At the Sea Slug Singles Bar, Mating is a Sure Thing

Newswise — The word "chemistry" is often used to describe the spark of sexual attraction between two people, generally functioning as shorthand for the mysterious, apparently unexplainable mechanisms that drive the choice of a mate. But while human sexual chemistry may seem beyond rational explanation, scientific understanding of the sexual chemistry of sea slugs has just taken a great leap forward.

In a paper to be published next week in the Proceedings of the National Academy of Sciences (PNAS), researchers at the University of Texas Medical Branch at Galveston report the results of six years of biochemical, structural biological and behavioral studies of the water-borne protein pheromones that sea slugs " members of the genus Aplysia—use to attract mates. UTMB scientists led by Gregg Nagle and Sherry D. Painter (also authors on this paper) discovered these "attractins" " the first water-borne invertebrate pheromones to be found " in the late 1990s. The chemicals are so powerful that even a teaspoonful in a swimming-pool-sized tank would be enough to elicit sea-slug mating behavior.

Strangely, they also affect members of different Aplysia species than the one that made them, drawing together large mixed-species groups of mating animals (called "brothels" by marine biologists). Using both amino acid sequence data and nuclear magnetic resonance-derived solution structures, the UTMB team figured out why five different species of Aplysia respond to each other's attractins in the wild and in the lab (including Aplysia from the Gulf of Mexico responding to attractin from a different species of Aplysia found off California). Certain structural features recur in attractins produced by all five species, suggesting that those particular regions are critical to activating the animal's unusual mating behavior.

Although hermaphroditic, Aplysia almost never fertilize themselves. Instead, they make attractins, which perform the dual function of summoning a mate (no easy task for a nearly blind animal in dark waters) and stimulating it to take on a male role in copulation " unless, as often happens, more than one Aplysia shows up, in which case the creatures form chains with the Aplysia in the middle acting as both male and female. This paper offers the first solid evidence for why these "brothels" often include more than one species of Aplysia—a common amino acid sequence found in all their attractins, which the researchers showed to be essential in T-maze experiments in which Aplysia failed to respond to mutated attractins that lacked the sequence.

The structural motif generated by this sequence of amino acids, abbreviated IEECKTS, is thought to be involved in the water-borne sexual signals of other gastropods, and similar motifs are seen in the pheromones of single-celled organisms known as ciliated protozoans, leading researchers to suggest that attractin-like signals could be quite old in evolutionary terms. (The authors of the PNAS paper even suggest the possibility that water-borne pheromones, which can bind to mammalian interleukin receptors, could be the ancestors of cytokines in higher animals.) Within the genus Aplysia, they theorize, the response of different species to each other's attractin does no harm and actually creates benefits, since a larger "brothel" can send out a stronger pheromone signal, drawing still more sea slugs to the party and increasing the chances of each finding a suitable mate or mates.