A master regulatory loop that activates genes in a temporally coordinated manner in muscle cells of ascidian embryos

Ascidian larval muscle cells present a classic example of autonomous development. A regulatory mechanism for these cells has been extensively investigated, and the regulatory gene circuit has been documented from maternal factors to a muscle-specific gene. In the present study, we comprehensively identified genes expressed specifically in ascidian muscle cells, and found that all of them are under control of a positive regulatory loop of Tbx6-r.b and Mrf, the core circuit identified previously. We also found that several transcription factors under control of the Tbx6-r.b/Mrf regulatory loop resulted in various temporal expression profiles, which are probably important for creating functional muscle cells. These results, together with results of previous studies, provide an exhaustive view of the regulatory system enabling autonomous development of ascidian larval muscle cells. It shows that the Tbx6-r.b/Mrf regulatory loop, but not a single gene, serves a “master” regulatory function. This master regulatory loop not only controls spatial gene expression patterns, but also governs temporal expression patterns in ascidian muscle cells.