Fig. 8. Regulatory network of somatic myogenesis. The scheme depicts the interactions among the major genes and/or their products that regulate the development of larval body wall muscles from gastrulation to the specification of muscle founder cells, as explained in detail in the text. After the establishment of the high Twi domain in the mesoderm underneath the posterior portions of parasegments by Slp, domains competent for somatic myogenesis are specified by `competence domain genes', such as l(1)sc, tin (the hypothetical role of tin as competence domain gene is indicated by parentheses) and Poxm, regulated by Twi and the ectodermal signals Wg and Dpp. These signals, in combination with the localized EGF (Spi) and FGF (Pyr, Ths) signals and through remote inhibition of Spi signaling by Aos (Freeman, 1997), determine the promuscular clusters, which express l(1)sc and activate the MAPK signaling pathway. Singling out of muscle progenitors and separation from fusion-competent myoblasts (FCMs) occurs by lateral inhibition, which is mediated by N signaling that is coupled to MAPK signaling through multiple feedback loops. At this stage, or subsequently, in muscle founder cells generated from progenitors by asymmetric division mediated by N signaling, muscle identity genes, such as Poxm, Kr and slou, are activated by the integration on their enhancers of competence domain gene products with the effectors of the mentioned signals (for references, see text). Hypothetical interactions are indicated by dashed lines.