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Eukaryotic flagella collectively form metachronal waves that facilitate the ability to cause flow or swim. Among such flagellated and planktonic swimmers, large volvocine genera such as Eudorina, Pleodorina and Volvox form bundles of small male gametes (sperm) called "sperm packets" for sexual reproduction. Although these sperm packets reportedly have flagella and the ability to swim, previous studies on volvocine motility have focused on asexual forms and the swimming characteristics of sperm packets remain unknown. However, it is important to quantify the motility of sperm packets and sperm in order to gain insights into the significance of motility in the sexual reproduction of planktonic algae. In this study, we quantitatively described the behavior of three flagellated forms of a male strain of Pleodorina starrii—asexual colonies, sperm packets, and single dissociated sperm—with emphasis on comparison of the two multicellular forms. Despite being smaller, sperm packets swam approximately 1.4 times faster than the asexual colonies of the same male strain. Body length was approximately 0.5 times smaller in the sperm packets than in asexual colonies. The flagella from sperm packets and asexual colonies showed asymmetric waveforms, whereas those from dissociated single sperm showed symmetric waveforms, suggesting the presence of a switching mechanism between sperm packets and dissociated sperm. Flagella from sperm packets were approximately 0.5 times shorter and had a beat period approximately twice as long as those from asexual colonies. The flagella of sperm packets were densely distributed over the anterior part of the body, whereas the flagella of asexual colonies were sparse and evenly distributed. The distribution of flagella, but not the number of flagella, appear to illustrate a significant difference in the speeds of sperm packets and asexual colonies. Our findings reveal novel aspects of the regulation of eukaryotic flagella and shed light on the role of flagellar motility in sexual reproduction of planktonic algae. [ABSTRACT FROM AUTHOR]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2007.03.036 Byline: Christopher D. Wood (a), Takuya Nishigaki (a), Yoshiro Tatsu (b), Noboru Yumoto (b), Shoji A. Baba (c), Michael Whitaker (d), Alberto Darszon (a) Keywords: Calcium; Sea urchin; Sperm; Motility; Flagella Abstract: Speract, an egg-derived sperm-activating peptide, induces changes in intracellular Ca.sup.2+, Na.sup.+, pH, cAMP, cGMP, and membrane potential in sperm of the sea urchin Strongylocentrotus purpuratus. Ca.sup.2+ is a key regulator of motility in all sperm and, in many marine species, is required for generating turns interspersed with straighter swimming paths that are essential for chemotaxis towards the egg. We show that speract triggers a train of increases in flagellar Ca.sup.2+, and that each individual Ca.sup.2+ fluctuation induces a transient increase in flagellar asymmetry that leads to a turn. We also find that modifying the amplitude, duration and interval between individual Ca.sup.2+ fluctuations by treating sperm with niflumic acid, an inhibitor of Ca.sup.2+-activated Cl.sup.- channels, correspondingly alters the properties of the sperm turns. We conclude that Ca.sup.2+ entry through a fast flagellar pathway not only induces sperm turns, but the kinetics of Ca.sup.2+ entry may shape the nature of these turns, and that these kinetics are tuned by other channels, possibly including Cl.sup.- channels. In addition, the speract-induced changes in sperm motility closely resemble those seen during chemotaxis in other marine organisms, yet speract is not a chemoattractant. This implies the Ca.sup.2+-induced motility changes are necessary but not sufficient for chemotaxis. Author Affiliation: (a) Departamento de Genetica del Desarrollo y Fisiologia Molecular, Instituto de Biotecnologia, Universidad Nacional Autonoma de Mexico, UNAM, Apdo Postal 510-3, Cuernavaca, Morelos 62250, Mexico (b) National Institute of Advanced Industrial Science and Technology (AIST), Midorigaoka, Ikeda 563-8577, Japan (c) Department of Biology, Ochanomizu University, Tokyo 112-8610, Japan (d) Institute of Cell and Molecular Biosciences, Faculty of Medical Sciences, University of Newcastle, Newcastle upon Tyne, UK Article History: Received 15 November 2006; Revised 22 March 2007; Accepted 22 March 2007

Eggs of many marine and mammalian species attract sperm by releasing chemoattractants that modify the bending properties of flagella to redirect sperm paths toward the egg. This process, called chemotaxis, is dependent on extracellular [Ca.sup.2+]. We used stroboscopic fluorescence imaging to measure intracellular [Ca.sup.2+] concentration ([[Ca.sup.2+]]i) in the flagella of swimming sea urchin sperm. Uncaging of cyclic GMP induced [Ca.sup.2+] entry via at least two distinct pathways, and we identified a nimodipine-sensitive pathway, compartmentalized in the flagella, as a key regulator of flagellar bending and directed motility changes. We found that, contrary to current models, the degree of flagellar bending does not vary in proportion to the overall [[Ca.sup.2+]]i. Instead we propose a new model whereby flagella bending is increased by [Ca.sup.2+] flux through the nimodipine-sensitive pathway, and is unaffected by [[Ca.sup.2+]]i increases through alternative pathways.

Speract, a sperm-activating peptide (SAP) from sea urchin eggs, increases the intracellular concentration of [Ca.sup.2+] ([[Ca.sup.2+]]i) and modulates sperm motility. We measured the initial sperm response to speract using its caged analog and observed, for the first time, a small but significant decrease in sperm [[Ca.sup.2+]]i before the increase. Both directions of the [[Ca.sup.2+]]i change were co completely blocked in high [K.sup.+] seawater. Using membrane-permeant caged cyclic nucleotides (cNMP), only cGMP induced the decrease in [[Ca.sup.2+]]i although both cGMP and cAMP increased the [[Ca.sup.2+]]i. The decrease in the [[Ca.sup.2+]]i induced by cGMP was more notable following a second photolytic event, once [[Ca.sup.2+]]i had been elevated by an initial flash. This pattern of [[Ca.sup.2+]]i change was confirmed in individual sperm. These results together with pharmacological evidence suggest that the initial [[Ca.sup.2+]]i decrease is due to a [Na.sup.+]/[Ca.sup.2+] exchanger activity, stimulated by hyperpolarization mediated by [K.sup.+] efflux through cGMP-regulated [K.sup.+] channels. Keywords: Speract; Sperm activation; Caged peptide Caged cyclic nucleotides, Time-resolved [Ca.sup.2+] measurement, [Ca.sup.2+] imaging, Sperm chemotaxis; [Na.sup.+]/[Ca.sup.2+] exchanger; Membrane potential; Ion channel