Katz Sigrid, Klepal Waltraud, Bright Monika
Department of Marine Biology, Faculty of Life Sciences, University of Vienna, Austria.
Biol Bull. 2011 Apr;220(2):128-39. doi: 10.1086/BBLv220n2p128.
The polychaete family Siboglinidae, which is currently construed as comprising the Frenulata, Monilifera (composed of Sclerolinum), Vestimentifera, and Osedax, has become known for its specialized symbiont-housing organ called the trophosome. This organ replaced the digestive system of the worms and is located in the elongated trunk region in Frenulata, Sclerolinum, and Vestimentifera. Currently two types of trophosomes have been described: in the taxa Frenulata and Sclerolinum the bacteriocytes originate from endoderm, and in Vestimentifera they originate from mesoderm. In Osedax, a trophosome was described as lacking (Rouse et al., 2004), but bacteriocytes are located in Osedax's characteristic root tissue. Here, we argue for a consistent name for the symbiont-housing tissue, namely trophosome, as in other siboglinids. In this study we provide morphological evidence that in Osedax the bacteriocytes are derived from somatic mesoderm. We show that the trophosome in Osedax is an apolar tissue composed of bacteriocytes and nonsymbiotic cells. As in vestimentiferans, a specific cell cycle was identified; however, in this case it is directed from the posterior to the anterior end of the worms instead of from the center toward the periphery. Comparison of all siboglinid trophosomes and re-evaluation of their body regions allows us to discuss whether the trophosomes are homologous and to hypothesize about the organization of the last common ancestor of Siboglinidae.
多毛纲西伯格林虫科目前被认为包括缨鳃虫目、念珠虫目（由硬管虫属组成）、须腕虫目和食骨蠕虫目，该科以其特殊的共生体容纳器官——营养体而闻名。这个器官取代了蠕虫的消化系统，位于缨鳃虫目、硬管虫属和须腕虫目的细长躯干区域。目前已描述了两种类型的营养体：在缨鳃虫目和硬管虫属分类群中，细菌细胞起源于内胚层，而在须腕虫目中，它们起源于中胚层。在食骨蠕虫目中，曾有描述称其没有营养体（劳斯等人，2004年），但细菌细胞位于食骨蠕虫特有的根组织中。在此，我们主张为共生体容纳组织采用一个统一的名称，即营养体，如同其他西伯格林虫类。在本研究中，我们提供了形态学证据，表明在食骨蠕虫中细菌细胞源自体中胚层。我们发现食骨蠕虫的营养体是一种由细菌细胞和非共生细胞组成的无极性组织。与须腕虫类一样，我们识别出了一个特定的细胞周期；然而，在这种情况下，它是从蠕虫的后端向前端进行，而非从中心向周边进行。对所有西伯格林虫类营养体的比较以及对它们身体区域的重新评估，使我们能够讨论营养体是否同源，并对西伯格林虫科最后一个共同祖先的组织结构进行假设。
