The Hidden World of Rickettsiales Symbionts: “Candidatus Spectririckettsia obscura,” a Novel Bacterium Found in Brazilian and Indian Paramecium caudatum

Abstract

Symbioses between bacteria and eukaryotes are widespread and may have significant impact on the evolutionary history of symbiotic partners. The order Rickettsiales is a lineage of intracellular Alphaproteobacteria characterized by an obligate association with a wide range of eukaryotic hosts, including several unicellular organisms, such as ciliates and amoebas. In this work, we characterized the Rickettsiales symbionts associated with two different genotypes of the freshwater ciliate Paramecium caudatum originated from freshwater environments in distant geographical areas. Phylogenetic analyses based on 16S rRNA gene showed that the two symbionts are closely related to each other (99.4% identity), belong to the family Rickettsiaceae, but are far-related with respect to previously characterized Rickettsiales. Consequently, they were assigned to a new species of a novel genus, namely "Candidatus Spectririckettsia obscura." Screening on a database of short reads from 16S rRNA gene amplicon-based profiling studies confirmed that bacterial sequences related to the new symbiont are preferentially retrieved from freshwater environments, apparently with extremely scarce occurrence (< 0.1% positive samples). The present work provides new information on the still under-explored biodiversity of Rickettsiales, in particular those associated to ciliate host cells.

Deianiraea, an extracellular bacterium associated with the ciliate Paramecium, suggests an alternative scenario for the evolution of Rickettsiales

“…In recent years, an increasing amount of research was produced on neglected members of the order Rickettsiales (e.g. [24,38,41,74,75,101,102]), which, contrarily to those thoroughly studied in the past, are not of direct medical or veterinary concern. Such investigations represented a major contribution to understand the phylogenetic and genomic diversity of Rickettsiales and are gradually reshaping our interpretation of their evolution.…”

Detection of a new bacterium of the family Holosporaceae (Alphaproteobacteria: Holosporales) associated with the oribatid mite Achipteria coleoptrata

“…Furthermore, we included the 16S rDNA sequences of endosymbiont of X. muta and bacteria noted in other marine hosts: Porites compressa Dana, 1846 and Hippocampus guttulatus Cuvier, 1829. In order to make the data comprehensive, we applied in the analysis also the sequences of other bacteria published in similar works (Hess et al 2016;Lanzoni et al 2016;Szokoli et al 2016a, b;Castelli et al 2018b;Potekhin et al 2018;Schrallhammer et al 2018;Tashyreva et al 2018;Fokin et al 2019) using Rickettsiales sequences as an outgroup. The bacterial sequences used in the analysis were presented in the Fig.…”

“…We conducted phylogenetic analysis based on 16S rDNA sequences of the bacterium from A. coleoptrata and endosymbionts of other host that showed sequence identity by BLASTn. Additional sequence data were included in the analysis according to literature data (Boscaro et al 2013;Schulz et al 2014;Hess et al 2016;Lanzoni et al 2016;Szokoli et al 2016a, b;Castelli et al 2018b;Potekhin et al 2018;Schrallhammer et al 2018;Tashyreva et al 2018).…”

The extracellular association of the bacterium “CandidatusDeianiraea vastatrix” with the ciliateParameciumsuggests an alternative scenario for the evolution ofRickettsiales

“…In recent years, an increasing amount of research was produced on neglected members of the order Rickettsiales (e.g. Wang & Wu 2014;Martijn et al 2015;Schulz et al 2016;Szokoli et al 2016b;Castelli et al 2018;Floriano et al 2018;Yurchenko et al 2018), which, contrarily to those thoroughly studied in the past, are not of direct medical or veterinary concern. Such investigations represented a major contribution to understand the phylogenetic and genomic diversity of Rickettsiales, and are gradually reshaping our interpretation of their evolution.…”