Cloning and embryonic expression of Hrsna, a snail family gene of the ascidian Halocynthia roretzi: implication in the origins of mechanisms for mesoderm specification and body axis formation in chordates.

snail family genes encode a transcription factor with specific zinc finger motifs. In vertebrates, they are expressed in the entire mesoderm in early embryogenesis and later in the paraxial mesoderm and the tail-bud, suggesting roles in specification and morphogenesis of the paraxial mesoderm. In the present study, a snail family gene Hrsna from a member of the chordates, an ascidian (Halocynthia roretzi), was cloned to obtain an insight into the origin of the mechanisms of mesoderm specification and body axis formation as observed in vertebrates. Expression of Hrsna during ascidian embryogenesis was found to be quite similar to that of vertebrate snail genes. First, before gastrulation, Hrsna was initially expressed in most precursors of mesodermal tissues including the notochord where As-T, the ascidian homolog of brachyury, was expressed. Hrsna expression persisted in the paraxial mesoderm, the mesenchyme and muscle, but not in the notochord precursors. Also, just as vertebrate snail family genes are expressed in the border of the neural plate that develops into dorsal neural tube and neural crest cells, so Hrsna expression was detected in the precursors of lateral and dorsal regions of the neural tube. However, Hrsna expression was not detected in the tip of the tail, unlike in vertebrate counterparts. In the light of the present findings, similarity and dissimilarity of mechanisms governing mesoderm specification and body axis formation between ascidians and vertebrates are discussed.