Conservation of cis-regulatory codes over half a billion years of evolution.

Identifying homologous cell types across species is essential for understanding cell type evolution. The retina is ideal for comparative analysis because its six major cell classes have persisted since the origin of vertebrates more than half a billion years ago. Here, we show that the retina's conserved cellular architecture is mirrored by deep conservation of the cis-regulatory codes that govern gene expression. Through single-cell chromatin accessibility analysis of lamprey, fish, bird, and mammalian retinas, we demonstrate cross-species conservation of cis-regulatory codes in all retinal cell classes despite extensive turnover of cis-regulatory regions. Conservation manifests as clustering of high-affinity transcription factor binding sites in cell class-specific open chromatin regions. Thus, the retina's cellular Bauplan is controlled by cis-regulatory codes, which predate the divergence of extant vertebrates.