Host plant-dependent effects of microbes and phytochemistry on the insect immune response
Herbivorous insects can defend themselves against pathogens via an immune response, which is influenced by the nutritional quality and phytochemistry of the host plant. However, it is unclear how these aspects of diet interact to influence the insect immune response and what role is played by ingested foliar microbes. We examined dietary protein, phytochemistry, and the caterpillar microbiome to understand variation in immune response of the Melissa blue butterfly, Lycaeides melissa. We also asked if these factors have host plant-specific effects by measuring L. melissa immune response when reared on a recently colonized exotic host plant (Medicago sativa) as compared to the immune response on an ancestral, native host (Astragalus canadensis). L. melissa did not experience immunological benefits directly related to consumption of the novel plant M. sativa. However, we did find negative, direct effects of phytochemical diversity and negative, direct effects of diet-derived microbial diversity on constitutive immune response for caterpillars fed M. sativa, as measured by phenoloxidase activity. Foliar protein did not directly influence the immune response, but did do so indirectly by increasing weight gain. Our results highlight the important effects of host diet on caterpillar physiology and raise the possibility that foliar microbiota, despite being rapidly passed through the gut, can affect the caterpillar immune response.
KeywordsLepidoptera Microbiome Novel host Phenoloxidase Secondary metabolites
We would like to thank The Center for Chemical Ecology at the University of Nevada Reno. This research was funded by the National Science Foundation Graduate Research Fellowship Program awarded to SY (DGE-1447692), NSF (IOS-1456354) and NSF DEB grant awarded to MF (DEB-1638793).
Author contribution statement
SY, MF, and AS conceived the ideas and experimental design; SY performed the experiment and field work and was in charge of immune assays and statistical analyses. JH processed and analyzed sequencing data; IW and DJ processed protein samples; CP and CD processed and analyzed phytochemical data. SY and JH prepared the manuscript; all authors reviewed the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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