engleski

Algal endosymbionts in european Hydra strains reflect multiple origins of the zoochlorella symbiosis

Symbiotic associations are of broad significance in evolution and biodiversity. Green Hydra is a classic example of endosymbiosis. In its gastrodermal myoepithelial cells it harbours endosymbiotic unicellular green alga, most commonly from the genus Chlorella. Hydra is a single freshwater polyp that inhabits shallow lakes and calm, slow-moving waters. It belongs to the hydrozoan clade Aplanulata within the deep- branching eumetazoan phylum Cnidaria. It provides useful model system for comparative research in development and evolution, both for investigations of early branching metazoans and for the study of plant-animal symbioses. Recent phylogenetic analyses that included most globally identified Hydra species demonstrated they can be divided into four groups. Here we investigate the phylogeny of algal endosymbionts from green Hydra strains, collected from six different geographical sites. All strains were endosymbiotic algae isolated from green Hydra ; four from different localities in Croatia, one from Israel and one from Germany. We used nuclear (18S rDNA, the ITS region) and chloroplast markers (16S, rbcL) for maximum likelihood phylogenetic analyses of 547 different sequences spanning chlorophyte diversity. We focussed on the question of whether native symbionts of Croatian H. viridissima strains descend from two or more symbiotic events or whether symbiosis with Chlorella occurred only once in the distant past followed by subsequent cospeciation and the secondary origin of free- living algal strains from escaped endosymbionts. Resulting phylogenetic trees based on ITS region, rbcL gene, 16S rRNA gene and 18S rDNA gene do not support a monophyletic origin of endosymbiotic algal strains isolated from Croatian green Hydra hosts. It thus appears that Hydra – algal endosymbioses have been established multiple times during the evolution of these strains

endosymbiotic algae; green Hydra; molecular phylogeny; symbiosis

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