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

Alex P Arp,Wayne B Hunter,Kirtesn S Pelz-Stelinski

doi:10.3389/fphys.2016.00570

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.

Highlights

Insects transmit a wide range of animal and plant pathogens
The structure of peptidoglycan is conserved between classes of bacteria with Gram-positive having Llysine type peptidoglycan and Gram-negative bacteria and Gram-positive bacilli are made of Dap-type peptidoglycan in their respective cell walls (Schleifer and Kandler, 1972)
Peptidoglycan recognition proteins (PGRPs) were not evident in the D. citri genome, though one gene coding for a lysM peptidoglycan-binding domain-containing protein was identified, though lacking the PGRP domain (Table 1)

Summary

Introduction

Insects transmit a wide range of animal and plant pathogens. The majority of vector-borne pathogen research has focused on viral zoonotic and plant pathogens, with commensurate attention given to understanding the physiological factors that promote interactions between the pathogen and vector (Fereres and Moreno, 2009; Sim et al, 2014). Insects lack the adaptive immune system found in chordates, and instead rely exclusively on an innate immune system to regulate interactions with invading microorganisms. The insect innate immune system consists of humoral and cellular defense responses activated by pattern recognition receptors (PRRs), which detect and bind to conserved microbial surface structures, called pathogen-associated molecular patterns (PAMPs), (Pili-Floury et al, 2004; Dziarski and Gupta, 2006; Wang et al, 2011). Three hemimetabolous insects, the pea aphid, Acyrthosiphon pisum (Family: Aphididae), kissing bug, Rhodnius prolixus, (Family: Reduviidae) and human louse, Pediculus humanus, (Family: Pediculidae) possess comparatively reduced innate immune systems that lack genes for recognition and killing of Gram-negative bacteria (Gerardo et al, 2010; Kim et al, 2011; Mesquita et al, 2015)

Methods

Results

Conclusion