A chordate species lacking <i>Nodal</i> utilizes calcium oscillation and <i>Bmp</i> for left-right patterning.

Abstract

Larvaceans are chordates with a tadpole-like morphology. In contrast to most chordates of which early embryonic morphology is bilaterally symmetric and the left-right (L-R) axis is specified by the Nodal pathway later on, invariant L-R asymmetry emerges in four-cell embryos of larvaceans. The asymmetric cell arrangements exist through development of the tailbud. The tail thus twists 90° in a counterclockwise direction relative to the trunk, and the tail nerve cord localizes on the left side. Here, we demonstrate that larvacean embryos have nonconventional L-R asymmetries: 1) L- and R-cells of the two-cell embryo had remarkably asymmetric cell fates; 2) Ca²⁺ oscillation occurred through embryogenesis; 3) <i>Nodal</i>, an evolutionarily conserved left-determining gene, was absent in the genome; and 4) bone morphogenetic protein gene (<i>Bmp</i>) homolog <i>Bmp.a</i> showed right-sided expression in the tailbud and larvae. We also showed that Ca²⁺ oscillation is required for <i>Bmp.a</i> expression, and that BMP signaling suppresses ectopic expression of neural genes. These results indicate that there is a chordate species lacking <i>Nodal</i> that utilizes Ca²⁺ oscillation and <i>Bmp.a</i> for embryonic L-R patterning. The right-side <i>Bmp.a</i> expression may have arisen via cooption of conventional BMP signaling in order to restrict neural gene expression on the left side.