The chonotrichs are sessile ciliated protozoa that are ectosymbiotic on the body parts of a variety of crustaceans. They have long been considered a separate group because their sessile habit has resulted in the evolution of a very divergent body form and reproductive strategy compared to free-living ciliates. In the mid-20th Century, the free-living dysteriid cyrtophorian ciliates were proposed as a potential sister clade because the chonotrich bud or daughter cell showed similarities during division morphogenesis (i.e. ontogeny) to these free-living dysteriids. A single small subunit (SSU) rRNA gene sequence is available for the chonotrich Isochona sp. However, its authenticity has recently been questioned, and the placement of this sequence within the dysteriid clade has added to this controversy. In this report, the SSUrRNA gene sequence of the chonotrich Chilodochona carcini, ectosymbiotic on the green crab Carcinus maenas, is provided. Topology testing of the SSUrRNA gene phylogeny, constructed by Bayesian Inference, robustly supports the sister-group relationship of Isochona sp. and Chilodochona carcini, the monophyly of these two chonotrichs, and the divergence of the chonotrich clade within the dysteriid clade.
The family Beijerinckiaceae was circumscribed in 2005 to accommodate four genera of phylogenetically related alphaproteobacteria: Beijerinckia , Chelatococcus , Methylocella and Methylocapsa . Later, four additional genera, i.e. Methylovirgula , Methyloferula , Methylorosula and Camelimonas , were described and assigned to this family, which now accommodates 21 species with validly published names. Members of this family possess strikingly different lifestyles, including chemoheterotrophy, facultative methylotrophy, obligate methanotrophy and facultative methanotrophy. Levels of 16S rRNA gene sequence similarity among most of these bacteria range from 96 to 98 %, suggesting a common evolutionary origin. The genera Chelatococcus and Camelimonas , however, are not monophyletic with the other described genera based on 16S rRNA gene sequence phylogeny, and instead form a distant cluster more closely related to the Methylobacteriaceae . Physiologically these two genera also lack several properties common to the other Beijerinckiaceae . On the other hand, the genus Rhodoblastus , presently considered a member of the Bradyrhizobiaceae , affiliates with high confidence to the Beijerinckiaceae . Here, we propose to transfer the genera Chelatococcus and Camelimonas to the family Chelatococcaceae fam. nov., and present an emended description of the family Beijerinckiaceae , including the genus Rhodoblastus .