Induction of genetic heterogeneity with stem cells populations in planarian flatworm chimeras

Stem cells are the basic self-replicating units that maintain tissue homeostasis in adult animals. But how the body coordinates the division and differentiation of its stem cell populations to maintain dynamic tissue homeostasis is still unclear. The unusual adult stem cell system of planarians exemplifies the challenge of understanding dynamic tissue homeostasis. Planarians possess pluripotent adult stem cells that continuously renew all cell types and of which a single one can make a complete animal body. The additional continuous divisions of the stem cells, extreme longevity and lack of a single cell generational bottleneck due to asexual reproduction by fission/regeneration exemplify a profound question: How do planarians maintain dynamic homeostasis in the face of inevitable genetic drift? To address this question, we experimentally induced genetic heterogeneity by tissue transplants between a diverse collection of genetically distinct Schmidtea polychroa strains. By following the host and donor cell populations in the resulting chimeric animals over time and space, we can distinguish different donor-host cell dynamics between specific strain combinations. These results provide first glimpses of the processes that might contribute to the maintenance of tissue homeostasis in the face of inevitable genetic drift.