Galbut virus infection minimally influences Drosophila melanogaster fitness traits in a strain and sex-dependent manner

Abstract

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. But on average only about 60% of individual flies are infected. One possible explanation for this apparent paradox is that a subset of flies are resistant to infection. Although galbut virus infected flies seem 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 slightly decreased average lifespan and total offspring production, but these decreases were mostly not statistically significant. Galbut virus DGRP-517 flies pupated and eclosed slightly faster than their uninfected counterparts. Some galbut virus infected flies exhibited altered sensitivity to viral, bacterial, and fungal pathogens. Galbut virus infection produced minimal changes to host mRNA levels as measured by RNA sequencing, consistent with minimal phenotypic changes. 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 were generally 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. ImportanceVirology has largely focused on viruses that cause unmistakable phenotypic changes. But metagenomic surveys are revealing that persistent virus infections are extremely common, even in apparently healthy organisms. The extent to which these persistent viruses impact host fitness and evolution remains largely unclear. Here we study fitness impacts of a partitivirus named galbut virus that is ubiquitous in wild D. melanogaster populations. Despite efficient biparental vertical transmission, galbut virus is present in only just over half of wild flies. We quantified various fitness-related traits in galbut virus infected and uninfected fly lines and found that infection produced small but measurable changes in host phenotype that in aggregate may reduce fly fitness. Further studies that take advantage of this virus that naturally infects a premier model organism that is easy to study in the wild will shed further light on the persistent virus-host dynamics that may actually represent most viral infections.