Annotation of the Asian Citrus Psyllid Genome Reveals a Reduced Innate Immune System

Abstract

Citrus production worldwide is currently facing significant losses due to citrus greening disease, also known as Huanglongbing. The citrus greening bacteria, Candidatus Liberibacter asiaticus (CLas), is a persistent propagative pathogen transmitted by the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae). Hemipterans characterized to date lack a number of insect immune genes, including those associated with the Imd pathway targeting Gram-negative bacteria. The D. citri draft genome was used to characterize the immune defense genes present in D. citri. Predicted mRNAs identified by screening the published D. citri annotated draft genome were manually searched using a custom database of immune genes from previously annotated insect genomes. Toll and JAK/STAT pathways, general defense genes Dual oxidase, Nitric oxide synthase, prophenoloxidase, and cellular immune defense genes were present in D. citri. In contrast, D. citri lacked genes for the Imd pathway, most antimicrobial peptides, 1,3-β-glucan recognition proteins (GNBPs), and complete peptidoglycan recognition proteins. These data suggest that D. citri has a reduced immune capability similar to that observed in A. pisum, P. humanus, and R. prolixus. The absence of immune system genes from the D. citri genome may facilitate CLas infections, and is possibly compensated for by their relationship with their microbial endosymbionts.

Improved annotation of the insect vector of Citrus greening disease: Biocuration by a diverse genomics community

“…In the D. citri genome, we identified two pasha homologs (Supplementary Note 22) and two loqs homologs but were initially unable to identify a true r2d2 ortholog ( Supplementary Note 23) in the genome. The apparent absence of r2d2 was consistent with previous reports (22,104). However, a search of the MCOT (MCOT18647.0.CO) transcriptome identified a gene with similarity to R2D2 orthologs from bed bug, Tribolium castaneum and mosquitoes.…”

“…However, bloodfeeding mosquitoes actually show an increase in immune genes and this expansion has been attributed to the likelihood of encountering pathogens in their food source. Arp et al (22) pointed out additional inconsistencies with the diet hypothesis, including the presence of a full immune system in an insect species that develops in a sterile environment.…”

Infection with phytopathogenic bacterium inhibits melatonin biosynthesis, decreases longevity of its vector, and suppresses the free radical‐defense

“…Treatments (healthy vs infected) were compared using two‐tailed t test, and statistical significance was established as P ≤ .05. The listed genes were assembled based on Arp et al and recent available data in National Center for Biotechnology Information website ( NCBI , http://www.ncbi.nlm.nih.gov/gene/). The full list of expressed genes, names, accession numbers, and primers are available in Table…”

“…We investigated the transcript levels of 18 genes involved in free radical‐defense in D. citri (Figure C and Table ). These genes were assembled based on the previous study of Arp et al and recent available data in NCBI website. The studied genes include superoxide dismutase ( SODs ), glutathione S ‐transferase ( GSTs ), peroxidase ( PODs ), and phospholipid hydroperoxide glutathione peroxidase ( PHGPXs ).…”

“…Samples were analyzed in triplicate for each biologic replicate (5 biologic replicates/treatment; n = 15). The listed genes were assembled based on previous study of Arp et al 61 and recent available data in national center for biotechnology information website (NCBI, http:// www.ncbi.nlm.nih.gov/gene/). The full list of expressed genes, names, accession numbers, and primers are available in Table S2 and uninfected D. citri adults; 56 however, it was detected as a trace in D. citri nymphs reared on healthy or CLas-infected Valencia sweet orange flushes without any significant differences between them.…”