Disease-causing pathogens can adapt to their host, becoming more specialized and thus better able to infect a particular host species or genotype. If the pathogen interacts with a host specific to the same geographic area, then as both hosts and pathogens co-evolve, this can lead to local adaptation. Alternatively, standing variation in pathogens might occur in a single geographic area without specialization to particular host types. I studied the host/pathogen relationship between Agraulis vanillae (Nymphalidae), commonly known as the Gulf fritillary butterfly, and nucleopolyhedrovirus (NPV), a lethal virus, transmitted when it contaminates passion flower (Passiflora spp.) vines, the food plant of the host larvae. Twenty one virus isolates were collected from various locations in San Diego county to look for evidence of regional diversity in this pathogen. I extracted DNA from each isolate and sent samples for library construction and Illumina sequencing (Retrogen). I then analyzed full genomes using the software Geneious, first examining highly conserved genes, with important implications for disease transmission and pathogen fitness. These methods could detect the presence of any genotypic variation in the pathogen population and variation that could be critical in determining the outcomes of competition between virus strains in a single host population.
Results/Conclusions
I found evidence of geographic structure on a very small spatial scale, with two distinct groups of genetically different strains found in North San Diego County versus the city of San Diego, a distance of less than 50 km. Preliminary results show genetic variation in key genes. Some single nucleotide polymorphisms observed code for amino acid substitutions, as in the chitinase gene, and an amino acid deletion mutation was observed in the P-74 gene. These non-synonymous substitutions may have effects on the structure or function of the protein, with potential implications for ecologically-important traits such as virulence and speed of kill. Additionally, strong geographic structure was observed in these key genes: strains collected in the northern region were genetically similar to each other and distinct from strains collected in the central region, which were also genetically similar to those in their own region. This genetic variation is consistent with pathogen evolution of specialization to its location-specific host, or with geographic barriers to dispersal of the pathogen. To determine whether local adaptation is in fact occurring here, future work is needed to compare infection rates of northern and central San Diego strains in larvae from those same or different populations.