running title: Ras signaling in muscle development key words: myogenesis, signal integration, founder cell, muscle progenitor, fusion competent myoblast 2 1 SUMMARY 2 A fundamentally different mechanism is shown for how Ras signaling governs cell fate 3 specification in the Drosophila somatic versus visceral mesoderms, providing insight into how 4 signaling specificity is achieved.Pleiotropic signaling pathways must somehow engender specific cellular responses. In the 7 Drosophila mesoderm, Ras pathway signaling specifies muscle founder cells from among the 8 broader population of myoblasts. For somatic muscles, this is an inductive process mediated by 9 the ETS-domain downstream Ras effectors Pointed and Aop (Yan). We demonstrate here that for 10 the circular visceral muscles, despite superficial similarities, a significantly different 11 specification mechanism is at work. Not only is visceral founder cell specification not dependent 12 on Pointed or Aop, but Ras pathway signaling in its entirety can be bypassed. Our results show 13 that de-repression, not activation, is the predominant role of Ras signaling in the visceral 14 mesoderm and that accordingly, Ras signaling is not required in the absence of repression. The 15 key repressor acts downstream of the transcription factor Lameduck and is likely a member of 16 the ETS transcription factor family. Our findings fit with a growing body of data that point to a 17 complex interplay between the Ras pathway, ETS transcription factors, and enhancer binding as 18 a critical mechanism for determining unique responses to Ras signaling. 19 20 21 22Embryonic development requires that individual cells within a field acquire new, distinct fates. 23The Drosophila larval musculature has emerged as an exemplary system for investigating this 24 process, revealing important insights into the acquisition of developmental competence, 25 progressive determination of cell fate, and the integration of multiple signals at the 26 transcriptional level. It has proven to be a particularly effective model for understanding how 27 specific outcomes can be obtained from the activation of the receptor tyrosine kinase 28
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