Expression of the sea urchin MyoD homologue, SUM1, is not restricted to the myogenic lineage during embryogenesis

SUM1 (sea urchin myogenic factor 1) is a sea urchin homologue of the myogenic basic helix-loop-helix transcription factors of the MyoD family. SUM1 was initially cloned from Lytechinus variegatus where immunocytochemistry demonstrated restricted expression in precursors of the circumesophageal muscles, the only identified muscle cells in the early embryo. Subsequent in situ hybridization analysis indicates that SUM1 embryonic expression is not restricted to the myogenic lineage; a distinct population of nonmyogenic cells also expresses SUM1. For comparative purposes, we cloned the SUM1 orthologue in the distantly related sea urchin, Strongylocentrotus purpuratus, where we found SpSUM1 transcripts in the same population of nonmyogenic cells.