Fig. 4. Loss of hdl and tcf3b function leads to progressive loss of rostral gene expression domains and concomitant expansion of more caudal domains. All embryos are shown in rostral view with dorsal towards the right. (A1-A3) pax6 expression at 1-3 somites. (B1-B5) pax2.1 and (C1-C5) gbx1 expression at tailbud stage. In hdl mutants, rostral pax6 expression occupies a smaller domain (A2, bar), pax2.1 shows moderate expansion (B2), whereas gbx1 is unchanged (C2). hdl embryos injected with tcf3b MO show a range of phenotypes, suggesting further caudalization of the embryos. Moderately caudalized embryos show complete loss of rostral pax6 (A3), further expansion of pax2.1 (B3) and expansion of gbx1 (C3) expression. In more severely caudalized embryos, pax2.1 expression begins to be lost (B4) as gbx1 expands further (C4). In the most severely caudalized embryos, pax2.1 expression is completely lost (B5) and gbx1 expression shifts to a more rostral domain (C5). (D) Simultaneous examination of pax6 (rostral purple), pax2.1 (red) and gbx1 (caudal purple) expression at the tailbud stage. (E) gbx1 (purple) expression expands rostrally around the pax2.1 (red) expression domain in moderately caudalized embryos.

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