Expression and functional analysis of Citiff1, an ascidian NK-2 class gene, suggest its role in endoderm development

In solitary ascidians the fate of endoderm is determined at a very early st age of development and depends on cytoplasmic factors whose nature has not been determined. We have isolated a member of the NK-2 gene family, Cititf1 , from the ascidian Ciona intestinalis, showing high sequence homology to m ammalian TITF1, The Cititf1 gene was expressed in all endodermal precursors at the pregastrula and gastrula stages, and is thus the first specific reg ulatory endodermal marker to be isolated from an ascidian, Cititf1 expressi on was downregulated at the end of gastrulation to reappear at middle tailb ud and larval stages in the most anterior and ventral parts of head endoder m, regions,which give rise, after metamorphosis, to the adult endostyle, wh ere Cititf1 mRNA was still present. Microinjection of Cititf1 mRNA into fer tilized eggs resulted in tadpole larvae with abnormalities in head-trunk de velopment consequent to the formation of excess endoderm, perhaps due to re cruitment of notochord precursors to an endodermal fate. These data suggest that Cititf1 plays an important role in normal endoderm differentiation du ring ascidian embryogenesis.