期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2020
卷号：117
期号：8
页码：4188-4198
DOI：10.1073/pnas.1916858117
出版社：The National Academy of Sciences of the United States of America
摘要：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) Ca 2+ oscillation occurred through embryogenesis; 3) Nodal , an evolutionarily conserved left-determining gene, was absent in the genome; and 4) bone morphogenetic protein gene ( Bmp ) homolog Bmp.a showed right-sided expression in the tailbud and larvae. We also showed that Ca 2+ oscillation is required for Bmp.a expression, and that BMP signaling suppresses ectopic expression of neural genes. These results indicate that there is a chordate species lacking Nodal that utilizes Ca 2+ oscillation and Bmp.a for embryonic L–R patterning. The right-side Bmp.a expression may have arisen via cooption of conventional BMP signaling in order to restrict neural gene expression on the left side.
