Expression and function of a starfish Otx ortholog, AmOtx: a conserved role for Otx proteins in endoderm development that predates divergence of the eleutherozoa

The sea urchin orthodenticle (Otx)-related transcription factor is an early activator of other endomesodermally expressed transcription factors. Its normal function is required for the development of the archenteron and to lock cells into endomesodermal fate. To determine if this is a basal Otx function in echinoderms we have studied the role of an Otx ortholog in a starfish, Asterina miniata. The patterns of AmOtx expression are found to be similar, in many details, to those reported for other indirectly developing echinoderms and hemichordates, suggestive of a conserved function both in endoderm development and ciliary band formation in these two phyla. When downstream targets of the AmOtx protein are suppressed using a dominant engrailed repressor strategy, embryos fail to develop the endodermal component of the archenteron, though initial phases of mesoderm development proceed normally. The function of Otx proteins in endodermal development at least predated the evolution of the free-living echinoderms (Eleutherozoa).