CNIDARIANS AND THE NOVELTY OF STEM CELL IDENTITY

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Yes!  Several, in fact. A recent study using single-cell RNAseq in the freshwater polyp Hydra characterized the complete developmental trajectories three distinct stem cell lineages in this model cnidarian: two tissue-restricted stem cell lineages and one multipotent lineage (Siebert et al., 2019). Indeed, the multipotent stem cells (interstitial stem cells or "i-cells") of Hydra been tauted as an example of mammalian-like stem cells that evolved before the diversification of cnidarians and bilaterians from their common ancestor (Bosch 2009). The only problem is: i-cells appear to be unique to Hydrozoa. Across cnidarians the dynamics of stem cell biology (e.g., their embryonic origins, their contribution to regeneration, and their spatio-temporal progression through differentiation into daughters) varies widely (Gold and Jacobs, 2013). These observations make efforts to define stem cell identity in other cnidarian lineages increasingly important.   

Cnidocytes are single-use cells – after the cnidocyst is discharged, the cell dies. Because cnidocytes are used in prey capture, defense, and adhesion, these cells must be replaced throughout the lifetime of a cnidarian. Like other somatic cells, new cnidocytes differentiate from the daughters of proliferative precursors. Thus, the evolution of cnidocytes must have occurred alongside the origin of stem cells. We have developed a technique for manually discharging cnidocytes using the low-power setting on an infrared laser ablation system. Using a combination of pharmacological techniques and assays of gene expression, we are investigating the process by which cells decide to maintain stemness or differentiate into a new cnidocyte. 

Bosch TC. (2009) Hydra and the evolution of stem cells. Bioessays. 31(4):478-486  Link

Gold DA, Jacobs DK. (2013) Stem cell dynamics in Cnidaria: are there unifying principles? Dev Genes Evol. 223(1-2):53-66 Link 

Siebert S, Farrell JA, Cazet JF, et al. (2019) Stem cell differentiation trajectories in Hydra resolved at single-cell resolution. Science. 365(6451):eaav9314.   Link

Watch us discharge cnidocytes using an infrared laser on our YouTube channel!