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First published online March 27, 2009
Journal of Experimental Biology 212, 1092-1100 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.027029

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Endogenous signaling pathways and chemical communication between sperm and egg

Patrick J. Krug¹, Jeffrey A. Riffell² and Richard K. Zimmer^3,4,*

¹ Department of Biological Sciences, California State University, Los Angeles, CA 90032, USA
² ARL Division of Neurobiology, University of Arizona, Tucson, AZ 85721, USA
³ Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA 90095, USA
⁴ Neurosciences Program and Brain Research Institute, University of California, Los Angeles, CA 90095, USA

^* Author for correspondence (e-mail: z{at}biology.ucla.edu)

Accepted 28 January 2009

Red abalone (Haliotis rufescens) sperm detect a waterborne chemical cue released by conspecific eggs, and change their swimming behavior to increase the likelihood of fertilization success. Previously, we isolated the natural sperm attractant by bioassay-guided fractionation and high-performance liquid chromatography, and chemically identified it as the free-amino acid L-tryptophan (L-Trp). In the present study, levels of this ecologically meaningful compound were quantified in various abalone tissues, and in freshly spawned eggs. Tryptophan was the least abundant of 19 dissolved free amino acids (DFAAs) in ovary, testis, foot muscle, gill, stomach and hemolymph. As a proportion of the DFAA pool, however, Trp concentrations were significantly elevated in eggs (three- to seven-times higher) relative to all other sampled tissues. Natural rates of Trp release from eggs also were measured and correlated with fertility. Fertilization success peaked during an initial 30 min period (post-spawn), but decreased to nil over the next 50 min. Closely paralleling these events, Trp accumulated in seawater around freshly spawned eggs for the first 45 min (post-spawn) before decaying rapidly from solution. Older eggs stopped releasing Trp approximately when they became infertile, revealing a critical link between gamete physiology and chemical signaling. This apparent negative feedback loop did not arise from tryptophan oxidation, uptake by bacteria in seawater, or a degrading enzyme released by eggs. As a metabolic precursor critical to development of the larval nervous system, Trp could be an honest indicator of egg fitness for prospective sperm suitors. Our results suggest that endogenous signaling pathways have been co-opted for external communication between gametes, as an adaptation to increase reproductive success by promoting sperm navigation towards fertile eggs.

Key words: fertilization, sexual reproduction, gamete interactions, sperm, egg, attractant, chemotaxis, tryptophan, amino acid, abalone, Haliotis rufescens

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