Galbut virus (family Partitiviridae) infects Drosophila melanogaster and can be transmitted vertically from infected mothers or infected fathers with near perfect efficiency. This form of super-Mendelian inheritance should drive infection to 100% prevalence, and indeed, galbut virus is ubiquitous in wild D. melanogaster populations. However, on average, only about 60% of individual flies are infected. One possible explanation for this is that a subset of flies are resistant to infection. Although galbut virus-infected flies appear healthy, infection may be sufficiently costly to drive selection for resistant hosts, thereby decreasing overall prevalence. To test this hypothesis, we quantified a variety of fitness-related traits in galbut virus-infected flies from two lines from the Drosophila Genetic Reference Panel (DGRP). Galbut virus-infected flies had no difference in average lifespan and total offspring production compared to their uninfected counterparts. Galbut virus-infected DGRP-517 flies pupated and eclosed faster than their uninfected counterparts. Some galbut virus-infected flies exhibited altered sensitivity to viral, bacterial, and fungal pathogens. The microbiome composition of flies was not measurably perturbed by galbut virus infection. Differences in phenotype attributable to galbut virus infection varied as a function of fly sex and DGRP strain, and differences attributable to infection status were dwarfed by larger differences attributable to strain and sex. Thus, galbut virus infection does produce measurable phenotypic changes, with changes being minor, offsetting, and possibly net-negative.

Partitiviruses are segmented, multipartite dsRNA viruses that until recently were only known to infect fungi, plants, and protozoans. Metagenomic surveys have revealed that partitivirus-like sequences are also commonly associated with arthropods. One arthropod-associated partitivirus, galbut virus, is extraordinarily common in wild populations of Drosophila melanogaster fruit flies. To begin to understand the processes that underlie this virus’s high global prevalence, we established colonies of wild-caught infected flies. Infection remained at stably high levels over three years, with between 63-100% of individual flies infected. Galbut virus infects fly cells and replicates in tissues throughout infected adults, including reproductive tissues and the gut epithelium. We detected no evidence of horizontal transmission via ingestion but vertical transmission from either infected females or infected males was ~100% efficient. Vertical transmission of a related partitivirus, verdadero virus, that we discovered in a laboratory colony of Aedes aegypti mosquitoes was similarly efficient. This suggests that efficient biparental vertical transmission may be a feature of at least a subset of insect-infecting partitiviruses. To study the impact of galbut virus infection free from the confounding effect of other viruses, we generated an inbred line of flies with galbut virus as the only detectable virus infection. We were able to transmit infection experimentally via microinjection of homogenate from these galbut-only flies. This sets the stage for experiments to understand the biological impact and possible utility of partitiviruses infecting important model organisms and disease vectors. Importance: Galbut virus is a recently discovered partitivirus that is extraordinarly common in wild populations of the model organism Drosophila melanogaster . Like most viruses discovered through metagenomics, most of the basic biological questions about this virus remain unanswered. We found that galbut virus, along with a closely related partitivirus found in Aedes aegypti mosquitoes, is transmitted from infected females or males to offspring with ~100% efficiency and can be maintained in laboratory colonies over years. This represents one of the most efficient means of virus transmission described, and likely underlies the successful spread of these viruses through insect populations. We created Drosophila lines that contained galbut virus as the only virus infection and showed that these flies can be used as a source for experimental infections. This provides insight into how arthropod-infecting partitiviruses may be maintained in nature and sets the stage for exploration of their biology and potential utility.

Located 50 miles west of Fort Collins, Colorado, Colorado State University’s Mountain Campus in Pingree Park hosted the 23rd annual Rocky Mountain Virology Association meeting in 2023 with 116 participants. The 3-day event at the end of September consisted of 28 talks and 43 posters that covered the topics of viral evolution and surveillance, developments in prion research, arboviruses and vector biology, host–virus interactions, and viral immunity and vaccines. This year’s Randall Jay Cohrs keynote presentation covered the topic of One Health and emerging coronaviruses. This timely discussion covered the importance of global disease surveillance, international collaboration, and trans-disciplinary research teams to prevent and control future pandemics. Peak fall colors flanked the campus and glowed along the multiple mountain peaks, allowing for pristine views while discussing science and networking, or engaging in mountain activities like fly fishing and hiking. On behalf of the Rocky Mountain Virology Association, this report summarizes select presentations from the 23rd annual meeting.

Aedes aegypti mosquitoes can be infected by arboviruses that can be transmitted to and cause disease in vertebrates. Ae. aegypti can also be infected by insect-specific viruses, which do not infect vertebrates, but may impact mosquito biology and vector competence. In this study we characterized the viromes of Ae. aegypti populations that had been maintained in laboratory colonies for 4 to 18 years. Mosquitoes were originally collected in Tapachula, Poza Rica and Merida, Mexico and New Orleans, USA. We used metagenomics to characterize the viruses present in the colonies, then quantified the vertical transmission efficiencies of the viruses by performing crosses between populations. The viruses infecting these colonies included Aedes anphevirus, Guadeloupe mosquito virus, and verdadero virus. These viruses were at high prevalence in infected populations, with over 75% of individual adult mosquitoes infected. All three viruses exhibited biparental vertical transmission: both infected mothers and infected fathers transmitted infection to their offspring, but in all cases maternal transmission was more efficient than paternal. We also identified evidence of possible horizontal transmission between adult mosquitoes that had cohabited during crosses. Efficient transgenerational transmission likely contributes to the ability of these viruses to persist in laboratory populations and nature. Because of their efficient vertical transmission and minimal apparent fitness costs, these viruses could be good candidates for gene delivery to mosquito populations, including for vector control. Author summary: Aedes aegypti mosquitoes are responsible for transmission of many dangerous pathogens, including dengue virus, Zika virus, and yellow fever virus. These mosquitoes also carry viruses that do not pose a threat to public health, but might influence mosquito biology. In this study, we identified viruses that persistently infected 4 long-established Ae. aegypti colonies in our insectary. These viruses were at high prevalence in these laboratory populations, and did not appear to harm infected mosquitoes. The viruses exhibited an ability to efficiently transmit from parents to offspring, both from infected mothers and infected fathers. This provided an explanation for how these mosquitoes were able to persist so successfully in laboratory populations, and by extension in nature. Given their ability to efficiently spread and minimal apparent fitness costs, these viruses represent promising candidates for use in mosquito control or other practical applications.

Drosophila melanogaster , the common fruit fly, has been instrumental to our understanding of evolution, genetics and disease. There are benefits to studying these flies in the wild, including assessment of their naturally occurring microbiota. To facilitate efforts to catch wild D. melanogaster , we designed two fly traps and evaluated several candidate attractants. The first trap utilized a stable food substrate that can be used to catch live flies to establish new lab colonies. The second trap was designed to be reusable and easy to ship to enable the collection of flies over time from diverse locations. We evaluated several chemical attractants derived from banana and from marula fruit, which is the proposed ancestral food host of D. melanogaster . We found that wild flies were preferentially attracted to banana-based odorants over marula-derived ones. Overall, these traps and attractants represent an inexpensive and simple option for the collection of wild D. melanogaster and related species for sampling or colony establishment.