Evolution of the ability to form a siliceous test of protists based on the molecular phylogeny

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Living radiolarians generally possess algal symbionts. They are generally recognized as yellow-green or yellow-brown, minute spheres, and variable in size, about 3 to 10 micrometers. The majority of modern symbionts-bearing radiolarians appear to depend on their symbionts to provide photosynthetically fixed carbon and to maintain the radiolarians in low nutrient environments (e.g., Anderson, 1978). In this study, Dinoflagellates, chlorophytes, and haptophytes were identified as endosymbionts of radiolarians of Euchitonia elegans (Ehrenberg), Dictyocoryne truncatum (Ehrenberg), and Spongaster tetras Ehrenberg by molecular and ultrastructural analyses. Moreover, I made clear that Dictyocoryne profunda Ehrenberg, which have been considered to not have algal symbionts, harbor many cyanobacteria symbionts on the surface.The present results show that the different species of radiolarians can have different symbionts and they can be strong specificity. If the same radiolarian species does associate with different algae, this might be the flexibility in choice of symbionts ; however, the present results possibly reject this hypothesis. The non-flexibility of these interactions probably suggests Radiolaria have a common mechanism for the recognition and selection of symbionts.