The molecular basis of a neuroendocrine fixed action pattern: egg laying in Aplysia.

We describe a gene family that encodes the proteins controlling the egg-laying behavior of Aplysia. The family evolved by duplication and divergence from a common ancestral gene. The ELH gene family is expressed in the atrial gland, in the bag cells, and in a small network of neurons in the central ganglia. The bag cells and the atrial gland express distinct members of the family that encode different precursor proteins. These contain one or more biologically active peptides that can be released by proteolytic cleavage. The bag cell precursor releases several peptides that have multiple sites of action and may generate different components of the egg-laying behavior. Coordination of the full stereotyped behavior is achieved by simultaneous release of these peptides from the bag cell processes.