Evolution of insect innate immunity through domestication of bacterial toxins

Abstract

Toxin cargo genes are often horizontally transferred by phages between bacterial species and are known to play an important role in the evolution of bacterial pathogenesis. Here, we show how these same genes have been horizontally transferred from phage or bacteria to animals and have resulted in novel adaptations. We discovered that two widespread bacterial genes encoding toxins of animal cells, <i>cytolethal distending toxin subunit B</i> (<i>cdtB</i>) and <i>apoptosis-inducing protein of 56 kDa</i> (<i>aip56)</i>, were captured by insect genomes through horizontal gene transfer from bacteria or phages. To study the function of these genes in insects, we focused on <i>Drosophila ananassae</i> as a model. In the <i>D. ananassae</i> subgroup species, <i>cdtB</i> and <i>aip56</i> are present as singular (<i>cdtB</i>) or fused copies (<i>cdtB::aip56</i>) on the second chromosome. We found that <i>cdtB</i> and <i>aip56</i> genes and encoded proteins were expressed by immune cells, some proteins were localized to the wasp embryo's serosa, and their expression increased following parasitoid wasp infection. Species of the <i>ananassae</i> subgroup are highly resistant to parasitoid wasps, and we observed that <i>D. ananassae</i> lines carrying null mutations in <i>cdtB</i> and <i>aip56</i> toxin genes were more susceptible to parasitoids than the wild type. We conclude that toxin cargo genes were captured by these insects millions of years ago and integrated as novel modules into their innate immune system. These modules now represent components of a heretofore undescribed defense response and are important for resistance to parasitoid wasps. Phage or bacterially derived eukaryotic toxin genes serve as macromutations that can spur the instantaneous evolution of novelty in animals.