former mechanism, and mice the latter.…”

“…Like mice, all mammals generate their germ cells by induction, but other vertebrates such as zebrafish and Xenopus set aside a GERM PLASM by preformation, while many invertebrates, including annelid worms, molluscs, and many insects, segregate their germline using inductive mechanisms. 5 'Preformation' in worms and flies. The division between preformation and inductive germ cell specification depends on whether there is physical continuity of germ cell cytoplasm between one generation and the next.…”

Germline development in amniotes: A paradigm shift in primordial germ cell specification

“…In the animal kingdom, PGCs can form via two different mechanisms: maternal specification (or predetermination) and induction (or epigenesis) (reviewed in Ref. 2). …”

“…PGCs and pPGCs are distinct from somatic cells because they contain molecules that inhibit gene expression (transcriptional and translational repression) often concentrated in granules 4, 5, 6. Interestingly, these inhibitory factors are often species‐specific, which is suggestive of convergent evolution 2.…”

“…Comparative analyses between taxa lead to the current view that the ancestral mechanism of PGC formation is induction 2, 20, 21, 22, 23, with the use of maternal specification and development of germ plasm occurring in several independent events during evolution (convergent evolution). In vertebrate anamniotes, maternal specification is employed by the teleost zebrafish 24, the sturgeon 25 (which belongs to the Acipenseriforms, considered ‘primitive fish’), and amphibian anurans 26, 27.…”