Abstract Pathogens continue to emerge from increased contact with novel host species. Whilst these hosts can represent distinct environments for pathogens, the impacts of host genetic background on how a pathogen evolves post-emergence are unclear. In a novel interaction, we experimentally evolved a pathogen (Staphylococcus aureus) in populations of wild nematodes (Caenorhabditis elegans) to test whether host genotype and genetic diversity affect pathogen evolution. After ten rounds of selection, we found that pathogen virulence evolved to vary across host genotypes, with differences in host metal ion acquisition detected as a possible driver of increased host exploitation. Diverse host populations selected for the highest levels of pathogen virulence, but infectivity was constrained, unlike in host monocultures. We hypothesise that population heterogeneity might pool together individuals that contribute disproportionately to the spread of infection or to enhanced virulence. The genomes of evolved populations were sequenced, and it was revealed that pathogens selected in distantly-related host genotypes diverged more than those in closely-related host genotypes. S. aureus nevertheless maintained a broad host range. Our study provides unique empirical insight into the evolutionary dynamics that could occur in other novel infections of wildlife and humans.
Microbes are vital to the functioning of multicellular organisms.This realisation has fuelled great interest in the effects of microbes on the health of plant [1-3] and animal hosts [4-6] and has revealed that microbe-mediated protection against infectious disease is a widespread phenomenon (Table 1) [7][8][9][10][11].Defensive microbes can protect hosts from infection by parasites (including pathogens and parasitoids) by direct or host-mediated means (Box 1).Such protective traits have made these microbes attractive candidates for disease control.In fact, defensive microbes are already being applied in phage therapy and bacteriotherapy for humans, as well as to control vector-borne and agricultural diseases (Table 2).Despite the impact defensive microbes can have on host and parasite fitness, our current perspective of host-parasite evolution is largely based upon pairwise species interactions [12].By combining knowledge of defensive microbe-parasite interactions at the mechanistic level with evolutionary theory, we can predict how defensive microbes might alter the evolution of host and parasite traits, such as resistance and virulence.This will not only shape how we understand patterns of host-parasite coevolution in nature but will inform our decisisons in utilising defensive microbes as disease control agents.We propose three potential evolutionary implications of defensive microbes on host-parasite interactions.
Abstract One approach to control dengue virus transmission is the symbiont Wolbachia , which limits viral infection in mosquitoes. Despite plans for its widespread use in Aedes aegypti , Wolbachia 's mode of action remains poorly understood. Many studies suggest that the mechanism is likely multifaceted, involving aspects of immunity, cellular stress and nutritional competition. A previous study from our group used artificial selection to identify a new mosquito candidate gene related to viral blocking; alpha‐mannosidase‐2a ( alpha‐Mann‐2a ) with a predicted role in protein glycosylation. Protein glycosylation pathways tend to be involved in complex host–viral interactions; however, the function of alpha‐mannosidases has not been described in mosquito–virus interactions. We examined alpha‐Mann‐2a expression in response to virus and Wolbachia infections and whether reduced gene expression, caused by RNA interference, affected viral loads. We show that dengue virus (DENV) infection affects the expression of alpha‐Mann‐2a in a tissue‐ and time‐dependent manner, whereas Wolbachia infection had no effect. In the midgut, DENV prevalence increased following knockdown of alpha‐Mann‐2a expression in Wolbachia ‐free mosquitoes, suggesting that alpha‐Mann‐2a interferes with infection. Expression knockdown had the same effect on the togavirus chikungunya virus, indicating that alpha‐Mann‐2a may have broad antivirus effects in the midgut. Interestingly, we were unable to knockdown the expression in Wolbachia ‐infected mosquitoes. We also provide evidence that alpha‐Mann‐2a may affect the transcriptional level of another gene predicted to be involved in viral blocking and cell adhesion; cadherin87a . These data support the hypothesis that glycosylation and adhesion pathways may broadly be involved in viral infection in Ae. aegypti .
Abstract Microbiota can protect their hosts from infection. The short timescales in which microbes can evolve presents the possibility that “protective microbes” can take-over from the immune system of longer-lived hosts in the coevolutionary race against pathogens. Here, we found that coevolution between a protective bacterium (Enterococcus faecalis) and a virulent pathogen (Staphylococcus aureus) within an animal population (Caenorhabditis elegans) resulted in more disease suppression than when the protective bacterium adapted to uninfected hosts. At the same time, more protective E. faecalis populations became costlier to harbor and altered the expression of 134 host genes. Many of these genes appear to be related to the mechanism of protection, reactive oxygen species production. Crucially, more protective E. faecalis populations downregulated a key immune gene, , known to be effective against S. aureus infection. These results suggest that a microbial line of defense is favored by microbial coevolution and may cause hosts to plastically divest of their own immunity.
Abstract Microbes that protect their hosts from pathogenic infection are widespread components of the microbiota of both plants and animals. It has been found that interactions between ‘defensive’ microbes and pathogens can be genotype‐specific and even underlie the variation in host resistance to pathogenic infection. These observations suggest a dynamic co‐evolutionary association between pathogens and defensive microbes, but direct evidence of co‐evolution is lacking. We tested the hypothesis that defensive microbes and pathogens could co‐evolve within host populations by co‐passaging a microbe with host‐defensive properties ( Enterococcus faecalis ) and a pathogen ( Staphylococcus aureus) within Caenorhabditis elegans nematodes. Using both phenotypic and genomic analyses across evolutionary time, we found patterns of pathogen local adaptation and defensive microbe–pathogen co‐evolution via fluctuating selection dynamics. Moreover, co‐evolution with defensive microbes resulted in more rapid and divergent pathogen evolution compared to pathogens evolved independently in host populations. Taken together, our results indicate the potential for defensive microbes and pathogens to co‐evolve, driving interaction specificity and pathogen evolutionary divergence in the absence of host evolution.
Abstract Microbes that protect against infection inhabit hosts across the tree of life. It is unclear whether and how the host immune system may affect the formation of new protective symbioses. We investigated the transcriptomic response of Caenorhabditis elegans following novel interactions with a protective microbe ( Enterococcus faecalis ) able to defend against infection by pathogenic Staphylococcus aureus . We have previously shown that E. faecalis can directly limit pathogen growth within hosts. In this study, we show that colonisation by protective E. faecalis caused the differential expression of 1,557 genes in pathogen infected hosts, including the upregulation of immune genes such as lysozymes and C-type lectins. The most significantly upregulated host lysozyme gene, lys-7, impacted the competitive abilities of E. faecalis and S. aureus when knocked out. E. faecalis has an increased ability to resist lysozyme activity compared to S. aureus , suggesting that the protective microbe could gain a competitive advantage from this host response. Our finding that protective microbes can benefit from immune-mediated competition after introduction opens up new possibilities for biocontrol design and our understanding of symbiosis evolution. Crosstalk between the host immune response and microbe-mediated protection should favour the continued investment in host immunity and avoid the potentially risky evolution of host dependence.
Wolbachia is an intracellular bacterium that blocks virus replication in insects and has been introduced into the mosquito, Aedes aegypti for the biocontrol of arboviruses including dengue, Zika and chikungunya. Despite ongoing research, the mechanism of Wolbachia-mediated virus blocking remains unclear. We recently used experimental evolution to reveal that Wolbachia-mediated dengue blocking could be selected upon in the A. aegypti host and showed evidence that strong levels of blocking could be maintained by natural selection. In this study, we investigate the genetic variation associated with blocking and use these analyses to generate testable hypotheses surrounding the mechanism of Wolbachia-mediated dengue blocking. From our results, we hypothesise that Wolbachia may block virus replication by increasing the regeneration rate of mosquito cells via the Notch signalling pathway. We also propose that Wolbachia modulates the host’s transcriptional pausing pathway either to prime the host’s anti-viral response or to directly inhibit viral replication.
Microbes can defend their host against virulent infections, but direct evidence for the adaptive origin of microbe-mediated protection is lacking. Using experimental evolution of a novel, tripartite interaction, we demonstrate that mildly pathogenic bacteria (Enterococcus faecalis) living in worms (Caenorhabditis elegans) rapidly evolved to defend their animal hosts against infection by a more virulent pathogen (Staphylococcus aureus), crossing the parasitism-mutualism continuum. Host protection evolved in all six, independently selected populations in response to within-host bacterial interactions and without direct selection for host health. Microbe-mediated protection was also effective against a broad spectrum of pathogenic S. aureus isolates. Genomic analysis implied that the mechanistic basis for E. faecalis-mediated protection was through increased production of antimicrobial superoxide, which was confirmed by biochemical assays. Our results indicate that microbes living within a host may make the evolutionary transition to mutualism in response to pathogen attack, and that microbiome evolution warrants consideration as a driver of infection outcome.
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