Extreme conservation of cnidarian stinging cell identity despite 600 million years of evolution

Understanding how cells specialize is essential for reconstructing the diversification of life on earth. Cnidocytes (stinging cells) have a single origin in the stem cnidarian ([~]800mya) and have since specialized into extremes in morphology and function. Using single-cell RNA sequencing and transcriptional lineage reconstruction in a coral and a sea anemone, we show that a single gene (FoxL2) controls a critical switch point in the evolution of cnidocyte diversity: the decision to be a piercing cell (nematocyte) or an ensnaring cell (spirocyte). Nematocytes are lowly conserved between species and express younger genes, reflecting the continued diversification of this cell lineage. Surprisingly, ensnaring cells are one of the most highly conserved differentiated cell types and express older genes than nematocytes. This suggests spirocytes reached an adaptive peak early and have changed little during the 600 million years since corals and sea anemones last shared a common ancestor, making them living fossil cells.