Decoupling brain from nerve cord development in the annelid Capitella teleta: Insights into the evolution of nervous systems.

In the deuterostomes and ecdysozoans that have been studied (e.g. chordates and insects), neural fate specification relies on signaling from surrounding cells. However, very little is known about mechanisms of neural specification in the third major bilaterian clade, spiralians. Using blastomere isolation in the annelid Capitella teleta, a spiralian, we studied to what extent extrinsic versus intrinsic signals are involved in early neural specification of the brain and ventral nerve cord. For the first time in any bilaterian, we found that brain neural ectoderm is autonomously specified. This occurs in the daughters of first-quartet micromeres, which also generate anterior neural ectoderm in other spiralians. In contrast, isolation of the animal cap, including the 2d micromere, which makes the trunk ectoderm and ventral nerve cord, blocked ventral nerve cord formation. When the animal cap was isolated with the 2D macromere, the resulting partial larvae had a ventral nerve cord. These data suggest that extrinsic signals from second-quartet macromeres or their daughters, which form mesoderm and endoderm, are required for nerve cord specification in C. teleta and that the 2D macromere or its daughters are sufficient to provide the inductive signal. We propose that autonomous specification of anterior neural ectoderm evolved in spiralians in order to enable them to quickly respond to environmental cues encountered by swimming larvae in the water column. In contrast, a variety of signaling pathways could have been co-opted to conditionally specify the nerve cord. This flexibility of nerve cord development may be linked to the large diversity of trunk nervous systems present in Spiralia.