The myogenic potency of HLH-1 reveals wide-spread developmental plasticity in early C. elegans embryos

In vertebrates, striated muscle development depends on both the expression of members of the myogenic regulatory factor family (MRFs) and on extrinsic cellular cues, including Wnt signaling. The 81 embryonically born body wall muscle cells in C. elegans are comparable to the striated muscle of vertebrates. These muscle cells all express the gene hlh-1, encoding HLH-1 (CeMyoD) which is the only MRF-related factor in the nematode. However, genetic studies have shown that body wall muscle development occurs in the absence of HLH-1 activity, making the role of this factor in nematode myogenesis unclear. By ectopically expressing hlh-1 in early blastomeres of the C. elegans embryo, we show that CeMyoD is a bona fide MRF that can convert almost all cells to a muscle-like fate, regardless of their lineage of origin. The window during which ectopic HLH-1 can function is surprisingly broad, spanning the first 3 hours of development when cell lineages are normally established and non-muscle cell fate markers begin to be expressed. We have begun to explore the maternal factors controlling zygotic hlh-1 expression. We find that the Caudal-related homeobox factor PAL-1 can activate hlh-1 in blastomeres that either lack POP-1/TCF or that have down-regulated POP-1/TCF in response to Wnt/MAP kinase signaling. The potent myogenic activity of HLH-1 highlights the remarkable developmental plasticity of early C. elegans blastomeres and reveals the evolutionary conservation of MyoD function.