Formation of the chordamesoderm in the amphioxus embryo: Analysis with Brachyury and fork head/HNF-3 genes.

The embryonic development of amphioxus (cephalochordates) has much in common with that of vertebrates, suggesting a close phylogenetic relationship between the two chordate groups. To gain insight into alterations in the genetic cascade that accompanied the evolution of vertebrate embryogenesis, we investigated the formation of the chordamesoderm in amphioxus embryos using the genes Brachyury and fork head/HNF-3 as probes. Am(Bb)Bra1 and Am(Bb)Bra2 are homologues of the mouse Brachyury gene isolated from Branchiostoma belcheri. Molecular phylogenetic analysis suggests that the genes are independently duplicated in the amphioxus lineage. Both genes are initially expressed in the involuting mesoderm of the gastrula, then in the differentiating somites of neurulae, followed by the differentiating notochord and finally in the tail bud of ten-somite stage embryos. On the other hand, Am(Bb)fkh/HNF3-1, an amphioxus (B. belcheri) homologue of the fork head/HNF-3 gene, is initially expressed in the invaginating endoderm and mesoderm, then later in the differentiating notochord and in the tail bud. With respect to these two types of genes, the formation of the notochord and tail bud in amphioxus embryos shows similarity and dissimilarity with that of the notochord and tail bud in vertebrate embryos.