About the Cover
Cover
The larger image on the cover of this issue shows three specimens of Anthopleura elegantissima, a sea anemone (disk diameter ~2 cm); the animals were photographed on the rocky shore of San Juan Island, off the coast of Washington. The different colors of the anemones are due to their algal symbionts: the single brown anemone primarily contains the familiar dinoflagellate symbionts from the genus Symbiodinium (commonly called "zooxanthellae"), whereas the two green anemones contain algae from the phylum Chlorophyta. The proportion of algal partners in each host is influenced by light and temperature; in particular, the number of anemones hosting chlorophytes increases in the shade and at higher latitudes. But the anemones on the cover are obviously living together and, indeed, could be clone mates; so the basis for the distribution of symbiont types is still uncertain.
Also obscure is the taxonomy of the green algal symbionts in Anthopleura species. Previous investigations of these symbionts led to the presumption that they are marine species of the genus Chlorella, and that they are therefore related to similar algae associated with freshwater animals such as hydras and sponges, whence the designation "zoochlorellae." But Chorella is a paraphyletic taxon; i.e., the species within it do not have a common origin. Thus, the identity of the green symbionts within Anthopleura and their relationship with freshwater algae are in question.
In this issue of The Biological Bulletin (p. 87), Louise A. Lewis and Gisele Muller-Parker report on a phylogenetic analysis of 18S rDNA and the rbcL gene, which are encoded, respectively, by the nucleus and plastid of green symbionts isolated from Anthopleura elegantissima. The results suggest that these symbionts are members of a well-supported but highly divergent clade comprising species of small, spherical green algae-both symbiotic and free-living-which are distinct from species of Chorella in freshwater. The clade includes a lichen symbiont (Coccomyxa glaronensis), a free-living alga (Paradoxia), and the small green endophytes in the gametophytic tissues of Ginkgo biloba, a gymnosperm "living fossil." (Ginkgo leaves yellowing against a fall sky appear on the cover.)
Because this newly identified clade of green algae participates in a very wide range of symbiotic interactions, Lewis and Muller-Parker propose that members of the lineage share a genetic predisposition to form symbiotic relationships and are, therefore, potential models for investigating how such associations are established and maintained. Studies of the growth rates, nutrition, and physiological requirements of these algae, both in the host and in culture, should provide insight into the features that promote their symbioses.
The photograph of Anthopleura elegantissima was taken in the field by Christopher Wojcik, and the image of the ginkgo leaves was provided by Louise A. Lewis (University of Connecticut). Beth Liles (Marine Biological Laboratory) designed the cover.
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Copyright © 2005 by the Marine Biological Laboratory.
