Distel Daniel L, Beaudoin David J, Morrill Wendy
Department of Biochemistry, Microbiology, and Molecular Biology, University of Maine, Orono 04469, USA.
Appl Environ Microbiol. 2002 Dec;68(12):6292-9. doi: 10.1128/AEM.68.12.6292-6299.2002.
Wood-boring bivalves of the family Teredinidae (commonly called shipworms) are known to harbor dense populations of gram-negative bacteria within specialized cells (bacteriocytes) in their gills. These symbionts are thought to provide enzymes, e.g., cellulase and dinitrogenase, which assist the host in utilizing wood as a primary food source. A cellulolytic, dinitrogen-fixing bacterium, Teredinibacter turnerae, has been isolated from the gill tissues of numerous teredinid bivalves and has been proposed to constitute the sole or predominant symbiont of this bivalve family. Here we demonstrate that one teredinid species, Lyrodus pedicellatus, contains at least four distinct bacterial 16S rRNA types within its gill bacteriocytes, one of which is identical to that of T. turnerae. Phylogenetic analyses indicate that the three newly detected ribotypes are derived from gamma proteobacteria that are related to but distinct (>6.5% sequence divergence) from T. turnerae. In situ hybridizations with 16S rRNA-directed probes demonstrated that the pattern of occurrence of symbiont ribotypes within bacteriocytes was predictable and specific, with some bacteriocytes containing two symbiont ribotypes. However, only two of the six possible pairwise combinations of the four ribotypes were observed to cooccur within the same host cells. The results presented here are consistent with the existence of a complex multiple symbiosis in this shipworm species.
船蛆科的钻木双壳贝类(通常称为船蛆)已知在其鳃中特化细胞(含菌细胞)内含有密集的革兰氏阴性细菌群体。这些共生体被认为能提供酶,例如纤维素酶和固氮酶,帮助宿主将木材作为主要食物来源加以利用。一种能分解纤维素、固定氮的细菌——特纳氏船蛆杆菌,已从多种船蛆科双壳贝类的鳃组织中分离出来,并被认为是该双壳贝类家族唯一的或主要的共生体。在此我们证明,一种船蛆物种——柄海笋,在其鳃含菌细胞内至少含有四种不同的细菌16S rRNA类型,其中一种与特纳氏船蛆杆菌的相同。系统发育分析表明,新检测到的三种核糖体类型源自γ-变形菌,它们与特纳氏船蛆杆菌相关,但又不同(序列差异>6.5%)。用16S rRNA导向探针进行的原位杂交表明,含菌细胞内共生体核糖体类型的出现模式是可预测且特定的,有些含菌细胞含有两种共生体核糖体类型。然而,在同一宿主细胞内仅观察到四种核糖体类型六种可能的两两组合中的两种同时出现。此处给出的结果与这种船蛆物种中存在复杂的多重共生关系相符。