Evidence of autonomous neural specification for both brain and ventral nerve cord tissue in Annelida

Evolution of nervous systems is a long debated topic, and similar mechanisms of conditional neural specification linked to dorsal-ventral (D-V) axis formation across some taxa have been used to support homology. We tested for autonomous versus conditional neural specification in two distantly related annelids, Capitella teleta and Platynereis dumerilii, using blastomere isolations. Our results support previous work in C. teleta and further demonstrate that the autonomous specification of anterior neural tissue and for the first time in trunk neural tissue for both annelids. In C. teleta, we found evidence for conditional pro-neural and anti-neural signals for the VNC. Animal caps lacking vegetal macromeres at the 16-cell stage form a brain and a D-V axis but not a VNC while the addition of any single macromere rescues VNC fate. This suggests that animal micromeres other than 2d produce an anti-neural signal while a pro-neural signal is produced vegetally and that VNC specification is decoupled from D-V axis formation. Taken together, our study suggests possible conservation of autonomous specification of the brain and VNC within Annelida, raising interesting questions of how mechanisms controlling neural specification evolved in Spiralia.