Conserved signalling pathways can induce different outcomes in different developmental contexts. In the ascidian vegetal hemisphere, notochord and mesenchyme are induced in the anterior and posterior margins, respectively, by the same fibroblast growth factor (FGF). To become mesenchyme, the posterior blastomeres must inherit posterior-vegetal cytoplasm from the egg. Kobayashi and co-workers used gain- and loss-of-function experiments to demonstrate that the action of the posterior-vegetal egg cytoplasm is due to maternal mRNA encoding the putative transcription factor and muscle determinant Macho-1 (see ). Expression of macho-1 is both necessary and sufficient to determine the mesenchymal fate of the posterior vegetal hemisphere; in its absence,notochord is induced. Kobayashi et al. suggest that Macho-1 acts upstream of the transcriptional repressor Snail, which suppresses notochord induction by suppressing the transcription factor Brachury.
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