The Asian citrus psyllid (ACP, Diaphorina citri, Hemiptera: Liviidae) is the principal vector of Candidatus Liberibacter asiaticus (Las), the putative bacterial agent of citrus greening/huanglongbing (HLB); currently the most serious citrus disease worldwide. Las is transmitted in a persistent-propagative manner by ACP, and the salivary glands and midgut have been suggested as transmission barriers that can impede translocation of Las within the vector. However, no detailed ultrastructural studies have been reported on these organs in this or other psyllid species, although some bacterium-like structures have been described in them and assumed to be the causal agents of HLB. In this study, we describe the ultrastructure of the salivary glands, filter chamber, other parts of the alimentary canal, and other organs and tissues of ACP including the compound ganglionic mass (in the thorax) and the bacteriome (in the abdomen). Furthermore, in addition to two ultrastructurally apparently different symbiotic bacteria found in the bacteriome, other morphological types of bacteria were found in the gut epithelial cells and salivary glands of both Las-infected (quantitative polymerase chain reaction positive) and noninfected (quantitative polymerase chain reaction negative) ACP. These results show the importance of immunolabeling, fluorescence in situ hybridization, or other labeling techniques that must be used before identifying any bacterium-like structures in ACP or other vectors as Las or other possible agents of HLB. This ultrastructural investigation should help future work on the cellular and subcellular aspects of pathogen-psyllid relationships, including the study of receptors, binding sites, and transmission barriers of Las and other pathogens within their psyllid vectors.
Keywords: alimentary canal; bacteriome; bacterium-like structures; salivary gland; symbionts.