Fourth International Bemisia Workshop International Whitefly Genomics Workshop
Squash Leaf Curl Virus Localizes in Primary Salivary Gland Compartments, and at Midgut and Filter Chamber Brush Border Cells In Viruliferous Bemisia Tabaci
Department of Plant Sciences, The University of Arizona, Tucson, AZ, USA. Correspondence: joecic@yahoo.com
Bemisia tabaci acquires begomovirus (genus Begomovirus, family Geminiviridae) particles during feeding on the phloem sap of virus-infected plants. Virions follow a persistent, circulative pathway in the whitefly vector’s body to reach the primary salivary glands, which upon gaining entrance, are transmitted to another host plant in the salivary contents that are egested during feeding. However, this circulative route, or ‘the transmission pathway’ is not well understood. Knowledge of the specific pathway of virons and of the relationship with the principle whitefly vector organs that comprise the transmission pathway are important requisites to identifying and characterizing interactive whitefly- and presumably endosymbiont-encoded proteins essential to the transmission process and the basis for virus-vector specificity. The alimentary canal of B. tabaci and other Homoptera is modified to accommodate the high concentration of water in its’ food sources. Anterior and posterior ends of the midgut are connected to each other in the anterior abdominal hemocoel. Housed within the posterior end, where the hindgut starts, is a minute, conical, filter organ. The oesophagus complexes with, and empties directly into, this organ rather than the primitive location at the anterior midgut. Also associated with this complex are cells interpreted to be malpighian. The filter organ separates food from water, stores the food in the midgut loop, and allows the water to drain directly down the hindgut so that it doesn’t dilute the blood or the food reserves as in other fluid-feeding insects. From in vivo observations and huge numbers of instances in dissections and sectioned material, the midgut loop was found capable of passing from the abdomen, into the thorax, and back again. When in the thorax, it can reach the base of the head, where the salivary glands are located. B. tabaci has two pairs of salivary glands, primary and accessory, tethered to each other by their canals, which in turn are joined to common ducts that empty into the buccal cavity. In this study, in-situ hybridization with digoxygenin-labelled oligonucleotide viral DNA probes localized virus DNA in whole viruliferous whiteflies embedded in paraffin sections, revealing segments of the circulative pathway that have been pointed to by previous studies, were not entirely confirmed, or were altogether unknown. In this study, heavy deposits of silver-enhanced, ultrasmall gold-conjugated-anti-digoxygenin were observed on the microvilli, whereas more sparse deposits were observed in the cytoplasm and along the basement membrane of the midgut and filter chamber for virtually all specimens examined. Heavy deposition also occurred in two lobate, probably cellular, compartments at the apex of each primary salivary gland, at the same general region where the salivary canals are adjoined. Labeling of the analogous locations was not observed in identically treated, nonviruliferous adult whiteflies. Even though the accessory gland has long been thought to serve a key function in the pathway and in virus-vector specificity, no analogous labeling was seen in the accessory glands that could be considered above the threshold of sensitivity when using this approach to localize virus DNA in the whitefly. In addition to aortal-directed circulation, we posit that hemolymph is reciprocally displaced, volume for volume, whenever the midgut translocates from abdomen to thorax and back again, and that this action therefore assists in the diffusion of blood-borne virions. Further, virions might also be transported directly to the salivary gland membrane from contact with the midgut when it is anteriorized into the thorax. In contrast to the aphid-luteovirus model, these results suggest that the primary glands, instead of the accessory glands are organs in which virions are concentrated while awaiting discharge in salivary contents during subsequent feeding and egestion.
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