Insects are known to have varying immune responses to protect themselves from pathogen infection. In Lepidoptera infected by nuclear polyhedrosis viruses (NPVs), larval coloration has been found to correlate with disease resistance in some species. My research focused on measuring phenotypic variation in coloration of Agraulis vanillae (Gulf fritillary) larvae, and whether that coloration predicted A. vanillae larval susceptibility to infection with a host-specific baculovirus, Agva NPV. Healthy lab-raised larvae bred from field-collected A.vanillae individuals were used in this experiment. In total, 192 healthy 3rd to 4th instar larvae were photographed for coloration analysis and subsequently infected with a dose of 0 (control), 100, 1000, or 10000 viral particles. Larvae were raised in the lab after virus exposure until they had either died of viral infection or survived and pupated. Prior to infection, the larvae were photographed from above and from the side with a color standard. Images were then processed in Adobe Lightroom and Photoshop to standardize color across images, and L*, a*, and b* coloration values for each pixel were obtained in R. Non-metric multidimensional scaling (NMDS) was used to look for individual-level variation in color and whether that variation corresponded to disease susceptibility in the infection experiment.
Results/Conclusions
From the 192 Agraulis vanillae larvae that were infected, I found substantial variation in coloration among individual larvae in the NMDS analysis. Along with evidence of variation in susceptibility to NPV infection based on differences in coloration values for each larva, I found that there was an increasing difference in the coloration of A. vanillae larvae that died from NPV infection versus larvae that survived infection exposure as NPV dosage increased. In particular, A. vanillae larvae that died from either 1000 or 10000 OBs of NPV had significantly different coloration than larvae that survived infection in those respective experimental groups. Based on these preliminary results, I can conclude that larval coloration of the A. vanillae varies among individual larvae, and that the image analysis method I used was able to successfully capture these differences. Furthermore, this phenotypic variation in coloration was predictive of their susceptibility to Agva NPV infection. Further research is needed to determine whether the observed variation in color and disease susceptibility is due to genetic variation or to environmental factors such as diet, which is known to affect both coloration and disease susceptibility in other Lepidoptera species.