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.

Non-retroviral Endogenous Viral Elements in Tephritid Fruit Flies Reveal Former Viral Infections Not Related to Known Circulating Viruses.

A wide variety of insect-specific non-retroviral RNA viruses specifically infect insects. During viral infection, fragments of viral sequences can integrate into the host genomes creating non-retroviral endogenous viral elements (nrEVEs). Although the exact function of nrEVEs is so far unknown, some studies suggest that nrEVEs may interfere with virus replication by producing PIWI-interacting RNAs (piRNAs) that recognize and degrade viral RNAs through sequence complementarity. In this article, we identified the nrEVEs repertoire of ten species within the dipteran family Tephritidae (true fruit flies), which are considered a major threat to agriculture worldwide. Our results suggest that each of these species contains nrEVEs, although in limited numbers, and that nrEVE integration may have occurred both before and after speciation. Furthermore, the majority of nrEVEs originated from viruses with negative single-stranded RNA genomes and represent structural viral functions. Notably, these nrEVEs exhibit low similarity to currently known circulating viruses. To explore the potential role of nrEVEs, we investigated their transcription pattern and the production of piRNAs in different tissues of Ceratitis capitata. We successfully identified piRNAs that are complementary to the sequence of one nrEVE in C. capitata, thereby highlighting a potential link between nrEVEs and the piRNA pathway. Overall, our results provide valuable insights into the comparative landscape of nrEVEs in true fruit flies, contributing to the understanding of the intimate relation between fruit flies and their past and present viral pathogens.

Expanding the Medfly Virome: Viral Diversity, Prevalence, and sRNA Profiling in Mass-Reared and Field-Derived Medflies.

The Mediterranean fruit fly (medfly), Ceratitis capitata, is an agricultural pest of a wide range of fruits. The advent of high-throughput sequencing has boosted the discovery of RNA viruses infecting insects. In this article, we aim to characterize the RNA virome and viral sRNA profile of medfly. By means of transcriptome mining, we expanded the medfly RNA virome to 13 viruses, including two novel positive ssRNA viruses and the first two novel dsRNA viruses reported for medfly. Our analysis across multiple laboratory-reared and field-collected medfly samples showed the presence of a core RNA virome comprised of Ceratitis capitata iflavirus 2 and Ceratitis capitata negev-like virus 1. Furthermore, field-collected flies showed a higher viral diversity in comparison to the laboratory-reared flies. Based on the small RNA sequencing, we detected small interfering RNAs mapping to all the viruses present in each sample, except for Ceratitis capitata nora virus. Although the identified RNA viruses do not cause obvious symptoms in medflies, the outcome of their interaction may still influence the medfly's fitness and ecology, becoming either a risk or an opportunity for mass-rearing and SIT applications.