Abstract
Tapeworms grow at rates that rival all metazoan tissues, including during embryonic and neoplastic growth. For example, the rat intestinal tapeworm, Hymenolepis diminuta, produces up to 2,200 proglottids (segments), increasing in length up to 3,400 fold, and weight up to 1.8 million fold within the first 15 days of infection1. Tapeworms also regenerate: they shed large parts of their body, releasing their embryos to continue their life cycle, yet are able to continuously replenish proglottids and maintain an equilibrium length. Such remarkable growth and regeneration are fueled by adult somatic stem cells, which have yet to be characterized molecularly. Using H. diminuta as a laboratory model, we find that regeneration is limited to the tapeworm neck, making this tissue a prime source to identify stem cell genes. Using transcriptomic analyses and RNA interference (RNAi), we characterize and functionally validate the first molecular regulators of tapeworm growth and regeneration. However, we find no evidence that stem cells are restricted to the regeneration-competent neck. Instead, we find that lethally irradiated tapeworms can be rescued from death when cells from both regeneration-competent and regeneration-incompetent regions are transplanted into the neck. Furthermore, the persistence of regenerative ability over time requires signal(s) from the head, even though the head itself cannot regenerate and the head is not necessary for initial regeneration. Together, the head and neck tissue provide a microenvironment that regulates stem cells to enable region-specific regeneration in this tapeworm.