Lineage-specific plasmid acquisition and the evolution of specialized pathogens in Bacillus thuringiensis and the Bacillus cereus group
Meric, G; Mageiros, L; Pascoe, B; et al.Woodcock, D; Mourkas, E; Lamble, S; Bowden, R; Jolley, K; Raymond, B; Sheppard, S
Date: 6 March 2018
Journal
Molecular Ecology
Publisher
Wiley
Publisher DOI
Abstract
Bacterial plasmids have roles that range from large secondary chromosomes to small selfish
genetic elements. Distinct, but not necessarily mutually exclusive theories have been
proposed to resolve plasmid bacteria relationships: plasmids may facilitate evolutionary
novelty and maintain beneficial genes via hitchhiking, while ...
Bacterial plasmids have roles that range from large secondary chromosomes to small selfish
genetic elements. Distinct, but not necessarily mutually exclusive theories have been
proposed to resolve plasmid bacteria relationships: plasmids may facilitate evolutionary
novelty and maintain beneficial genes via hitchhiking, while plasmid mobility may be
opposed by coevolutionary relationships with chromosomes or encouraged via the infectious
sharing of genes encoding public goods. Here, we sought to explore a range of these
hypotheses through a large-scale examination of the association between plasmids and
genomes in the phenotypically diverse Bacillus cereus group. This complex group is rich in
plasmids, many of which encode essential virulence factors (Cry toxins) that are known
public goods. We aimed to characterize population genomic structure, examine the dynamics
of plasmid distribution and gene content and the role of mobile elements in diversification..
We analysed coding sequence within the core and accessory genome of 190 B. cereus group
isolates, including 23 novel sequences, including plasmid genes from a reference collection
of 410 plasmid genomes. While cry genes were widely distributed, those with invertebrate
toxicity were predominantly associated with one sequence cluster (clade 2) and
phenotypically defined Bacillus thuringiensis. Cry toxin plasmids in clade 2 showed evidence
of recent horizontal transfer and dynamic gene content, a pattern of plasmid segregation
consistent with transfer during infectious cooperation. Nevertheless, comparison between
clades suggests that coevolutionary interactions may drive association of plasmids and
chromosomes and limit wider transfer of key virulence traits. Proliferation of successful
plasmid and chromosome combinations is a feature of specialized pathogens with
characteristic niches (Bacillus anthracis, B. thuringiensis) and has occurred multiple times in
the B. cereus group
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