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.
Highlights
Antagonisms are often at the heart of rapid evolutionary change
We found that almost all coevolved and many one-sided adapted populations showed a high prevalence of BT-679 (Fig 3C), which is known to have stronger nematocidal effects than other pathogenic B. thuringiensis strains, such as BT-246 and BT-247, which are both present in the ancestral population [21,35]
Our analysis confirmed that loss of virulence in the toxin-plasmid-lacking variant could be reconstituted to almost wildtype levels by reintroduction of a plasmid with either of the two toxin genes or by addition of a high concentration of a Cry21Aa2-expressing E. coli. These results strongly suggest that the two toxin genes, and possibly their copy number, account for the nematocidal effects in BT-679 and may have been under positive selection under one-sided adaptation, and especially under during coevolution, where high virulence is favoured by selection (Fig 1B, 1C, and 1F)
Summary
Evolution can be extremely fast and dramatic, especially when infectious disease agents such as bacterial pathogens engage in a continuous arms race with their host organism. Rounds of novel pathogen attack strategies and associated host counterdefenses conspire to drive host–pathogen coevolution and biological innovation. To better understand the underlying genetic mechanisms and the exact trait characteristics under selection, we conducted experimental evolution using a simple host–pathogen model system (nematode versus bacterium) under controlled laboratory conditions. We analysed the associated adaptive changes in real time using large-scale phenotyping, population whole genome sequencing, and genetic analysis of the identified candidate genes. We show that coevolution (rather than one-sided adaptation) favors and maintains pathogen virulence, and that two specific toxin genes significantly influence this virulence during coevolution
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