Storage cell proliferation during somatic growth establishes that tardigrades are not eutelic organisms

Abstract

Tardigrades, microscopic ecdysozoans renowned for their resilience to extreme environments, have long been thought to maintain a constant cell number after completing embryonic development, a phenomenon known as eutely. However, sporadic reports of dividing cells have raised questions about this assumption. In this study, we investigated whether tardigrades truly exhibit a fixed cell number during somatic growth using the model speciesHypsibius exemplaris. Comparing hatchlings to adults, we observed an overall increase in the number of storage cells, a tardigrade cell type involved in nutrient storage. To assess cell proliferation, we monitored DNA replication via the incorporation of the thymidine analog 5-ethynyl-2'-deoxyuridine (EdU). A significantly higher number of storage cells incorporated EdU while animals were still growing. Starvation halted both animal growth and storage cell proliferation, linking the two processes. Additionally, we found that EdU incorporation in storage cells is associated with molting, a critical process in tardigrade post-embryonic development, since it involves cuticle renewal to enable further growth. Finally, we show that hydroxyurea, a drug that slows down DNA replication progression, strongly reduces the number of EdU+cells and results in molting-related fatalities. Our data not only provide a comprehensive picture of replication events during tardigrade growth but also highlight the critical role of proper DNA replication in tardigrade molting and survival. This study definitively challenges the notion of eutely in tardigrades, offering promising avenues for exploring cell cycle, replication stress, and DNA damage management in these remarkable creatures as genetic manipulation techniques emerge within the tardigrade research field.

Pigment-dispersing factor neuropeptides act as multifunctional hormones and modulators in tardigrades

“…Localization of pdfr-expressing target cells in all major tissues and organs of H. exemplaris 46 further suggests that PDFs might control or synchronize various functions in tardigrades, such as detection of light, neural processing, locomotion, feeding, digestion, osmoregulation, growth, development, oogenesis/reproduction, and formation of stylets and claws-a process related to molting 47 . The role of PDF/PDFR signaling in storage cells is unclear, but since these cells have been mainly associated with the production, storage and transport of lipids, polysaccharides and proteins 48,49 , PDFs might additionally regulate the storage and distribution of nutrients in the tardigrade body. This wide variety of functions parallels the reported multiple roles of PDFs in nematodes, ranging from the control of locomotion and mate searching to mechano-and chemosensation, including the sensation of oxygen 19,29,30 .…”