The quest for a non-vector psyllid: Natural variation in acquisition and transmission of the huanglongbing pathogen 'Candidatus Liberibacter asiaticus' by Asian citrus psyllid isofemale lines

Authors: Ammar, Eldesouky; Hall, David; HOSSEINZADEH, SAEED - Boyce Thompson Institute; Heck, Michelle

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/29/2018
Publication Date: 4/13/2018
Citation: Ammar, E., Hall, D.G., Hosseinzadeh, S., Heck, M.L. 2018. The quest for a non-vector psyllid: Natural variation in acquisition and transmission of the huanglongbing pathogen 'Candidatus Liberibacter asiaticus' by Asian citrus psyllid isofemale lines. PLoS One. 13(4):e0195804. https://doi.org/10.1371/journal.pone.0195804.
DOI: https://doi.org/10.1371/journal.pone.0195804

Interpretive Summary: Huanglongbing is a citrus disease involving citrus host trees, an insect vector Diaphorina citri, known as the Asian citrus psyllid, and a bacterial pathogen ‘Candidatus Liberibacter asiaticus.’ Huanglongbing, also known as citrus greening disease, is considered to be the most devastating of all citrus diseases, and there is currently no adequate control strategy. In this work, ARS scientists collected psyllid male and female pairs from throughout the state of Florida and have maintained these lines in the lab for over two years. The results show that different psyllid populations vary in their ability to spread the citrus greening pathogen from tree to tree. Some populations spread the pathogen very efficiently and others not at all. The ability or inability to transmit the pathogen is passed on from parents to offspring, proving that psyllid genes regulate tree-to-tree spread of the pathogen. These populations will enable scientists to develop new ways of blocking the spread of citrus greening by preventing psyllid spread of the citrus greening pathogen.

Technical Abstract: Genetic variability in insect vectors is valuable to study vector competence determinants and to select non-vector populations that may help reduce the spread of vector-borne pathogens. We collected and tested vector competency of 15 isofemale lines of Asian citrus psyllid (ACP) Diaphorina citri, vector of Candidatus Liberibacter asiaticus (CLas). CLas is associated with citrus greening (huanglongbing), the most serious citrus disease worldwide. ACP adults were collected from Murraya (orange jasmine) hedges in Florida, and individual pairs (females and males) were caged on healthy Murraya plants for egg laying. The progeny from each pair that tested CLas-negative by qPCR were maintained on Murraya plants and considered an isofemale line. Six acquisition tests on ACP adults that were reared as nymphs on CLas-infected citrus, from various generations of each line, were conducted to assess their acquisition rates (percentage of qPCR-positive adults). Three lines with mean acquisition rates of 28 to 32 %, were classified as ‘good’ acquirers and three other lines were classified as ‘poor’ acquirers, with only 5 to 8 % acquisition rates. All lines were further tested for their ability to inoculate CLas by confining CLas-exposed psyllids onto healthy citrus leaves for two weeks (6-10 adults/leaf/week), and testing the leaves for CLas by qPCR. Mean inoculation rates were 19-28% for the three good acquirer lines and 0 to 3 % for the three poor acquirer lines. Statistical analyses indicated positive correlations between CLas acquisition and inoculation rates, as well as between CLas titer in the psyllids and CLas acquisition or inoculation rates. Phenotypic and molecular characterization of one of the good and one of the poor acquirer lines revealed differences between them in color morphs and hemocyanin expression. Understanding the genetic architecture of CLas transmission will enable the development of new tools for combating this devastating citrus disease.