Fig. 3

Inhibition of mesodermal involution in vertebrates recapitulates amphioxus A/P patterning. In situ hybridization of gsc, delta and bra in amphioxus (a) and Xenopus (b) embryos. The arrowheads for delta-2 in the late gastrula Xenopus embryo indicate expression in somites. ae anterior endoderm, bc blastocoel. c Schematic diagram of relative size increase of the dorsal mesoderm during gastrulation in amphioxus and vertebrates. d Overview of the labelling study of the dorsal mesoderm in Xenopus. CellMask Orange (fluorescent dye) was injected into the dorsal mesoderm at stage (st) 10.5 and the embryos were cultured until stage 12. Fixed embryos were cut in half and then subjected to in situ hybridization. e, g, i At stage 12 in the control, labelled cells migrated anteriorly to form the head mesoderm (n = 21, 100 %) and expressed gsc (n = 2, 100 %) but not delta-2 (n = 2, 100 %) or bra (n = 3, 100 %). f, h, j At stage 12 in the embryos that had been injected with 400 pg/cell Xldd1 mRNA at the four-cell blastomere stage, the labelled cells stayed near the blastopore region (n = 40, 100 %), and gsc (n = 2, 100 %), delta-2 (n = 6, 100 %), and bra (n = 3, 100 %) expression overlapped along the A/P axis. st stage. The dotted circle indicates the cells labelled by CellMask Orange. External morphology (k, l), histological sections (m, n) and 3-D reconstruction (o, p) of control (n = 22, 100 %; left) and Xldd1 mRNA–injected embryos (n = 35, 94 %; right). nc notochord, s somite, br brain, cg cement gland, ov otic vesicle, nt neural tube. Expression of myoD (muscle differentiation marker) (q; n = 15, 100 %, r; n = 14, 100 %) and tbx1 (t; n = 12, 100 %, u; n = 20, 90 %) in control and Xldd1 mRNA–injected Xenopus embryos, respectively. s mrf1 and (v) tbx1/10 (pharynx and somite marker) expression in amphioxus embryos. w Schematic diagram of amphioxus-like developmental patterns in Xldd1 mRNA–injected Xenopus embryos. Scale bars, 1 mm (k, q) and 500 μm (m)