ABSTRACT
Germ cells, which carry the hereditary information for the next generation, retain the capacity to differentiate into each and every cell type of the organs. Although the ability to segregate a germ cell line is considered to be fundamental for the evolution of eukaryotes, only animals segregate their germ cells during embryogenesis. The mechanisms by which animals determine their germ cell line are quite varied and involve maternal determination and induction and a variety of intermediate forms. Interestingly, the question about how prevalent germline segregation is across animal groups is still a matter of debate, given that several animal groups (sponges, placozoans, cnidarians, ctenophores, acoels, and planarians), although possessing the necessary molecular machinery for the germline, do not seem to segregate their germline in an obvious way. In turn, these animals could produce their germline de novo throughout their life. Curiously, these animal groups also happen to have astonishing regeneration capacities, many of which have been well-studied. Here, we review the published evidence for morphological and molecular aspects of germline conservation across animal groups where the data remains controversial today.
