Abstract
Insect-specific ascoviruses with a circular genome are distributed in the USA, France, Australia and Indonesia. Here, we report the first ascovirus isolation from Spodoptera exigua in Hunan, China. DNA-DNA hybridization to published ascoviruses demonstrated that the new China ascovirus isolate is a variant of Heliothis virescens ascovirus 3a (HvAV-3a), thus named HvAV-3h. We investigated the phylogenetic position, cell infection, vesicle production and viral DNA replication kinetics of HvAV-3h, as well as its host-ranges. The major capsid protein (MCP) gene and the delta DNA polymerase (DNA po1) gene of HvAV-3h were sequenced and compared with the available ascovirus isolates for phylogenetic analysis. This shows a close relationship with HvAV-3g, originally isolated from Indonesia, HvAV-3e from Australia and HvAV-3c from United States. HvAV-3h infection induced vesicle production in the SeE1 cells derived from S. exigua and Sf9 cells derived from S. frugiperda, resulting in more vesicles generated in Sf9 than SeE1. Viral DNA replication kinetics of HvAV-3h also demonstrated a difference between the two cell lines tested. HvAV-3h could readily infect three important insect pests Helicoverpa armigera (Hübner), Spodoptera exigua (Hübner) and Spodoptera litura (Fabricius) from two genera in different subfamilies with high mortalities.
Highlights
Among viruses that infect insects, ascovirus is a recently discovered group of viruses with a circular double-stranded DNA genome ranging in size of 100-186 kbp [1,2,3,4,5]
Among all ascovirus species discovered in America, the HvAV3a species is the only ascovirus widely distributed in other areas including Australia, Indonesia and China
From the phylogenetic analysis in this study, Heliothis virescens Ascovirus 3h (HvAV-3h) has a close relationship with HvAV-3g isolated from S. exigua in Indonesia, and HvAV-3g is closely related to HvAV-3e from Australia
Summary
Among viruses that infect insects, ascovirus is a recently discovered group of viruses with a circular double-stranded DNA genome ranging in size of 100-186 kbp [1,2,3,4,5]. Unlike the well-studied insect-specific baculovirus that often causes larvae to liquefy, making it relative easy to identify, the apparent scarcity of ascoviruses is likely due to the poor manifestation of infection symptoms in insects [6]. Another fact that may contribute to the late discovery of ascoviruses is the poor per os infectivity to insects resulting in their scarcity in nature [2,7]. Despite the difficulties in ascovirus isolation, ascoviruses were first discovered in the Southeastern and Western United States in the late 1970’s [10,11,12,13]. Other ascoviruses have been reported in France, Australia and Indonesia [14,15,16]
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