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Chemical communication between sperm and egg is a key factor mediating sexual reproduction. Dissolved signal molecules that cause sperm to orient and accelerate towards an egg could play pivotal roles in fertilization success, but such compounds are largely undescribed. This investigation considered the behavioral responses of red abalone (Haliotis rufescens) sperm to soluble factors released into sea water by conspecific eggs. Sperm in proximity to individual live eggs swam significantly faster and oriented towards the egg surface. Bioassay-guided fractionation was employed to isolate the chemoattractant, yielding a single pure, fully active compound after reversed-phase and size-exclusion high-performance liquid chromatography. Chemical characterization by nuclear magnetic resonance spectroscopy indicated that the free amino acid L-tryptophan was the natural sperm attractant in H. rufescens.

Eggs released L-tryptophan at concentrations that triggered both activation and chemotaxis in sperm, exhibiting significant activity at levels as low as 10-8 mol l-1. The D-isomer of tryptophan was inactive, showing that the sperm response was stereospecific. Serotonin, a potent neuromodulator and tryptophan metabolite, had no effect on sperm swim speeds or on orientation. In experimental treatments involving an elevated, uniform concentration of tryptophan (10-7 mol l-1) or the addition of tryptophanase, an enzyme that selectively digests tryptophan, sperm failed to navigate towards live eggs. A natural gradient of L-tryptophan was therefore necessary and sufficient to promote recruitment of sperm to the surface of eggs in red abalone.

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