Somatic cell conversion to a germ cell lineage: A violation or a revelation?

The germline is unique and immortal (or at least its genome is). It is able to perform unique jobs (meiosis) and is selected for genetic changes. Part of being this special also means that entry into the germline club is restricted and cells of the soma are always left out. However, the recent evidence from multiple animals now suggests that somatic cells may join the club and become germline cells in an animal when the original germline is removed. This “violation” may have garnered acceptance by the observation that iPScells, originating experimentally from somatic cells of an adult, can form reproductively successful eggs and sperm, all in vitro. Each of the genes and their functions used to induce pluripotentiality are found normally in the cell and the in vitro conditions to direct germline commitment replicate conditions in vivo. Here, we discuss evidence from three different animals: an ascidian, a segmented worm, and a sea urchin; and that the cells of a somatic cell lineage can convert into the germline in vivo. We discuss the consequences of such transitions and provide thoughts as how this process may have equal precision to the original germline formation of an embryo. The classic model of August Weismann (The Germ‐Plasm, A Theory of Heredity, 1893; purple diagram) defines two types of cells in an organism: the Germ and the Soma. Weismann argued that all hereditary information came from the germ cells, which had continuity through generations of natural selection. Recent results in a variety of animals though suggest that at least in experimental situations, cells of the soma can replace a germline lost (red arrow). This essay reviews the evidence and the impact of this new concept. HIGHLIGHTS The germline is thought to exclude somatic cell lineages. Recent results though show somatic cells of diverse animals do convert to a functional germline. We discuss the consequences of such transitions and its precision relative to the original germline.