Whatever size you are, you'll have to navigate your way around your world at some time or another. Bugs orientate towards decomposing food, and migrating birds aim towards remote destinations. Some species' rely for survival on their sperm successfully navigating through open water towards a single egg. But how tiny sperm locate their own species' eggs was largely a mystery, until Richard Zimmer's lab began analysing the essential essence released by abalone eggs. After purifying litres of egg-enhanced seawater,Patrick Krug and Jeffrey Riffell found that the eggs were secreting nothing more exotic than tryptophan to lure the sperm in their direction(p. 1439).
More is known about how birds navigate the heavens than how a single sperm cell locates its target. Creatures that release their gametes into the ocean must have developed some way of attracting sperm from their own species,otherwise evolution would have consigned them to the wastebasket. Krug explains that `there had to be some chemoattractant' to direct the sperm.
Riffell and Krug decided to see if abalone sperm knew which way to go when tempted with a fresh egg, and if they could find the molecule that lured the sperm in the right direction. They placed an egg in a container of seawater and watched to see how the sperm reacted. The moment the sperm picked up the scent, they instantly doubled their speed and aimed directly at the egg. That's when the really hard work started. Zimmer's team had no idea what sort of compound they were searching for.
They flushed egg-conditioned seawater through chromatography columns and tested how the separated chemicals affected the sperm's speed and sense of direction. Krug explains that they had an early stroke of luck when the compound that the sperm responded to stuck to the first column they tried! It took several more purification steps before they had isolated a component that attracted the sperm, but they still didn't know what the mysterious compound was.
Nuclear magnetic resonance spectroscopy revealed the molecule's identity;the chemoattractant was simply tryptophan! Krug admits that he'd been secretly hoping that it might be something more glamorous, but after recovering from his initial disappointment, the advantages became clear. Krug and Riffell could test the sperm's response to a host of readily available tryptophan derivatives and really get to the bottom of the chemical's attraction.
First they discovered that the sperm were sensitive to L-tryptophan, and completely disinterested in D-tryptophan. They also tested how the sperm reacted to a uniform distribution of L-tryptophan versus a gradient of the chemical. When presented with a gradient, the sperm raced along it towards the egg, but when the sperm just had a uniformly high concentration of tryptophan, the gametes went wild, swimming all over the place, without locating the egg. Sperm that were presented with an egg that had lost its tryptophan gradient were completely disinterested in it, and failed to reach their goal.
Of course, this is still far from the sperm's real world, where they locate their target egg through swirling currents, but Krug and Riffell have gone a long way to solving the mystery of the identity of the chemical that lies at the heart of sperm navigation.