In Silico Characterization and Gene Expression Analysis of Toll Signaling Pathway-Related Genes in Diaphorina citri

SIMPLE SUMMARY: Diaphorina citri, the vector of the citrus greening disease pathogen ‘Candidatus Liberibacter asiaticus’, has a reduced immune system in comparison to the non-hemipteran insects, including those of Lepidoptera, Dipterans, and Hymenoptera. Other hemipterans, such as Acyrthosiphon pisum, and Rhodnius prolixus, possess a partial or absent immune deficiency (IMD) pathway. D. citri does not have the IMD pathway, but has a complete Toll system that protects them from Gram-positive bacteria. Here we characterized the Toll signaling system genes in D. citri. In addition, we studied the expression profile of Toll genes in different life stages. Discovering this system and its dynamic expression may help in designing an innovative strategy by which we can control this insect vector. ABSTRACT: The Asian citrus psyllid, Diaphorina citri is the main vector of citrus greening disease, also known as Huanglongbing (HLB). Currently, mitigating HLB depends on the control of D. citri using insecticides. To design innovative control strategies, we should investigate various biological aspects of D. citri at the molecular level. Herein we explored the Toll signaling system-related proteins in D. citri using in silico analyzes. Additionally, the transcripts of the identified genes were determined in all life stages from eggs to adults. Our findings reveal that D. citri genome possesses Toll signaling pathway-related genes similar to the insect model, Drosophila melanogaster, with slight differences. These genes include cact, TI, Myd88, Dif/DI, pll, tub, and spz encoding Cactus, Toll, Myeloid differentiation factor 88, Dorsal related immunity factor/Dorsal, Pelle, Tube, and Spaetzle, respectively. Unlike D. melanogaster, in D. citri Dorsal, immunity factor and Dorsal are the same protein. In addition, in D. citri, Pelle protein possesses a kinase domain, which is absent in Pelle of D. melanogaster. Gene expression analysis showed the transcript for cact, TI, Myd88, pll, tub, and spz are maximum in adults, suggesting the immunity increases with maturity. Instead, Dif/DI transcripts were maximal in eggs and adults and minimal in nymphal stages, indicating its role in embryonic development. The overall findings will help in designing pioneering control strategies of D. citri based on repressing its immunity by RNAi or CRISPR and combining that with biological control.