Evolutionary origins and transcriptomic innovations of vertebrate Cajal-Retzius cells

Evolutionary changes in cortical development were instrumental in the emergence of the mammalian neocortex. Cajal-Retzius cells, a transient neuron type discovered over a century ago, are critical players in the development of mammalian-specific cortical features such as inside-out neurogenesis. However, it is unclear whether Cajal-Retzius cells exist only in mammals or whether they are ancestral in vertebrates but acquired new functions during mammalian brain development. To trace the evolution of this cell type, we probe the presence of Cajal-Retzius cells in chicken, salamander, zebrafish, and little skate. First, through comparative transcriptomics and spatial analysis, we show the presence of cells with a Cajal-Retzius molecular identity in non-mammalian vertebrates. However, only amniote Cajal-Retzius cells gained robust expression of Reelin, a secreted glycoprotein crucial for mammalian cortical development. Second, we find that Cajal-Retzius cells are part of a larger and diverse family of neuron types and are closely related to Tp73+ external tufted cells in the olfactory bulb, which express most of the "canonical" Cajal-Retzius transcription factors. Our results indicate that Cajal-Retzius cells emerged early in vertebrate history, using a regulatory program largely shared with neurons in the olfactory system, and that transcriptomic novelties underlie their specialized roles in mammalian cortical development. We uncover an extreme example of how cell types may diverge in function over the course of evolution, even when their core transcriptomic profile is broadly preserved.