Views on the parameters of vector competency
Sherwood JL
Department of Plant Pathology, The University of Georgia, Athens, Georgia, USA
Correspondence: sherwood@uga.edu
The number of thrips species that are known to transmit Tomato spotted wilt virus (TSWV) or other tospoviruses is limited, and the basis of competency of thrips to transmit tospoviruses has been investigated to any significance in only a few thrips-virus pathosystems. It has long been observed that only larval thrips that acquire tospoviruses or adults derived from such larvae are competent to transmit virus. Nonviruliferous adults do ingest virus particles when feeding upon virus infected plants, but such adult thrips have not been shown to transmit virus. Acquisition of virus by thrips has been shown to require the glycoprotein of the virus envelope (Sin S-H. 2005. Proceedings of the National Academy of Sciences USA 102:5168¨C5173), but if acquisition is a result of interaction with a ligand in the thrips midgut or a pH dependent process has not been fully elucidated. Upon entering the larval midgut, it has widely documented that virus replication ensues in the midgut tissues. This can be transient, with subsequent virus replication ensuing in the muscle fibers surrounding the midgut. Similar events have been noted when adult thrips and a non-vector species of thrips (see de Assis FM et al. 2004. Phytopathology 94:333¨C336, and references therein) have fed on virus-infected plants, but unlike when virus acquisition occurs in larval thrips of species that transmit the virus, virus is not subsequently found in the salivary glands. Several models have been proposed regarding how virus eventually becomes established in the salivary glands for subsequent transmission. The virus could transverse several membranes and move in the hemocoel to the salivary glands like other insect borne plant-infecting viruses that are persistently transmitted. A ligament-like structure that putatively forms a connection between the midgut and the salivary glands has been proposed as a tentative pathway for virus movement. Additionally, the proximity of the infected midgut organs to the salivary glands during thrips development may facilitate virus movement to the salivary glands. Each of these provides possibilities for understanding virus transmission, or lack thereof, by thrips. Additional insight on virus acquisition and vector competency can be obtained from virus isolates that form particles, but are not transmitted.
