Transcriptomic Profiling of Bean Aphid <i>Megoura crassicauda</i> upon Exposure to the Aphid-Obligate Entomopathogen <i>Conidiobolus obscurus</i> (Entomophthoromycotina) and Screening of CytCo-Binding Aphid Proteins through a Pull-Down Assay

Prolonged periods of host-lethal infection by entomopathogenic fungi pose challenges to the development of biological control agents. The obligate entomopathogen <i>C. obscurus</i>, however, rapidly kills aphid hosts, warranting investigation. This study investigated the interaction between <i>C. obscurus</i> and a bean aphid <i>Megoura crassicauda</i> during the incubation period of infection, using transcriptome analysis to map host gene expression profiles. Results indicate <i>C. obscurus</i>-inoculated aphid activation of the wound healing immune responses, alongside suppression of the key molecules involved in Toll signaling, melanization, and metabolism. Furthermore, neuromotor system-related genes were upregulated, paralleling the intoxication observed in a nematode pest treated with <i>C. obscurus</i>-derived CytCo protein. To deepen interaction insights, a His-tag pull-down assay coupled with mass spectrometry analysis was conducted using CytCo as a bait to screen for potential aphid protein interactors. The proteins were identified based on the assembled transcriptome, and eleven transmembrane proteins were predicted to bind to CytCo. Notably, a protein of putatively calcium-transporting ATPase stood out with the highest confidence. This suggests that CytCo plays a vital role in <i>C. obscurus</i> killing aphid hosts, implicating calcium imbalance. In conclusion, <i>C. obscurus</i> effectively inhibits aphid immunity and exhibits neurotoxic potential, expediting the infection process. This finding facilitates our understanding of the complex host–pathogen interactions and opens new avenues for exploring biological pest management strategies in agroforestry.