The regulatory pathway from genes directly activated by maternal factors to muscle structural genes in ascidian embryos

Abstract

Striated muscle cells in the tail of ascidian tadpole larvae differentiate cell-autonomously. Although several key regulatory factors have been identified, the genetic regulatory pathway has not fully been understood; comprehensive understanding of the regulatory pathway is essential for accurate modeling to deduce principles for gene regulatory network dynamics, and for comparative analysis how ascidians have evolved the cell-autonomous gene regulatory mechanism. Here, we revealed regulatory interactions among three key regulatory factors, Zic-r.b, Tbx6-r.b, and Mrf, and elucidated the mechanism by which these factors activate muscle structural genes. We revealed a cross-regulatory circuit among these regulatory factors, which maintained the expression of Tbx6-r.b and Mrf during gastrulation. While these two factors combinatorially activated muscle structural genes in late stage embryos, muscle structural genes were activated mainly by Tbx6-r.b before gastrulation. Time points when expression of muscle structural genes became first detectable were strongly correlated with the degree of Tbx6-r.b occupancy. Thus, the genetic pathway starting with Tbx6-r.b and Zic-r.b, which are activated by maternal factors and ending with expression of muscle structural genes, has been revealed.