Petersen Jillian M, Wentrup Cecilia, Verna Caroline, Knittel Katrin, Dubilier Nicole
Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, 28359 Bremen, Germany.
Biol Bull. 2012 Aug;223(1):123-37. doi: 10.1086/BBLv223n1p123.
Bathymodiolin mussels dominate hydrothermal vent and cold seep communities worldwide. Symbiotic associations with chemosynthetic sulfur- and methane-oxidizing bacteria that provide for their nutrition are the key to their ecological and evolutionary success. The current paradigm is that these symbioses evolved from two free-living ancestors, one methane-oxidizing and one sulfur-oxidizing bacterium. In contrast to previous studies, our phylogenetic analyses of the bathymodiolin symbionts show that both the sulfur and the methane oxidizers fall into multiple clades interspersed with free-living bacteria, many of which were discovered recently in metagenomes from marine oxygen minimum zones. We therefore hypothesize that symbioses between bathymodiolin mussels and free-living sulfur- and methane-oxidizing bacteria evolved multiple times in convergent evolution. Furthermore, by 16S rRNA sequencing and fluorescence in situ hybridization, we show that close relatives of the bathymodiolin symbionts occur on hosts belonging to different animal phyla: Raricirrus beryli, a terebellid polychaete from a whale-fall, and a poecilosclerid sponge from a cold seep. The host range within the bathymodiolin symbionts is therefore greater than previously recognized, confirming the remarkable flexibility of these symbiotic associations.
深海贻贝在全球的热液喷口和冷泉群落中占据主导地位。与为它们提供营养的化学合成硫氧化和甲烷氧化细菌的共生关系是它们生态和进化成功的关键。目前的范式认为这些共生关系是由两个自由生活的祖先进化而来的，一个是甲烷氧化细菌，另一个是硫氧化细菌。与之前的研究不同，我们对深海贻贝共生体的系统发育分析表明，硫氧化菌和甲烷氧化菌都属于多个进化枝，这些进化枝与自由生活的细菌穿插分布，其中许多是最近在海洋低氧区的宏基因组中发现的。因此，我们假设深海贻贝与自由生活的硫氧化和甲烷氧化细菌之间的共生关系在趋同进化中多次出现。此外，通过16S rRNA测序和荧光原位杂交，我们发现深海贻贝共生体的近亲存在于属于不同动物门的宿主上：来自鲸落的缨鳃虫多毛类动物Raricirrus beryli，以及来自冷泉的一种海绵动物。因此，深海贻贝共生体的宿主范围比以前认识的要广，这证实了这些共生关系具有显著的灵活性。
