Unraveling the regulatory development and molecular mechanisms of identical sea urchin twins

Since Hans Driesch’s pioneering work in 1891, it has been known that animal embryos can develop into complete individuals even when divided1–6. However, the developmental processes and molecular mechanisms enabling this self-organization remain poorly understood. In this study, we revisit Driesch’s experiments by examining the development of isolated 2-cell stage blastomeres in the sea urchin, Hemicentrotus pulcherrimus. Contrary to intact embryos, these isolated blastomeres initially form a flat, single layer of dividing cells that eventually round up to be a blastula. Live imaging and knockdown experiments reveal that actomyosin activity at the basal side of the cells and septate junctions drive this process. Intriguingly, we observed temporal disorganization of the anterior-posterior (A-P) and dorsal-ventral (D-V) axes, where the original A-P poles come into contact after sphere shape formation. The disrupted A-P axis is subsequently corrected as the embryos employ the Wnt/β-catenin signaling mechanisms assumed to be used in intact embryos to re-establish a normal axis. These findings suggest that axis re-organization through pre-existing developmental mechanisms is essential for the successful regulative development of divided embryos. In the process of elucidating the ability of the sea urchin Hemicentrotus pulcherrimus to develop into normal larvae even when blastomeres are separated at the 2-cell stage, we compared mRNA expression between embryos separated at the 2-cell stage and normal embryos using bulk RNA-seq. The comparisons were conducted at 11, 14, 18, and 24 hours post-fertilization, with the 2-cell stage prior to separation also analyzed. RNA-seq was performed in three batches for each condition.