NORMAL DEVELOPMENT AND EMBRYONIC GENE ACTIVITY OF THE ASCIDIAN HERDMANIA-MOMUS

Embryonic and post-larval development of the tropical solitary ascidia n Herdmania momus is shown to be similar to that of extensively studie d ascidian model systems. H. momus development is rapid and temperatur e-dependent, with hatching occurring 8.5 h after fertilization at 28 d egrees C. An increase in total embryonic gene transcription is detecte d at the 110-cell stage or the onset of gastrulation. Treatment of ear ly embryos with actinomycin D inhibits transcription and curtails morp hogenetic cell movement in the early gastrula without immediately inhi biting cell division. The prevalence of homeobox-containing transcript s increases around the 110-cell stage and later in development. Isolat ed H. momus homeobox genes, expressed at the tailbud stage, have great est sequence identity to members of Hox, otd/Otx, eve/Evx and cad/Cdx homeobox classes. Evidence from H. momus and other ascidians suggests that urochordates possess most of the homeobox genes of the chordate H OX cluster.