Host–Pathogen Coevolution: The Selective Advantage of Bacillus thuringiensis Virulence and Its Cry Toxin Genes

Abstract

Reciprocal coevolution between host and pathogen is widely seen as a major driver of evolution and biological innovation. Yet, to date, the underlying genetic mechanisms and associated trait functions that are unique to rapid coevolutionary change are generally unknown. We here combined experimental evolution of the bacterial biocontrol agent Bacillus thuringiensis and its nematode host Caenorhabditis elegans with large-scale phenotyping, whole genome analysis, and functional genetics to demonstrate the selective benefit of pathogen virulence and the underlying toxin genes during the adaptation process. We show that: (i) high virulence was specifically favoured during pathogen–host coevolution rather than pathogen one-sided adaptation to a nonchanging host or to an environment without host; (ii) the pathogen genotype BT-679 with known nematocidal toxin genes and high virulence specifically swept to fixation in all of the independent replicate populations under coevolution but only some under one-sided adaptation; (iii) high virulence in the BT-679-dominated populations correlated with elevated copy numbers of the plasmid containing the nematocidal toxin genes; (iv) loss of virulence in a toxin-plasmid lacking BT-679 isolate was reconstituted by genetic reintroduction or external addition of the toxins. We conclude that sustained coevolution is distinct from unidirectional selection in shaping the pathogen's genome and life history characteristics. To our knowledge, this study is the first to characterize the pathogen genes involved in coevolutionary adaptation in an animal host–pathogen interaction system.

Caenorhabditis elegans responses to bacteria from its natural habitats

“…The original selection of E. coli as a nutritional source for C. elegans was not based on knowledge of the natural microbes associated with C. elegans in its natural habitat, but on the availability of E. coli in research laboratories and in the history of Sydney Brenner, an E. coli bacteriophage geneticist, as he developed C. elegans as a model organism (12). Since the effect of diverse pathogenic bacteria has been studied in C. elegans (13)(14)(15)(16), however, little effort has been made to isolate ecologically relevant and not necessarily detrimental bacteria. A recent study has also made use of soil where C. elegans does not proliferate to isolate bacteria and study their community assembly (17).…”

Complete genome sequence of the nematicidal Bacillus thuringiensis MYBT18247

“…This toxin arsenal, especially the copy number of individual toxin genes, can be shaped by reciprocal co-adaptation with a nematode host, as previously demonstrated using controlled evolution experiments in the laboratory [41,42]. The B. thuringiensis strain MYBT18246 described herein and its host Caenorhabditis elegans have been selected as a model system for such co-evolution experiments [41]. One aim of this sequencing project was to provide a high-quality reference genome sequence for the original B. thuringiensis MYBT18246 in order to obtain a detailed phylogeny and shed light on the evolution of this microparasite, with a particular focus on the presence of virulence factors, elements of genome plasticity and host adaptation factors.…”

“…The worldwide distribution of B. thuringiensis and its capacity to adapt to a diverse spectrum of invertebrate hosts is explained by the formation of spores and a remarkable variability in crystal protein families [13]. This toxin arsenal, especially the copy number of individual toxin genes, can be shaped by reciprocal co-adaptation with a nematode host, as previously demonstrated using controlled evolution experiments in the laboratory [41,42]. The B. thuringiensis strain MYBT18246 described herein and its host Caenorhabditis elegans have been selected as a model system for such co-evolution experiments [41].…”

“…The genome sequence was analyzed to identify virulence factors and fitness factors contributing to the efficient infection of C. elegans. Additionally, the phylogenetic position of B. thuringiensis MYBT18246 in the Bcsl group was determined [41]. The complete genome sequence has been deposited in GenBank with the accession numbers (CP015350-CP015361) and in the integrated Microbial Genomes database with the Taxon ID 2671180122 [53].…”

Coevolution of parasitic fungi and insect hosts

“…This natural preservation is not possible in most systems, so researchers have turned to models that can be reared and tested in experimental settings. Experimental host-parasite coevolution is a powerful tool to investigate the evolution of host defense mechanisms and resistance as well as parasite virulence factors (e.g., Schulte et al, 2010;Masri et al, 2015;see Brockhurst and Koskella, 2013 for a review). Samples taken after different time points provide insight into evolutionary adaptations and counter-adaptations and such experiments have been carried out using model hosts such as Caenorhabditis elegans (Schulte et al, 2010;Masri et al, 2015), Daphnia spp.…”