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Cover

The animals shown on the cover are Ciona intestinalis, an ascidian (here attached to a mussel), and Muggiaea kochi, a siphonophore. Fertilization in these marine organisms, as in many others, is external. But sperm survive only briefly in seawater, so they would be more effective if they could locate the eggs rapidly. More than 50 years ago, Jean Dan reported, in this journal, a specific attraction between the sperm and eggs of a hydromedusa (1). Since then, attractive substances that affect sperm motility and behavior have been widely detected in eggs and related structures and, in a few species, have been chemically identified.

The effect of a sperm-attracting factor, partially purified from the eggs of Ciona intestinalis (2), is illustrated in a video image on the cover (upper left). A pipette is filled with agar impregnated with attractant, and its tip (diameter, ~30 m) is immersed in a drop of Ciona sperm suspension. In response to the gradient of factor, sperm swim in spiral paths with a constant radius and with a straight trajectory aimed toward the pipette tip-the source of attractant. Note that this characteristic behavior (chemotaxis) is unchanged as the sperm approaches the attractant and, therefore, as the local concentration of attractant increases. The chemotaxis of siphonophore sperm differs from that of Ciona: both the radius of the spiral and the direction of the trajectory change markedly as the sperm approaches the source of attractant. Thus, in their evolution, these two species have acquired different strategies for sperm chemotaxis.

As an approach to this phenomenon and, more generally, to the mechanisms underlying sperm chemotaxis, Makiko Ishikawa, Hidekazu Tsutsui, and their colleagues propose, in this issue of The Biological Bulletin (p. 95), two models of sperm chemotaxis-one for ascidians (e.g., Ciona intestinalis), and one for siphonophores (e.g., Muggiaea kochi). Both models are based on prior experimental evidence, and they share a common assumption: that the radius of the spiral path is inversely dependent upon the intracellular calcium ion concentration ([Ca²⁺]_i). Modeling the ascidian chemotaxis required one additional assumption: that the [Ca⁺²]_i, in turn, depend on the change in attractant concentration with time. The siphonophore model-in addition to the common assumption-requires also that [Ca⁺²]_i depend on the local concentration of attractant, and that the enzymatic efflux of Ca⁺² be substantially slower than its influx, raising the [Ca⁺²]_i.

On the cover, the two sets of calculated data are plotted on graphs representing (in the z axis) the profile of an attractant soon after it begins diffusing from a point source centered in a small area (4 mm²). Note that this concentration gradient is small until very near the source of attractant, when it spikes. The ascidian model (top plot) is a spiral path that changes little until the sperm reaches the steep portion of the attraction gradient, very close to the source. In contrast, the siphonophore model is a wide spiral path that narrows markedly and curves tightly toward the source as the sperm approaches it (lower plot). Both models approximate experimental results.

This study suggests that only a small number of critical parameters are required to model sperm chemotaxis. Presumably these parameters reflect mechanisms underlying the behavior, and they may be amenable to experimentation, eventually explaining species differences and their adaptive significance.

The video image was provided by Makiko Ishikawa, and the plots of the ascidian and siphonophore model trajectories by Hidekazu Tsutsui (Misaki Marine Biological Station, Tokyo University). The drawing of Ciona intestinalis is from Millar, 1970 (3), and that of Muggiaea kochi is from Chun, 1882 (4). Stalwarts in the search for illustrations were George Mackie (University of Victoria, Canada), Claudia Mills (University of Washington), Phil Pugh (Southampton Oceanography Centre), Casey Dunn (Yale University and l'Observatoire Oceanologique de Villefranche-sur-Mer), Gretchen Lambert (Seattle), Patricia Mather (Queensland Museum, Australia), and Nancy Stafford and Eleanor Uhlinger (MBL/WHOI Library, Woods Hole). The cover was designed by Beth Liles (Marine Biological Laboratory, Woods Hole).

1. Dan, J. C. 1950. Biol. Bull. 99:412-415.

2. Yoshida et al., 1994. Dev. Growth Differ. 36:589-595.

3. Millar, R. H. 1970. British Ascidians. Academic Press, London.

4. Chun, C. 1882. Sitzungsber. Preuss. Akad. Wiss. 52:1155-1172.


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