Convergent evolution of germ granule nucleators: A hypothesis

- Germ cells are often specified by a cytoplasmic determinant called germ plasm.
- Germ plasm is a collection of specific molecules and organelles.
- Proteins called germ plasm nucleators assemble germ plasm components.
- Germ plasm nucleators arose in different animal lineages by convergent evolution.
- Intrinsic disorder and rapid rates of evolution are conserved nucleator properties.

Germ cells have been considered “the ultimate stem cell” because they alone, during normal development of sexually reproducing organisms, are able to give rise to all organismal cell types. Morphological descriptions of a specialized cytoplasm termed 'germ plasm' and associated electron dense ribonucleoprotein (RNP) structures called 'germ granules' within germ cells date back as early as the 1800s. Both germ plasm and germ granules are implicated in germ line specification across metazoans. However, at a molecular level, little is currently understood about the molecular mechanisms that assemble these entities in germ cells. The discovery that in some animals, the gene products of a small number of lineage-specific genes initiate the assembly (also termed nucleation) of germ granules and/or germ plasm is the first step towards facilitating a better understanding of these complex biological processes. Here, we draw on research spanning over 100 years that supports the hypothesis that these nucleator genes may have evolved convergently, allowing them to perform analogous roles across animal lineages.