Evolutionary aspects of primordial germ cell formation

Animal embryos can be classified into three types depending on the time when the adult body form is specified--after metamorphosis, progressively by addition of posterior segments, or as a single event early in development. Segregation of germ cells correlates with specification of adult body form. When the adult body form is specified late in development, e.g. after metamorphosis (molluscs, echinoderms, cirripedes, hemichordates, cephalochordates and ascidians), germ cells appear in the early adult and at the site where the gonads will develop. When the adult body form is specified progressively during development by the sequential addition of posterior segments (annelids, onychophorans and most arthropods) germ cells are segregated either before or during addition of segments, in close association with the growth zone. In nematodes, chaetognaths, collembolans, higher holometabolous insects and vertebrates, the adult body form is specified early in development and germ cells are typically segregated correspondingly early and in extraembryonic regions. Therefore, as a general conclusion, germ cells appear to be segregated in locations and/or at times that exclude them from the process of specification of adult body form. Germ plasm is restricted to embryos in which exclusion of germ cells is difficult because the embryo is small or the signal specifying adult body form is pervasive. A possible role for germ plasm is thus as additional protection for the cells from the processes specifying adult body form.