RIFFELL, J.A.*; ZIMMER, R.K.; University of California, Los Angeles; University of California, Los Angeles: Sex and the single cell: Mechanisms of sperm chemoattraction in laminar shear flows
Sperm attractants have been implicated as key mediators of gamete recognition operating prior to cell contact, particularly for animals that broadcast their sperm and eggs into the sea. Such egg-derived factors may be ecologically important, increasing the effective target size of the egg by triggering activation and chemotaxis of sperm. However, the mechanisms by which attractant molecules and sperm behavior affect fertilization remain unknown, particularly in turbulent, flowing water. At the scale of sperm and egg, turbulent fluid motion exists as a laminar velocity gradient, or shear. Using a Taylor-Couette apparatus and a new application of infrared laser and computer-assisted video imaging technologies, the influence of chemosensory behavior by sperm of the red abalone, Haliotis rufescens, in mediating gamete encounters was determined. Our recent discovery of L-tryptophan as a potent attractant to abalone sperm provided the rare opportunity to quantify how navigation affects gamete encounters in flow. Chemotactic responses by live sperm (faster speeds and navigation towards an egg), allowed us to establish the shaped and sizes of attractant plumes from eggs, and to compare those distances to theoretical predictions on bulk transport of a solute in shear. Sperm behavioral responses to manipulations of the natural tryptophan gradient around individual eggs show that both chemotactic and chemokinetic effects significantly influence gamete encounters. Moreover, steady shear drastically stretches and modifies the concentration gradient of tryptophan around eggs, well beyond the broadcast distance of attractants produced by diffusion alone. Thus, the interplay between physical transport of the attractant, and the behavior of the sperm, both play key roles in controlling sperm-egg interactions and fertilization success in the red abalone.
