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First published online November 1, 2006
Journal of Experimental Biology 209, 4574-4579 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02500

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Eggs regulate sperm flagellar motility initiation, chemotaxis and inhibition in the coral Acropora digitifera, A. gemmifera and A. tenuis

Masaya Morita^1,2,*, Akira Nishikawa², Ayako Nakajima³, Akira Iguchi⁴, Kazuhiko Sakai², Akihiro Takemura² and Makoto Okuno³

¹ Department of Chemistry, Biology and Marine Sciences, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213, Japan
² Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus, 3422 Sesoko, Motobu, Okinawa 907-0227, Japan
³ Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Meguro-ku, Tokyo 153-8902, Japan
⁴ Comparative Genomics Centre, Molecular Science Building, James Cook University, Townsville, Queensland 4811, Australia

View larger version (46K): [in a new window]   Fig. 1. Sperm motility initiation factor in Acropora species. (A) Sperm of A. digitifera, A. gemmifera and A. tenuis were completely quiescent even after sperm were split from bundles. Scale bar, 50 m. (B) Sperm motility of A. digitifera in artificial seawater (ASW), ASW containing 20 mmol l-1 NH4Cl, Na-free ASW, and Na-free ASW containing 20 mmol l-1 NH4Cl. Other Acropora species also showed similar sperm motility. Values are mean ± s.d. N=5.

View larger version (19K): [in a new window]   Fig. 2. Trajectory of swimming sperm with or without eggs. Sperm movements in Acropora digitifera were recorded. The movement of sperm heads was traced by NIH images and the trajectories drawn. (A) The trajectories of sperm heads in the presence of an egg in ASW. Arrows indicate straight direct paths, a chemotactic behaviour observed close to eggs. (B) The trajectories of sperm heads when no egg was present in Na-free ASW containing 20 mmol l-1 NH4Cl. Other Acropora species, A. gemmifera and A. tenuis showed similar movement (data not shown). Scale bars, 50 µm.

View larger version (94K): [in a new window]   Fig. 3. Sperm motility suppression by the presence of an egg. Trajectories of sperm heads of Acropora digitifera were recorded by modulating the sensitivity of a CCD camera under dark-field illumination. Swimming speed was calculated from the length of trajectory determined using NIH Image. (A) Trajectories of sperm motility are shown with and without eggs. (B) Percentages of sperm motility. (C) Swimming speed. Other Acropora species showed similar patterns. Scale bars, 50 µm. Values are mean ± s.d. (N=150 sperm from three individuals).

View larger version (54K): [in a new window]   Fig. 4. Effect of Ca2+ on trajectories and swimming speed. Trajectories in the presence or absence of Ca2+ are shown. Sperm trajectories of Acropora digitifera in Na-free ASW containing (A) 5 mmol l-1 EGTA and 10 µmol l-1 of the Ca2+ ionophore A23187, (B) 10 mmol l-1 CaCl2 and 10 µmol l-1 A23187, and (C) 10 mmol l-1 CaCl2. (D) Swimming speed in each of the solutions. Other species showed similar patterns (data not shown). Values are mean ± s.d. (N=150 sperm from five individuals). Scale bars, 50 µm.

View larger version (13K): [in a new window]   Fig. 5. Schema of flagellar motility regulation in sperm of Acropora digitifera, A. gemmifera and A. tenuis.