Keywords: k. host/pathogen interactions; k. host/pathogen interactions
Advances in genomics have illuminated a potential role of horizontal gene transfer (HGT) as a source of evolutionary novelty in animals. We discovered the horizontal transfer of the cytolethal distending toxin B (cdtB) gene from bacteriophage (phage) donors into the nuclear genomes of several insects, including drosophilid flies. cdtB is a major virulence factor found in Proteobacteria and Actinobacteria that encodes a DNAse I-type nuclease. CdtB homologs encoded by bacterial pathogens of mammals typically induce cell cycle arrest and apoptosis of eukaryotic host cells. In secondary endosymbiotic bacteria of pea aphids (Ca. Hamiltonella defensa), phage-encoded cdtB may protect the insect host against parasitoid wasp attack. We hypothesize that HGT-derived cdtB is playing a similar protective role in the Drosophila lineages that harbor it. How insects are able to deploy HGT-derived cdtB to potentially kill parasitoids without harming their own cells remains unclear. Addressing this outstanding question is essential to understanding how a virulence gene that targets eukaryotic cells became functionalized within a eukaryotic genome. We hypothesize that drosophilid hosts may mitigate CdtB toxicity using a number of strategies, such as limiting its expression to particular tissues, or that the protein has evolved to not target the host genome (i.e., through modified substrate specificity or interactions with native host proteins). Here, we used the UAS-GAL4 system to test toxicity of cdtB expression in Drosophila melanogaster, which does not natively encode cdtB. We conducted in vitro nuclease assays to determine the specificity of drosophilid CdtB towards methylated vs. unmethylated DNA. We further investigated mechanisms by which drosophilid flies may evade CdtB toxicity by expressing the toxin in yeast. Overall, our work illuminates how prokaryotic genes whose products evolved to kill eukaryotic cells are integrated into animal gene networks.