Range of sperm attraction. (A) Determination of the diffusion coefficient of resact by two-focus fluorescence correlation spectroscopy. Resact was labeled at Cys 1 with Alexa Fluor 488 dye. The autocorrelation function of the first focus (black), the second focus (red), and the cross correlation of both foci (yellow and green) are shown. Symbols are experimental data points, and solid lines are global fits using three fit parameters (see Materials and methods). Residuals are shown on the bottom. (B) Shape of the resact gradient from a single A. punctulata egg for different times (1–60 min) after egg release. (C, top) Peak resact concentration at the center of release for different times after egg release. (bottom) Distance from the egg at which a cell captures one resact molecule during a swimming circle. (D) Cartoon of circular swimming of sperm in a chemoattractant gradient. The concentration of resact (blue) increases linearly from c₁ to c₂ during a complete circle. The cell compares the concentration of resact bound along both semicircles and computes the difference ΔN_abs. (E) Resact gradient (black) and the minimal gradient g_min (red) for the indicated times after egg release. For the effective range of sperm attraction, the resact gradient must be larger than g_min. (F) Effective range of sperm attraction versus time after egg release. At t > 30 min, g_resact ≤ g_min in the very vicinity of the egg. Hence, we observe two boundaries, a short one and a long one for the effective gradient. (G) Resact gradient g_resact (black) and minimal gradient g_min (red) for an egg that synthesizes 1,000 resact molecules/s. 60 min after egg release, the range of attraction is similar to that for an egg with no synthesis. However, near the egg, g_resact > g_min even after 60 min.