Diversity and environmental distribution of the cosmopolitan endosymbiont “Candidatus Megaira”

Abstract

Members of the order Rickettsiales are often found in association with ciliated protists. An interesting case is the bacterial endosymbiont “Candidatus Megaira”, which is phylogenetically closely related to the pathogen Rickettsia. “Candidatus Megaira” was first described as an intracellular bacterium in several ciliate species. Since then it has been found in association with diverse evolutionary distantly-related hosts, among them other unicellular eukaryotes, and also algae, and metazoa, such as cnidarians. We provide the characterization of several new strains of the type species “Candidatus Megaira polyxenophila”, and the multidisciplinary description of a novel species, “Candidatus Megaira venefica”, presenting peculiar features, which highlight the diversity and variability of these widespread bacterial endosymbionts. Screening of the 16S rRNA gene short amplicon database and phylogenetic analysis of 16S rRNA gene hypervariable regions revealed the presence of further hidden lineages, and provided hints on the possibility that these bacteria may be horizontally transmitted among aquatic protists and metazoa. The phylogenetic reconstruction supports the existence of at least five different separate species-level clades of “Candidatus Megaira”, and we designed a set of specific probes allowing easy recognition of the four major clades of the genus.

Assigning the Origin of Microbial Natural Products by Chemical Space Map and Machine Learning

“…Discussion with natural product chemists informed us that assigning NPs to their origin only from its chemical structure is not trivial, and can be problematic when isolating a new NP due to the occurrence of endosymbiosis, i.e., the fact that bacteria often live as symbionts within larger organisms [ 51 , 55 ]. We therefore asked the question whether our MAP4 SVM classifier would correctly predict the origin of NPs newly reported in 2020 and which are not part of NPAtlas ( Table 4 ).…”

The Obligate Symbiont “Candidatus Megaira polyxenophila” Has Variable Effects on the Growth of Different Host Species

“…About 20 ciliate cells were collected, washed three times in water and fixed with 2% paraformaldehyde (PFA, final concentration) on slides as described by Szokoli et al (2016). Hybridization and washing were carried out as described by Lanzoni et al (2019). Images were obtained with a Leica DMR microscope, equipped with an HBO 50W/AC-L2 fluorescent lamp, a Leica DFC490 video camera, and Leica IM1000 Software (v.1.0).…”

“Candidatus Trichorickettsia mobilis”, a Rickettsiales bacterium, can be transiently transferred from the unicellular eukaryote Paramecium to the planarian Dugesia japonica

“…In the last two decades, many new genera and species of Rickettsiales were found as symbionts (with ''symbiosis'' simply meaning the even-temporary association of organisms from different species) in a variety of other non-vector eukaryotic hosts, both from terrestrial and aquatic environments (reviewed in Perlman, Hunter & Zchori-Fein, 2006;Weinert et al, 2009;Gillespie et al, 2012;Castelli, Sassera & Petroni, 2016). Numerous such novel bacterial species were retrieved in aquatic protists (Kawafune et al, 2015;Schulz et al, 2016;Yang, Narechania & Kim, 2016;Hess, 2017;Yurchenko et al, 2018), including, notably, parasitic (Sun et al, 2009;Zaila et al, 2017) and various free-living ciliates (Ferrantini et al, 2009;Boscaro et al, 2013;Schrallhammer et al, 2013;Vannini et al, 2014;Senra et al, 2016;Szokoli et al, 2016a;Szokoli et al, 2016b;Castelli et al, 2019b;Lanzoni et al, 2019).…”