Spiders Seeking Sex

A male spider's sense of rhythm—specifically, his ability to drum a beguiling beat on his Valentine's silk web—was long thought to be the primary factor needed to spark a little eight-legged romance. Only recently have scientists come to appreciate the irresistible draw of spider sex pheromones. Still, the small number of molecules known to catalyze arachnid sex—just five, until now—trails behind the hundreds of analogous pheromones discovered for insects and mites.

Now, researchers led by Stefan Schulz, an organic chemist at the Braunschweig Institute of Technology, in Germany, report two new spider pheromones, including one that establishes a new class of natural products. The work will help researchers better understand spider mating rituals and may also provide a means to entice invasive spiders out of international shipping containers.

The first discovery exposes the love potion of the Australian redback spider, a poisonous member of the black widow family. Researchers who keep a close eye on the redback have long noticed that when males wander by the spider webs of females who haven't yet found success in love, the Romeos begin a "drumming and jerking" dance, Schulz says. Yet when male spiders encounter the webs of females who have already hooked up, the bachelors don't bother doing the spider salsa, he adds.

It turns out that available females spread a perfume of acylated serine derivatives on their webs to indicate their openness to romance (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200906312). These serine derivatives, which Schulz discovered in collaboration with Maydianne D. Andrade of the University of Toronto, have a molecular architecture never before seen in nature, Schulz notes.

To determine the pheromones' structure, the team did some "masterful analytical chemistry," figuring out the correct basic structure "from mass spectral data alone," comments Jocelyn G. Millar, a chemical ecologist at the University of California, Riverside. Because there was no molecular precedent for the new natural product, the researchers then "very cleverly made a combinatorial library of all possible isomers to verify the structure," he notes. The pheromone is an attractant, so Millar suggests that derivatives could be used to seduce the poisonous redback out of Australian shipping containers, preventing the species from being an invasive poisonous species in other countries.

In a second spider-sex advance, Schulz; Gabriele Uhl of the University of Greifswald, in Germany; and colleagues report another new arachnid pheromone, this time from the ubiquitous wasp spider, Argiope bruennichi. Although the newly discovered citrate derivative is similar to another known pheromone from the hunting spider Cupiennius salei, the team is the first to show, in field settings, that a synthesized version of the wasp spider pheromone can attract the right guys. Some 34 males were attracted in 30 minutes from several meters away, displaying typical courtship behavior including jerking and abdominal vibration, compared with zero spiders in the control study (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200906311).

As the pool of known spider pheromones increases to seven, Schulz notes that many of their structures are closely related to biologically important housekeeping molecules. The newly identified pheromones, for example, are built on scaffolds of serine, an amino acid, and citrate, a key molecule in energy metabolism. The diversion of important housekeeping molecules toward courtship is an interesting strategy that conflicts with previously held maxims. "Pheromone production was historically thought to entail relatively little physiological cost" to the organism, Schulz says. That concept may need to be rethought, he adds.

Both papers represent important steps forward in our understanding of how spiders pick their mates, Millar says. "We know very little so far about spider pheromones, despite the fact that spiders are very common and ecologically important organisms," he adds.