Horizontal gene transfer of a bacterial insect toxin gene into the Epichloe fungal symbionts of grasses

Horizontal gene transfer is recognized as an important factor in genome evolution, particularly when the newly acquired gene confers a new capability to the recipient species. We identified a gene similar to the makes caterpillars floppy (mcf1 and mcf2) insect toxin genes in Photorhabdus, bacterial symbionts of nematodes, in the genomes of the Epichloe fungi, which are intercellular symbionts of grasses. Infection by Epichloe spp. often confers insect resistance to the grass hosts, largely due to the production of fungal alkaloids. A mcf-like gene is present in all of the Epichloe genome sequences currently available but in no other fungal genomes. This suggests the Epichloe genes were derived from a single lineage-specific HGT event. Molecular dating was used to estimate the time of theHGTevent at between 7.2 and 58.8 million years ago. The mcf-like coding sequence from Epichloe typhina subsp. poae was cloned and expressed in Escherichia coli. E. coli cells expressing the Mcf protein were toxic to black cutworms (Agrotis ipsilon), whereas E. coli cells containing the vector only were non-toxic. These results suggest that the Epichloe mcf-like genes may be a component, in addition to the fungal alkaloids, of the insect resistance observed in Epichloe-infected grasses.