Sperm chemotaxis as revealed with live and synthetic eggs

Fertilization is one of the least understood fundamental biological processes. How sperm search for and find an egg remains enigmatic. Sperm attraction to egg-derived chemical cues may be significant evolutionarily for maintaining species barriers and important ecologically for increasing gamete encounters. New tools are needed, however, to resolve the functional consequences of these dissolved signal molecules. Freshly spawned eggs from red abalone (Haliotis rufescens) naturally release l-tryptophan, which stimulates chemotactic responses by conspecific sperm. Here, microspheres were manufactured to the approximate size and the same shape as female gametes and formulated to emit controlled doses of chemoattractant, imitating natural l-tryptophan release rates. When experimentally tested for effectiveness, male gametes did not distinguish between chemically impregnated mimics and live eggs, demonstrating that l-tryptophan alone is both necessary and sufficient to promote chemotaxis, and confirming the identity of a native sperm attractant. The techniques that we describe can be used to create synthetic eggs for most animal and plant species, including humans. Egg mimics increase the capacity for experimental manipulation and enable realistic studies of sperm behavior even in the absence of female gametes.