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

生活放射虫通常拥有藻类共生体。它们通常被认为是黄绿色或黄褐色的微小球体，大小可变，约3至10微米。大多数携带共生体的放射虫似乎依赖于它们的共生体来提供光合作用固定的碳，并将放射虫维持在低营养环境中（例如，安德森，1978年）。本研究通过分子生物学和超微结构分析，鉴定了秀丽真壳（Euchitonia elegans）、截形网伞（Dictyocoryne truncatum）和四角海绵（Spongaster tetras）放射虫的内共生体为甲藻、绿藻和附着菌。此外，笔者还发现，在过去一直被认为不存在藻类共生体的深网珊瑚（Dictyocoryne profunda Escherberg）表面，存在着大量的蓝藻共生体，这表明不同种类的放射虫可以有不同的共生体，并且它们可以具有很强的特异性。如果同一种放射虫确实与不同的藻类共生，这可能是共生体选择的灵活性;然而，目前的结果可能拒绝这一假设。这些相互作用的非灵活性可能表明放射虫有一个共同的识别和选择共生体的机制。

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.