Rearing zombie flies: Laboratory culturing of the behaviourally manipulating fungal pathogen Entomophthora muscae

Insect pathogenic fungi (IPF) and insects have ubiquitous interactions in nature. The extent of these interkingdom host-pathogen interactions are both complex and diverse. Some IPF, notably of the order Entomophthorales, manipulate their species-specific host before death. The fungus-induced altered insect behaviours are sequential and can accurately be repeatedly characterised temporally, making them a valuable model for understanding the molecular and chemical underpinnings of behaviour and host-pathogen co-evolutionary biology. Here, we present methods for the isolation and laboratory culturing of the emerging behaviourally manipulating model IPF Entomophthora muscae for experimentation.•E. muscae isolation and culturing in vitro.•Establishing and maintaining an E. muscae culture in vivo in houseflies (Musca domestica).•Controlled E. muscae infections for virulence experiments and quantification of conidia discharge per cadaver.

Robust manipulation of the behavior of Drosophila melanogaster by a fungal pathogen in the laboratory

Many microbes induce striking behavioral changes in their animal hosts, but how they achieve this is poorly understood, especially at the molecular level. Mechanistic understanding has been largely constrained by the lack of an experimental system amenable to molecular manipulation. We recently discovered a strain of the behavior-manipulating fungal pathogen Entomophthora muscae infecting wild Drosophila, and established methods to infect D. melanogaster in the lab. Lab-infected flies manifest the moribund behaviors characteristic of E. muscae infection: hours before death, they climb upward, extend their proboscides, affixing in place, then raise their wings, clearing a path for infectious spores to launch from their abdomens. We found that E. muscae invades the nervous system, suggesting a direct means by which the fungus could induce behavioral changes. Given the vast molecular toolkit available for D. melanogaster, we believe this new system will enable rapid progress in understanding how E. muscae manipulates host behavior.

Comparative transcriptomics reveal host-specific nucleotide variation in entomophthoralean fungi

Obligate parasites are under strong selection to increase exploitation of their host to survive while evading detection by host immune defences. This has often led to elaborate pathogen adaptations and extreme host specificity. Specialization on one host, however, often incurs a trade-off influencing the capacity to infect alternate hosts. Here, we investigate host adaptation in two morphologically indistinguishable and closely related obligate specialist insect-pathogenic fungi from the phylum Entomophthoromycota, Entomophthora muscae sensu stricto and E. muscae sensu lato, pathogens of houseflies (Musca domestica) and cabbage flies (Delia radicum), respectively. We compared single nucleotide polymorphisms within and between these two E. muscae species using 12 RNA-seq transcriptomes from five biological samples. All five isolates contained intra-isolate polymorphisms that segregate in 50:50 ratios, indicative of genetic duplication events or functional diploidy. Comparative analysis of dN/dS ratios between the multinucleate E. muscae s.str. and E. muscae s.l. revealed molecular signatures of positive selection in transcripts related to utilization of host lipids and the potential secretion of toxins that interfere with the host immune response. Phylogenetic comparison with the nonobligate generalist insect-pathogenic fungus Conidiobolus coronatus revealed a gene-family expansion of trehalase enzymes in E. muscae. The main sugar in insect haemolymph is trehalose, and efficient sugar utilization was probably important for the evolutionary transition to obligate insect pathogenicity in E. muscae. These results support the hypothesis that genetically based host specialization in specialist pathogens evolves in response to the challenge of using resources and dealing with the immune system of different hosts.