A single gene encodes multiple neuropeptides mediating a stereotyped behavior.

Egg laying in Aplysia is characterized by a stereotyped behavioral array which is mediated by several neuroactive peptides. We have sequenced two genes encoding the A and B peptides thought to initiate the egg-laying process, as well as a gene encoding egg-laying hormone (ELH) which directly mediates the behavioral array. The three genes share 90% sequence homology and are representatives of a small multigene family. Each gene encodes a protein precursor in which the active peptides are flanked by internal cleavage sites providing the potential to generate multiple small peptides. Each of the three genes consists of sequences homologous to A or B peptide as well as ELH. Although these genes share significant nucleotide homology, they have diverged such that different member genes express functionally related but nonoverlapping sets of neuroactive peptides in different tissues.