Infection of cotton bollworm by Helicoverpa armigera iflavirus decreases larval fitness

Covert RNA viruses in medflies differ in their mode of transmission and tissue tropism

Numerous studies have demonstrated the presence of covert viral infections in insects. These infections can be transmitted in insect populations via two main routes: vertical from parents to offspring, or horizontal between nonrelated individuals. Thirteen covert RNA viruses have been described in the Mediterranean fruit fly (medfly). Some of these viruses are established in different laboratory-reared and wild medfly populations, although variations in the viral repertoire and viral levels have been observed at different time points. To better understand these viral dynamics, we characterized the prevalence and levels of covert RNA viruses in two medfly strains, assessed the route of transmission of these viruses, and explored their distribution in medfly adult tissues. Altogether, our results indicated that the different RNA viruses found in medflies vary in their preferred route of transmission. Two iflaviruses and a narnavirus are predominantly transmitted through vertical transmission via the female, while a nodavirus and a nora virus exhibited a preference for horizontal transmission. Overall, our results give valuable insights into the viral tropism and transmission of RNA viruses in the medfly, contributing to the understanding of viral dynamics in insect populations.

IMPORTANCE

The presence of RNA viruses in insects has been extensively covered. However, the study of host-virus interaction has focused on viruses that cause detrimental effects to the host. In this manuscript, we uncovered which tissues are infected with covert RNA viruses in the agricultural pest Ceratitis capitata, and which is the preferred transmission route of these viruses. Our results showed that vertical and horizontal transmission can occur simultaneously, although each virus is transmitted more efficiently following one of these routes. Additionally, our results indicated an association between the tropism of the RNA virus and the preferred route of transmission. Overall, these results set the basis for understanding how viruses are established and maintained in medfly populations.

Genetic diversity of RNA viruses infecting invertebrate pests of rice

Invertebrate species are a natural reservoir of viral genetic diversity, and invertebrate pests are widely distributed in crop fields. However, information on viruses infecting invertebrate pests of crops is limited. In this report, we describe the deep metatranscriptomic sequencing of 88 invertebrate samples covering all major invertebrate pests in rice fields. We identified 296 new RNA viruses and 13 known RNA viruses. These viruses clustered within 31 families, with many highly divergent viruses constituting potentially new families and genera. Of the identified viruses, 13 RNA viruses clustered within the Fiersviridae family of bacteriophages, and 48 RNA viruses clustered within families and genera of mycoviruses. We detected known rice viruses in novel invertebrate hosts at high abundances. Furthermore, some novel RNA viruses have genome structures closely matching to known plant viruses and clustered within genera of several plant virus species. Forty-five potential insect pathogenic RNA viruses were detected in invertebrate species. Our analysis revealed that host taxonomy plays a major role and geographical location plays an important role in structuring viral diversity. Cross-species transmission of RNA viruses was detected between invertebrate hosts. Newly identified viral genomes showed extensive variation for invertebrate viral families or genera. Together, the large-scale metatranscriptomic analysis greatly expands our understanding of RNA viruses in rice invertebrate species, the results provide valuable information for developing efficient strategies to manage insect pests and virus-mediated crop diseases.

Characterization of Iflavirus in the Red Flour Beetle, Tribolium castaneum (Coleoptera; Tenebrionidae)

Iflavirus is a group of viruses distributed mainly in arthropod species. We surveyed Tribolium castaneum iflavirus (TcIV) in different laboratory strains and in Sequence Read Archives (SRA) in GenBank. TcIV is highly specific to only T. castaneum and is not found in seven other Tenebrionid species, including the closely related species T. freemani. The same strains from different laboratories and different strains displayed largely different degrees of infections in the examination of 50 different lines by using Taqman-based quantitative PCR. We found that ~63% (27 out of 43 strains) of T. castaneum strains in different laboratories are positive for TcIV PCR with large degrees of variation, in the range of seven orders of magnitude, indicating that the TcIV is highly fluctuating depending on the rearing conditions. The TcIV was prevalent in the nervous system with low levels found in the gonad and gut. The transovarial transmission was supported in the experiment with surface-sterilized eggs. Interestingly, TcIV infection did not show observable pathogenicity. TcIV offers an opportunity to study the interaction between the virus and the immune system of this model beetle species.

A Novel Iflavirus Was Discovered in Green Rice Leafhopper Nephotettix cincticeps and Its Proliferation Was Inhibited by Infection of Rice Dwarf Virus

The green rice leafhopper, Nephotettix cincticeps (Hemiptera: Cicadellidae), is a key insect vector transmitting rice dwarf virus (RDV) that causes rice dwarf disease. We discovered a novel iflavirus from the transcriptomes of N. cincticeps and named it as Nephotettix cincticeps positive-stranded RNA virus-1 (NcPSRV-1). The viral genome consists of 10,524 nucleotides excluding the poly(A) tail and contains one predicted open reading frame encoding a polyprotein of 3,192 amino acids, flanked by 5' and 3' untranslated regions. NcPSRV-1 has a typical iflavirus genome arrangement and is clustered with the members of the family Iflaviridae in the phylogenetic analysis. NcPSRV-1 was detected in all tested tissues and life stages of N. cincticeps and could be transmitted horizontally and vertically. Moreover, NcPSRV-1 had high prevalence in the laboratory populations and was widely spread in field populations of N. cincticeps. NcPSRV-1 could also infect the two-striped leafhopper, Nephotettix apicalis, at a 3.33% infection rate, but was absent in the zigzag leafhopper, Recilia dorsalis, and rice Oryza sativa variety TN1. The infection of RDV altered the viral load and infection rate of NcPSRV-1 in N. cincticeps, for which it seems that RDV has an antagonistic effect on NcPSRV-1 infection in the host.

Iflavirus Covert Infection Increases Susceptibility to Nucleopolyhedrovirus Disease in Spodoptera exigua

Naturally occurring covert infections in lepidopteran populations can involve multiple viruses with potentially different transmission strategies. In this study, we characterized covert infection by two RNA viruses, Spodoptera exigua iflavirus 1 (SeIV-1) and Spodoptera exigua iflavirus 2 (SeIV-2) (family Iflaviridae) that naturally infect populations of Spodoptera exigua, and examined their influence on susceptibility to patent disease by the nucleopolyhedrovirus Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) (family Baculoviridae). The abundance of SeIV-1 genomes increased up to ten-thousand-fold across insect developmental stages after surface contamination of host eggs with a mixture of SeIV-1 and SeIV-2 particles, whereas the abundance of SeIV-2 remained constant across all developmental stages. Low levels of SeIV-2 infection were detected in all groups of insects, including those that hatched from surface-decontaminated egg masses. SeIV-1 infection resulted in reduced larval weight gain, and an unbalanced sex ratio, whereas larval developmental time, pupal weight, and adult emergence and fecundity were not significantly affected in infected adults. The inoculation of S. exigua egg masses with iflavirus, followed by a subsequent infection with SeMNPV, resulted in an additive effect on larval mortality. The 50% lethal concentration (LC50) of SeMNPV was reduced nearly 4-fold and the mean time to death was faster by 12 h in iflavirus-treated insects. These results suggest that inapparent iflavirus infections may be able to modulate the host response to a new pathogen, a finding that has particular relevance to the use of SeMNPV as the basis for biological pest control products.