Host-pathogen infections and possible effects on co-evolutionary patterns depend on the genotypes of both host and pathogen. Obligate fungal pathogens of plants are often characterised by host-pathogen genotype-by-genotype (GxG) interactions, but whether these patterns exist in obligate-insect fungal pathogens are unclear. We take advantage of the obligate insect pathogenic fungus Entomophthora muscae, where individual isolates are specific to different dipteran host species in nature but can cross-infect multiple fly species in the laboratory. We collected three new isolates of E. muscae from Drosophila species. Phylogenetic analysis showed that Drosophila-isolated E. muscae represent a distinct geographically widespread Drosophila-lineage compared to house fly (Musca domestica) or Delia species-isolated E. muscae. We used the three new E. muscae isolates from Drosophila spp. together with a genetically distinct E. muscae isolate from house flies and assessed their virulence in a cross-infection experiment using one house fly, three D. suzukii and two D. melanogaster genotypes as hosts. All fungal isolates successfully infected hosts, induced behavioural manipulation, sporulated in all fly hosts, and differed in virulence between host genotypes, revealing GxG interactions. While house flies were most susceptible to fungal infection with 99% mortality, we found a lower virulence of 49% and 25% mortality in D. melanogaster and D. suzukii genotypes, respectively. Furthermore, all isolates harboured a specific mycovirus (family Iflaviridae), but co-phylogenetic branching patterns did not support fungus-virus co-speciation. We show that the genetic makeup of both fungal pathogen and fly host influence E. muscae infectivity, confirming GxG interactions in obligate fly fungal pathogens.

Funding provided by: European CommissionROR ID: https://ror.org/00k4n6c32Award Number: 859850 Funding provided by: Danmarks Frie ForskningsfondROR ID: https://ror.org/05svhj534Award Number: 8049-00086A Funding provided by: Carlsberg FoundationROR ID: https://ror.org/01kpjmx04Award Number: CF20-0609 Funding provided by: Danmarks Frie ForskningsfondROR ID: https://ror.org/05svhj534Award Number: DFF - 7014-00188 Funding provided by: Villum FondenROR ID: https://ror.org/05nqkay65Award Number: 10122