Epigenetic mechanisms mediate the experimental evolution of resistance against parasitic fungi in the greater wax moth Galleria mellonella

Krishnendu Mukherjee,Ekaterina Grizanova,Ivan Dubovskiy,Rüdiger Lehmann,Andreas Vilcinskas

doi:10.1038/s41598-018-36829-8

Abstract

Recent concepts in evolutionary biology suggest that epigenetic mechanisms can translate environmental selection pressures into heritable changes in phenotype. To determine whether experimental selection for a complex trait in insects involves epigenetic modifications, we carried out a generation-spanning experiment using larvae of the greater wax moth Galleria mellonella as a model host to investigate the role of epigenetics in the heritability of resistance against the parasitic fungus Metarhizium robertsii. We investigated differences in DNA methylation, histone acetylation and microRNA (miRNA) expression between an experimentally resistant population and an unselected, susceptible line, revealing that the survival of G. mellonella larvae infected with M. robertsii correlates with tissue-specific changes in DNA methylation and histone modification and the modulation of genes encoding the corresponding enzymes. We also identified miRNAs differentially expressed between resistant and susceptible larvae and showed that these regulatory molecules target genes encoding proteinases and proteinase inhibitors, as well as genes related to cuticle composition, innate immunity and metabolism. These results support our hypothesis that epigenetic mechanisms facilitate, at least in part, the heritable manifestation of parasite resistance in insects. The reciprocal adaptations underlying host–parasite coevolution therefore extend beyond the genetic level to encompass epigenetic modifications.

Highlights

Classical natural selection operates by favoring the survival and reproduction of the fittest phenotypes, which pass their adaptations to subsequent generations via genetic changes, i.e. mutations
To determine whether the evolution of resistance against the parasitic fungus M. robertsii in the insect host G. mellonella is associated with changes in DNA methylation, we experimentally selected larvae for resistance over multiple generations and compared the resistant line with an unselected susceptible line
We infected G. mellonella larvae by inoculation with M. robertsii conidia, and survivors were allowed to breed over six generations, under the same selection pressure

Summary

Introduction

Classical natural selection operates by favoring the survival and reproduction of the fittest phenotypes, which pass their adaptations to subsequent generations via genetic changes, i.e. mutations. The greater wax moth Galleria mellonella and its fungal parasites have recently been established as a model system for such generation-spanning experimental studies[6,7,8] Parasitic fungi such as Metarhizium robertsii (formerly M. anisopliae) differ from other entomopathogens by infecting insect hosts directly through the integument, a typically impenetrable physical barrier against microbial invaders[8,9]. As a further counter-adaptation, G. mellonella has evolved the capacity to sense the virulence-associated metalloproteinases and synthesize a specific inhibitor[19,20] These reciprocal responses and counter-responses reveal a co-evolved communication network between the fungal parasite and its insect host as a means to decide the outcome of infection[18]. Our results highlighted the involvement of all three mechanisms in the establishment of heritable resistance

Methods

Results

Discussion

Conclusion