A pan-metazoan view of germline-soma distinction challenges our understanding of how the metazoan germline evolves

A germline-soma distinction — irreversible differentiation from reproductive germline cells to sterile somatic cells — is a landmark of cellular cooperation in metazoans. Traditionally, this distinction was considered a property of only some metazoan taxa, such as vertebrates and insects. However, recent studies on a number of other metazoan taxa are challenging this traditional perspective, suggesting that a germline-soma distinction is widespread among metazoans. Here, we review recent molecular and cellular evidence supporting this suggestion and emphasise the difference between germline-soma distinction and germline segregation. We also outline the considerable diversity among metazoans in germline specification, segregation and regeneration. We finish by discussing how evolutionary explanations for this diversity can be investigated by harnessing theoretical modelling approaches.