Candidate pathogenicity islands in the genome of 'Candidatus Rickettsiella isopodorum', an intracellular bacterium infecting terrestrial isopod crustaceans.

Yadong Wang,Christopher Chandler

doi:10.7717/peerj.2806

Abstract

The bacterial genus Rickettsiellabelongs to the order Legionellales in the Gammaproteobacteria, and consists of several described species and pathotypes, most of which are considered to be intracellular pathogens infecting arthropods. Two members of this genus, R. grylliand R. isopodorum, are known to infect terrestrial isopod crustaceans. In this study, we assembled a draft genomic sequence for R. isopodorum, and performed a comparative genomic analysis with R. grylli. We found evidence for several candidate genomic island regions in R. isopodorum, none of which appear in the previously available R. grylli genome sequence.Furthermore, one of these genomic island candidates in R. isopodorum contained a gene that encodes a cytotoxin partially homologous to those found in Photorhabdus luminescensand Xenorhabdus nematophilus (Enterobacteriaceae), suggesting that horizontal gene transfer may have played a role in the evolution of pathogenicity in Rickettsiella. These results lay the groundwork for future studies on the mechanisms underlying pathogenesis in R. isopodorum, and this system may provide a good model for studying the evolution of host-microbe interactions in nature.

Highlights

IntroductionRickettsiella is a genus of bacteria that infects a range of arthropod hosts, including insects, crustaceans, and arachnids (Dutky & Gooden, 1952; Hall & Badgley, 1957; Vago & Martoja, 1963; Vago et al, 1970; Leclerque & Kleespies, 2008a; Leclerque & Kleespies, 2012; Tsuchida et al, 2010; Kleespies et al, 2011; Leclerque et al, 2011a)
We focused our subsequent analyses on the strongest candidate genomic islands, identified as those regions in R. isopodorum that had no apparent homology to any portion of either the R. grylli or D. massiliensis genomes
For the Rickettsiella strain found at high depth in the female individual, the results were especially impressive: the N50 was greater than 384 kb, and the longest contig was over 550 kb (Table 1)

Summary

Introduction

Rickettsiella is a genus of bacteria that infects a range of arthropod hosts, including insects, crustaceans, and arachnids (Dutky & Gooden, 1952; Hall & Badgley, 1957; Vago & Martoja, 1963; Vago et al, 1970; Leclerque & Kleespies, 2008a; Leclerque & Kleespies, 2012; Tsuchida et al, 2010; Kleespies et al, 2011; Leclerque et al, 2011a). There are several additional pathotypes thought to be synonyms of the previously recognized species, including R. tipulae (Leclerque & Kleespies, 2008a), R. agriotidis (Leclerque et al, 2011a), R. pyronotae (Kleespies et al, 2011), and R. costelytrae (Leclerque et al, 2012) Another bacterium designated as Diplorickettsia, found in ticks, is closely related and may belong in this genus (Mediannikov et al, 2010; Iasur-Kruh et al, 2013). There is probably much greater diversity in this genus than currently recognized; for instance, one genetic study identified multiple distinct lineages of Rickettsiella in just one species of tick, Ixodes uriae (Duron, Cremaschi & McCoy, 2015)

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