Light-modulated neural control of sphincter regulation in the evolution of through-gut.

The development of a continuous digestive tract, or through-gut, represents a key milestone in bilaterian evolution. However, the regulatory mechanisms in ancient bilaterians (urbilaterians) are not well understood. Our study, using larval sea urchins as a model, reveals a sophisticated system that prevents the simultaneous opening of the pylorus and anus, entry and exit points of the gut. This regulation is influenced by external light, with blue light affecting the pylorus via serotonergic neurons and both blue and longer wavelengths controlling the anus through cholinergic and dopaminergic neurons. These findings provide new insights into the neural orchestration of sphincter control in a simplified through-gut, which includes the esophagus, stomach, and intestine. Here, we propose that the emergence of the earliest urbilaterian through-gut was accompanied by the evolution of neural systems regulating sphincters in response to light, shedding light on the functional regulation of primordial digestive systems.