Stem cells: Autonomy interactors that emerge as causal agents and legitimate units of selection

Can stem cells be defined as legitimate units of selection (UOS)? Here I review traits of hallmark stem cells (many of which are manifested in marine organisms), which collectively reveal that some types of stem cells behave as real UOS entities. The two existing categories of stem cells maintain the soma and the germ line, have infallible high capacity for self-renewal, produce diverse lines of differentiated progenies and exist ad infinitum (within the organism's life span). Because of their primitive, undifferentiated state, stem cells are pliable to adopt totipotentiality, but that is not enough to be considered as genuine UOS agents. The discussion on highly transmitted cancer, germ and somatic stem cells, the autonomous traits of somatic stem cells and their stemness, stem cells consortia in biological chimerism, and parasitism vs. cooperation on the level of stem cells, point to a group of characters that denote some types of stem cells as UOS. Prime features of these stem cells include: (a) efficient motility within an organism and between organisms (conspecifics/different taxa; vertical/horizontal transmission); (b) ability to parasitize effectively host organisms, in the same species or in other species; (c) stemness and unlimited replication; (d) ability to blend with other cell lineages, blurring the line between distinct somatic and germ cell lineages; (e) ability to designate a variety of different soma as hosts; (f) ability to co-opt and manage somatic cells to collaborate in parasitic agenda; (g) lack of a distinct niche where they reside; and (h) manifestation of the power of holism. Events in which stem cells acquire the traits of legitimate UOS as real Darwinian individuals, are widespread in multicellular organisms, even when lacking structural cohesion; a phenomenon that confounded accepted dogma for UOS entity. It is therefore concluded that some stem cell lines foster the traits of UOS, as efficiently as whole organisms or genes.