Horizontal gene transfer rivals gene duplication as a source of anti-parasitoid immune innovation in the Drosophilidae
Macroparasites are among the most important agents of natural selection in their host populations, but anti-macroparasite immunity is poorly understood. Vinegar flies (Drosophilidae) and the parasitoid wasps that infect them are emerging models to study how animals defend against macroparasite attack. The canonical anti-parasitoid immune mechanism in insects is melanotic encapsulation, which…
In the world of insects, macroparasites play a significant role in shaping their populations, but the mechanisms that allow these insects to resist such parasitism remains largely unexplored. Among various insects, vinegar flies or Drosophilidae species and the parasites that infect them are becoming popular research subjects to delve into insect defense strategies against macroparasite attacks.
One well-known defense mechanism in insects is melanotic encapsulation, in which cells create a protective encapsulation around parasitoid embryos, coupled with the production of melanin, a pigment that helps in repelling parasites. Interestingly, researchers have now discovered that horizontal gene transfer (HGT) of a bacterially-derived humoral immune effector, Cytolethal distending toxin B (CdtB), has spread across various insects, including four different species of Drosophilidae.
The study involved analyzing 406 Drosophilidae species and four outgroup species to determine the prevalence of these two immune mechanisms. The results suggested that melanotic encapsulation was relatively rare among species with known immune responses. In contrast, numerous duplications of phenoloxidase (PPO) genes, the gene responsible for melanization, were detected in 88 different species.
However, it was found that the PPO3 gene, which is crucial for melanotic encapsulation, is only present in Drosophila melanogaster and its close relatives. Additionally, the PPO genes have lower copy numbers per genome in the Drosophilidae species compared to outgroup lineages, and these genes have evolved at a slower rate.
Furthermore, researchers discovered the presence of CdtB in the genomes of 93 Drosophilidae species, with at least 11 independent gains of this gene occurring throughout the evolutionary history of this group. The acquisition of CdtB was found to be linked to higher net diversification rates in some clades, suggesting that humoral immune effectors, like CdtB, may play a more significant role in anti-parasitoid immunity in insects than previously believed.
In conclusion, the findings of this study imply that humoral immune effectors may be more crucial in anti-parasitoid immunity in insects than previously thought. Furthermore, the repeated occurrence of HGT-mediated immune innovations could be linked to the evolutionary success of these insects.
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