Rediscovering the genus Lyticum, multiflagellated symbionts of the order Rickettsiales

Abstract

Among the bacterial symbionts harbored by the model organism Paramecium, many still lack a recent investigation that includes a molecular characterization. The genus Lyticum consists of two species of large-sized bacteria displaying numerous flagella, despite their inability to move inside their hosts' cytoplasm. We present a multidisciplinary redescription of both species, using the deposited type strains as well as newly collected material. On the basis of 16S rRNA gene sequences, we assigned Lyticum to the order Rickettsiales, that is intensely studied because of its pathogenic representatives and its position as the extant group most closely related to the mitochondrial ancestor. We provide conclusive proofs that at least some Rickettsiales possess actual flagella, a feature that has been recently predicted from genomic data but never confirmed. We give support to the hypothesis that the mitochondrial ancestor could have been flagellated, and provide the basis for further studies on these ciliate endosymbionts.

Disentangling the Taxonomy of Rickettsiales and Description of Two Novel Symbionts (“Candidatus Bealeia paramacronuclearis” and “Candidatus Fokinia cryptica”) Sharing the Cytoplasm of the Ciliate Protist Paramecium biaurelia

“…The families Rickettsiaceae, Anaplasmataceae, and “Candidatus Midichloriaceae” (RAM clade) represent highly supported monophyletic clades (12–19). The fourth family, Holosporaceae (49), formerly joined all remaining species of Rickettsiales, such as Holospora (50), Caedibacter caryophilus (42), “Candidatus Paracaedibacter symbiosus” (51), “Candidatus Odyssella thessalonicensis” (52), “Candidatus Captivus acidiprotistae” (53), “Candidatus Hepatincola porcellionum” (45), “Candidatus Paraholospora nucleivisitans” (54), “Candidatus Gortzia infectiva” (47), and “Candidatus Hepatobacter penaei” (55) and sequences of uncultured bacteria closely related to these species.…”

“…One follows the historical path, in which Rickettsiales includes the families Rickettsiaceae, Anaplasmataceae, the novel described “Candidatus Midichloriaceae,” and Holosporaceae, together with related sequences (“basal Rickettsiales”). This model is usually supported by phylogenetic analyses based on prokaryotic small-subunit (SSU) rRNA gene sequences (12–19). The opposing model does not recover the monophyly of all Rickettsiales and separates “basal Rickettsiales” to form a new order.…”

A Rickettsiales symbiont of amoebae with ancient features

“…Its 1.2 Mb genome encodes for features unusual for rickettsial genomes, such as flagellar genes and a cbb3 cytochrome oxidase, but overall shows a high similarity to the genomes of other Rickettsiales (Sassera et al ., ). Besides Midichloria , several other deeply branching rickettsiae have been identified in the past decades, mainly thriving in protist hosts (Horn et al ., ; Vannini et al ., 2005; 2010; Schmitz‐Esser et al ., ; Ferrantini et al ., ; Kawafune et al ., ; Boscaro et al ., 2013a,b; Schulz et al ., ). Several of these bacteria, including the amoeba symbiont Odyssella thessalonicensis (hereafter: Odyssella ), have been assigned to the family Holosporaceae , which was initially described as belonging to the Rickettsiales ; however, its correct phylogenetic placement is still a matter of debate (Ferla et al ., ; Santos and Massard, ).…”

“Candidatus Gortzia shahrazadis”, a Novel Endosymbiont of Paramecium multimicronucleatum and a Revision of the Biogeographical Distribution of Holospora-Like Bacteria

“…Molecular characterization shows a large biodiversity among these symbionts (Irbis and Ushida, 2004; Vannini et al, 2004, 2010, 2014; Rinke et al, 2006; Schrallhammer et al, 2006, 2011, 2013; Ferrantini et al, 2009; Boscaro et al, 2012, 2013a,b,c; Gong et al, 2014; Senra et al, 2015; Szokoli et al, 2016) and also the type of interaction with their hosts can range from mutualistic to parasitic (Kusch et al, 2002; Vannini et al, 2003, 2007; Fels and Kaltz, 2006; Fellous et al, 2011). In this work, we use De Bary's definition of “symbiosys” (de Bary, 1879), as “the living together of two differently named organisms”, independent of effects on the organisms involved, thereby including mutualism, parasitism, and commensalism.…”