An RFX transcription factor regulates ciliogenesis in the closest living relatives of animals

Cilia allowed our protistan ancestors to sense and explore their environment, avoid predation, and capture bacterial prey. Regulated ciliogenesis was likely critical for early animal evolution, and in modern animals, deploying cilia in the right cells at the right time is crucial for development and physiology. Two transcription factors, RFX and FoxJ1, coordinate ciliogenesis in animals but are absent from the genomes of many other ciliated eukaryotes, raising the question of how the regulation of ciliogenesis in animals evolved. By comparing the genomes of animals with those of their closest living relatives, the choanoflagellates, we found that the genome of their last common ancestor encoded at least three RFX paralogs and a FoxJ1 homolog. Disruption of the RFX homolog cRFXa in the model choanoflagellate Salpingoeca rosetta resulted in delayed cell proliferation and aberrant ciliogenesis, marked by the collapse and resorption of nascent cilia. In cRFXa mutants, ciliogenesis genes and foxJ1 were significantly downregulated. Moreover, the promoters of S. rosetta ciliary genes are enriched for DNA motifs matching those bound by the cRFXa protein in vitro. These findings suggest that an ancestral cRFXa homolog coordinated ciliogenesis in the progenitors of animals and choanoflagellates and that the selective deployment of the RFX regulatory module may have been necessary to differentiate ciliated from non-ciliated cell types during early animal evolution.