Fig. 7

Comparison of the three-strip expression patterns of homologous genes between zebrafish and amphioxus embryos. Dorsal view of a zebrafish embryo at early segmentation period (3ss) and an amphioxus embryo at mid-neurula (7ss). In zebrafish head mesoderm at this developmental stage, expression patterns of marker genes resolve into three parallel bilateral strips. The innermost region marked by foxc1a (dark blue) expression continues to the posterior paraxial mesoderm which is segmented by somites. In the amphioxus embryo, FoxC is expressed in the somites all along the body. Lateral to the paraxial mesoderm in zebrafish is the cranial lateral mesoderm (red) marked by the expression of tbx1, cyp26c1 and alx1 genes. In the amphioxus embryo, Tbx1/10 expression spans paraxial (somites) and ventral mesoderm. The latter is known to be homologous to vertebrate lateral mesoderm (wisteria), and Tbx1/10 expression here is limited to the most anterior pharyngeal region. In the somites, Tbx1/10 is expressed only in the ventral half of all the somites shown as red/blue stripes. Alx gene shows a similar expression pattern except that its somite expression is limited to the anteriormost region. In amphioxus, all three Cyp26 genes are expressed only in the most anterior somites, but not in the ventral mesoderm. The outermost strip is the anterior lateral plate mesoderm (ALPM, green) labelled with tbx20. In the amphioxus embryo, Tbx20 is expressed in the ventral mesoderm from anterior to posterior region. Note that the prechordal plate is not shown in this diagram