Fig. 4. Expression of Cm-derrière rescues the inversion of cardiac and visceral orientation induced by BVg1. (A) Effect of Cm-derrière on the ability of BVg1 to induce a secondary axis. Upper panels, embryos were injected with 300 pg of BVg1 mRNA or 300 pg of BVg1 mRNA plus 500 pg of Cm-derrière mRNA in the ventrovegetal blastomere at 8-cell stage and were harvested at stage 45 for morphological analysis. Induction of a partial secondary axis was observed in 71% (upper middle; N = 45) of BVg1-injected embryos and in 68% (upper right; N = 47) of BVg1 and Cm-derrière-co-injected embryos. Thus, Cm-derrière did not inhibit the ability of BVg1 to induce a partial secondary axis. Bottom panels, embryos were injected with 50 pg of derrière mRNA or 50 pg of derrière mRNA plus 500 pg of Cm-derrière mRNA in the ventrovegetal blastomere at 8-cell stage and were harvested at stage 45 for morphological analysis. Induction of a partial secondary axis was observed in 68% (bottom left; N = 57) of derrière-injected embryos, and this effect was completely rescued by co-injection of Cm-derrière (bottom right; N = 41). An arrowhead shows a partial secondary axis. (B and C) Effect of Cm-derrière on the ability of BVg1 to invert the left–right orientation. Embryos were injected with 300 pg of BVg1 mRNA or 300 pg of BVg1 mRNA plus 100, 500 or 1000 pg of Cm-derrière mRNA in R2 or R3 at 16-cell stage and were harvested at stage 45 for observation of cardiac and visceral left–right orientation. Injection of BVg1 mRNA into R2 or R3 resulted in the reversal of cardiac orientation (middle panel, B) in 44% (N = 27) or in 74% (N = 39) of the injected embryos, respectively. Co-injection of BVg1 mRNA and Cm-derrière mRNA into R2 or R3 resulted in the reversal of cardiac orientation (right panel, B). The results are summarized in (C), which indicates that Cm-derrière can rescue the alteration of cardiac orientation induced by expression of BVg1. Cm-derr, Cm-derrière.