Induction of copulatory behavior in Aplysia: atrial gland factors mimic the excitatory effects of freshly deposited egg cordons

Egg laying in the marine mollusc Aplysia is induced and coordinated by peptide products of the egg-laying hormone (ELH) gene expressed in the neuroendocrine bag cells of the central nervous system. At least three structurally related genes, belonging to the ELH family but distinct from the ELH gene, are expressed in the atrial gland, an exocrine organ of unknown function that secretes into the oviduct of Aplysia. The experiments described in this report were designed to test the hypothesis that the atrial gland gene products serve a pheromonal function for the animal, coordinating reproductive behavior among individuals. Our studies showed that there was a significantly shorter latency to copulation when an Aplysia was paired with an animal that was actively laying eggs than when it was paired with a sexually mature but nonlaying animal. Moreover, the addition of extracts or homogenates of the atrial gland to the seawater surrounding two nonlaying animals reduced the latency to mating compared to animals exposed only to seawater or to homogenates of other regions of the reproductive tract, including oviduct. These results suggest that atrial gland products, secreted onto the egg cordon as it passes through the oviduct, may play a pheromonal role and induce mating behavior between individuals. Experiments are in progress to determine whether the active atrial gland factor(s) are products of the ELH-family genes expressed in the gland.