Journal of General Virology (2002), 83, 663–671. Printed in Great Britain ................................................................................................................................................................................................................................................................................... Factors determining vector competence and specificity for transmission of Tomato spotted wilt virus Tatsuya Nagata, 1 † Alice K. Inoue-Nagata, 2 Jan van Lent, 1 Rob Goldbach 1 and Dick Peters 1 1 Department of Virology, Wageningen Agricultural University, Binnenhaven 11, 6709 PD, Wageningen, The Netherlands 2 EMBRAPA/Hortaliças, 70359-970, PO Box 218, Brası ! lia, DF, Brazil The competence of a Frankliniella occidentalis and a Thrips tabaci population to transmit Tomato spotted wilt virus (TSWV) was analysed. Adults of the F. occidentalis population transmitted this virus efficiently, whereas those of the thelytokous T. tabaci population failed to transmit. TSWV replicated in the midgut of the larvae of both populations after ingestion of virus ; however, lower amounts accumulated in T. tabaci larvae than in F. occidentalis larvae. The virus was almost undetectable in T. tabaci adults, whereas high titres were readily detected in the F. occidentalis adults. The first infections in F. occidentalis larvae were detected by immunocytochemical studies in midgut epithelial and subsequently in midgut muscle cells, the ligaments, and finally in the salivary glands. The infections were weaker in the midgut epithelial and muscle cells of T. tabaci larvae, followed by an almost complete absence of any infection in the ligaments, and a complete absence in the salivary glands. Studies by electron microscopy revealed the budding of some virus particles from the basal membrane of midgut epithelial cells of F. occidentalis larvae into the extracellular space of the basal labyrinth. Enveloped virus particles were also seen in midgut muscle cells of F. occidentalis larvae. They were not discerned in epithelial and muscle cells of T. tabaci larvae and adults. This study showed that the rate of virus replication in the midgut and the extent of virus migration from the midgut to the visceral muscle cells and the salivary glands are probably crucial factors in the determination of vector competence. Introduction Tomato spotted wilt virus (TSWV) is the type species of the genus Tospovirus within the Bunyaviridae. This virus family contains many animal-infecting viruses, including important human viruses (Regenmortel et al., 2000). These viruses are mainly transmitted by mosquitoes and ticks (Calisher, 1996). Members of the genus Tospovirus, one of the five Bunyaviridae genera, are transmitted by some thrips species, of which most are also known as important pests in various agricultural and horticultural crops (Goldbach & Peters, 1994). The tospo- viruses are transmitted by their vectors in a propagative–cir- culative way (Wijkamp et al., 1993; Ullman et al., 1993). TSWV has a unique vector relationship with thrips in that the virus is acquired by the larvae but not by adults, while the Author for correspondence : Dick Peters. Fax ›31 317 484820. e-mail Dick.Peters!viro.dpw.wau.nl † Present address : Biotecnologia Genomica, Universidade Catolica de Brası ! lia, Campus II, SGAN 916, 70790–160, Brası ! lia, DF, Brazil. ability to acquire virus rapidly declines during larval de- velopment (van de Wetering et al., 1996 ; Nagata et al., 1999). After a temperature-dependent latent period, larvae at the end of their second stage and adults are able to transmit the virus (Wijkamp et al., 1993). Until now, only eight thrips species of the genera Thrips and Frankliniella have been reported as vectors of tospoviruses (Mound, 1996 ; Webb et al., 1998). Interspecific as well as intraspecific differences were found in vector competence in a study using four tospoviruses and four thrips species of six distinct populations (Wijkamp et al., 1995). Of the four viruses studied, TSWV was transmitted by all four thrips species, while Impatiens necrotic spot virus was transmitted solely by F. occidentalis with a high efficiency. Distinct differences were found in the transmission of TSWV by four T. tabaci populations studied. Three thelytokous populations, which propagate parthenogenetically and pro- duce a progeny of only females (Moritz, 1997), did not transmit TSWV. An arrhenotokous population transmitted the virus with low efficiency (Wijkamp et al., 1995). Thrips of these populations lay two different types of eggs. Fertilized eggs, which are diploid, produce females ; unfertilized eggs give only 0001-7959 # 2002 SGM GGD