Fig. 3

Slow-specific gene expression is upregulated in sox6 morphant embryos. (A) sox6 is expressed in the fast-muscle precursors (red arrowheads) in wild-type (wt) embryos and ectopically expressed in the adaxial cells (green arrowheads) in ubo^tp39 embryos at the 12-somite (12s) and 15-somite (15s) stage. (B) Heat-shock-induced expression of Sox6 (marked with tdTomato (tdTOM)) inhibits Prox1 expression at 24 hpf (hours post fertilization) in adaxially derived cells marked by prdm1:GFP (arrowhead; 100% of the adaxially derived cells lacked Prox1 when ectopically expressing Sox6 (from the analysis of 14 fibres in 4 embryos)). (C) Lateral views of the tail at 24 hpf showing partial rescue of Prox1 expression in ubo^tp39 embryos injected with sox6MO (ubo^tp39: 0 rescued fibres in 20 embryos; ubo^tp39; sox6MO: mean of 58 rescued fibres per embryo, n=5, s.d.=29). (D) At 12s, ubo^tp39 mutants have reduced expression of smyhc1 and ectopic expression of fast-MyLC (recognized by the F310 antibody) in the adaxial cells marked with prdm1. sox6MO causes rescue of the smyhc1 expression in ubo^tp39 mutants but does not prevent ectopic fast-MyLC expression. (E) Transverse views of 24 hpf wt and smoothened (smo) embryos carrying the prdm1:GFP reporter gene (upper panel) and siblings of the same genotypes that have been injected with the sox6 morpholino (sox6MO; lower panels), showing that loss of sox6 causes Hedgehog-independent ectopic expression of stnnC (red) in fast muscle (identified by F310 antigen (blue)). (F) Lateral views of stnnC expression (red) in wt, smo and smo;sox6MO embryos at 24 hpf (9 out of 9 smo;sox6MO embryos, and 15 out of 15 wt embryos showed strong ectopic stnnC expression). Scale bars, 0.25 μm. GFP, green fluorescent protein; Prox1, prospero-related homeobox gene 1; smyhc1, slow myosin heavy chain 1; sox6MO, sox6 morpholino; stnnC, slow-specific Troponin C; ubo, u-boot.