Let's Talk About Sex…Cell Lineages

Sex is fundamental to many organisms. It is through sexual reproduction that humans, and many metazoans (multicellular eukaryotes in the animal kingdom), propagate our species. For more than 150 years, sexual reproduction within metazoans has been understood to rely on the existence of a discrete category of cells (germ cells) that are usually considered uniquely separate from all other cells in the body (somatic cells), and which form a cell lineage (germline) that is sequestered from all somatic cell lineages. The consideration of germ cells and germline as the lone source of reproductive potential within metazoans has allowed many investigators to place the hereditary and evolutionary burdens of sexually reproducing lineages solely within these cells and cell lineages, making them central to many important topics within biology, such as units of selection, transmission and population genetics, Darwinian evolution, and individuality. Regarding these topics, there is a predominant and shared understanding of germ cells, somatic cells, and the ways in which these two relate to each other that is rarely critically evaluated. In this article, I lay out how germ cells and germline within metazoans are understood by a majority of scientists and philosophers, both now and historically, by sketching out what I call the predominant epistemic framework of germ. I show how this framework conflicts with empirical evidence, propose a series of revisions to realign it with this evidence, and indicate why such revisions are urgently needed by highlighting the case of somatic cell genome editing.