Mechanisms of symbiont-conferred protection against natural enemies: an ecological and evolutionary framework
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An ecological framework for studying protection
Insects are infected by diverse microorganisms that vary both in their life-history strategies and in their effects on host fitness. Co-infections are common, and of particular interest are interactions between vertically-transmitted microbes (here termed ‘symbionts’) and natural enemies, including host-targeted pathogens and parasites and those vectored by insects to other hosts. Because the fitness of vertically-transmitted symbionts parallels the fitness of their hosts, these symbionts can
How evolution shapes symbiont-conferred protection
In most examples of symbiont-conferred protection, the functional mechanisms underlying protection still need to be identified, which will require a combination of experimental, immunological, genetic and genomic approaches (Box 1). Current examples, however, suggest that symbionts primarily protect insects against host-targeted pathogens either directly through toxin production (Figure 1a) or indirectly through exploitation of shared resources (Figure 1b), while symbionts confer resistance
Consideration of the gut microbiome
In addition to vertically-transmitted symbionts, many insects harbour complex gut microbiomes, and evidence suggests that these too can alter resistance. Mosquitoes, for example, harbour diverse gut flora that influence Plasmodium establishment [10, 11, 12, 51, 52]. A demonstrated relationship between mosquito gut bacterial species complexity and Plasmodium infection appears to be mediated by activation of an immune pathway (IMD) [13, 51, 52]. More recent work identified a gut-inhabiting
Conclusions
The mechanistic basis of symbiont-conferred protection against natural enemies can be complex, involving both direct interactions between microorganisms within hosts and more indirect interactions mediated through host-derived resources and host immune mechanisms (Figure 1). Symbiont-conferred protection can impact both host interactions with their natural enemies and insect vectoring capacity. The same mechanisms of protection can play a role in either case, though the evolutionary framework
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgments
N.M.G. is supported by National Science Foundation grant IOS-1149829. B.J.P. is supported by NSF fellowship DBI-1306387. Special thanks to the participants of the 2014 Keystone Symposium on Mechanisms and Consequences of Invertebrate-Microbe Interactions for stimulating this line of inquiry, and to Angela Douglas for helpful revisions.
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