Further Development 10.9: How Being Macho Can Give You a "Spine"

Sea Urchins and Tunicates: Deuterostome Invertebrates

The presence of Macho-1 in the posterior vegetal cytoplasm causes those posterior cells that will become mesenchyme to respond differently to the FGF signal than do the cells that will form neural structures (Figure 1; Kobayashi et al. 2003). Macho-1 prevents notochord induction in the mesenchymal cell precursors by activating the snail gene (which will in turn suppress the gene for Brachyury). Thus, Macho-1 is not only a muscle-activating determinant, but also a factor that distinguishes cell responses to the FGF signal. These FGF-responding cells do not become muscle because FGFs activate cascades that block muscle formation (a role for these factors that is conserved in vertebrates). As seen in Figure 1, the presence of Macho-1 changes the responses to endodermal FGFs, causing the anterior cells to form notochord while the posterior cells become mesenchyme.

**Figure 1** The two-step process for specifying the marginal cells of the tunicate embryo. The first step involves the cells’ acquisition (or nonacquisition) of the Macho-1 transcription factor. The second step involves the reception (or nonreception) of the FGF signal from the endoderm. (After K. Kobayashi et al. 2003. *Development* 130: 5179–5190.)

Literature Cited

Kobayashi, K., K. Sawada, H. Yamamoto, S. Wada, H. Saiga and H. Nishida. 2003. Maternal Macho-1 is an intrinsic factor that makes cell response to the same FGF signal differ between mesenchyme and notochord induction in ascidian embryos. Development 130: 5179–5190.
PubMed Link

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