Lineage-specific plasmid acquisition and the evolution of specialized pathogens in Bacillus thuringiensis and the Bacillus cereus group
© 2018 The Authors. Molecular Ecology Published by John Wiley & Sons Ltd. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited; https://creativecommons.org/licenses/by/4.0/
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
This work was supported by Medical Research Council (MRC) grants MR/M501608/1 and MR/L015080/1 awarded to SKS, and a NERC fellowship NE/E012671/1 and BBSRC BB/L00819X/1 grant to BR. GM was supported by a NISCHR Health Research Fellowship (HF-14-13). EM is supported by a University of Bath PhD studentship. Computational calculations were performed with HPC Wales (UK) and MRC CLIMB cloud-based computing servers.
This is the author accepted manuscript. The final version is available from Wiley via the DOI in this record
Published online 6 March 2018