Evolution of retinoic acid-dependent developmental regulatory system in chordates

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We demonstrated that retinoic acid activates expression of the Hox1 gene in ascidian embryos. We also showed that retinoic acid and Hox1 contributed to organogenesis of ascidian larvae. The larvacean Oikopleura dioica does not have retinoic acid synthase and retinoic acid receptor in their genome, even though it is a close relative of ascidians. We found in the present study that both larvaceans and ascidians have retinoic acid-independent mechanism to activate transcription of Hox1. We hypothesized that the common ancestor of larvaceans and ascidians had both retinoic acid-dependent and independent pathways for the Hox1 activation. Larvaceans are thought to have survived the loss of retinoic acid-related genes, probably because they had two redundant pathways.