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First published online December 10, 2003
Journal of Experimental Biology 207, 337-345 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00748

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Acclimation of sperm motility apparatus in seawater-acclimated euryhaline tilapia Oreochromis mossambicus

Masaya Morita¹, Akihiro Takemura² and Makoto Okuno^1,*

¹ Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Meguro-ku, Tokyo 153-8902, Japan
² Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus, 3422 Sesoko, Motobu, Okinawa 905-0227, Japan

^* Author for correspondence (e-mail: cokuno{at}mail.ecc.u-tokyo.ac.jp)

Accepted 10 October 2003

Euryhaline tilapia Oreochromis mossambicus can reproduce in freshwater and in seawater. Regulation of sperm motility appears to be modulated during acclimation of the fish from freshwater to seawater, being independent of extracellular Ca²⁺ in freshwater and dependent on extracellular Ca²⁺ in seawater. In the presence of extracellular Ca²⁺, sperm of seawater-acclimated tilapia (SWT) showed motility even in a hypertonic environment, whereas sperm of freshwater-acclimated tilapia (FWT) were not motile. The Ca²⁺ indicator, fluo-3, revealed that intracellular Ca²⁺ concentration, [Ca²⁺]i, of SWT sperm increased only in the presence of extracellular Ca²⁺ in hypotonic or hypertonic conditions. Since the increased [Ca²⁺]i in FWT sperm occurred under hypotonic conditions via intracellular Ca²⁺ stores, it is likely that tilapia modulate their source of increasing [Ca²⁺]i from intracellular stores (in FWT sperm) to extracellular stores (in SWT sperm). Experiments using demembranated sperm revealed that Ca²⁺ is necessary for activation of motility, suggesting that Ca²⁺ plays a key role in motility regulation in SWT sperm. We detected three phosphoproteins associated with the activation of sperm motility. Serine and threonine residues of two proteins of 15 kDa and 18 kDa became dephosphorylated in hypotonic conditions but remained phosphorylated in hypertonic conditions, suggesting that these protein phosphorylations were not only related to motility activation under hypertonic conditions but also resistant to osmotic pressure. The threonine residue(s) of a 41 kDa protein was also phosphorylated in dry sperm, even in FWT sperm in motility-feasible hypotonic conditions. It is likely that acclimation of the motility apparatus is associated with modulation of the flow of Ca²⁺ to increase [Ca²⁺]i and protein phosphorylation.

Key words: sperm motility, protein phosphorylation, Ca²⁺, osmolality, tilapia, Oreochromis mossambicus

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