Acquisition of dorsal structures, such as notochord and hollow nerve cord, is likely to have had a profound influence upon vertebrate evolution. Dorsal formation in chordate development thus has been intensively studied in vertebrates and ascidians. However, the present understanding does not explain how chordates acquired dorsal structures. Here we show that amphioxus retains a key clue to answer this question. In amphioxus embryos, maternal
nodal
mRNA distributes asymmetrically in accordance with the remodelling of the cortical cytoskeleton in the fertilized egg, and subsequently
lefty
is first expressed in a patch of blastomeres across the equator where
wnt8
is expressed circularly and which will become the margin of the blastopore. The
lefty
domain co-expresses zygotic
nodal
by the initial gastrula stage on the one side of the blastopore margin and induces the expression of
goosecoid
,
not-like, chordin
and
brachyury1
genes in this region, as in the oral ectoderm of sea urchin embryos, which provides a basis for the formation of the dorsal structures. The striking similarity in the gene regulations and their respective expression domains when comparing dorsal formation in amphioxus and the determination of the oral ectoderm in sea urchin embryos suggests that chordates derived from an ambulacrarian-type blastula with dorsoventral inversion.
Acquisition of the dorsal structures in chordate amphioxus
