Genetic Factors and Host Traits Predict Spore Morphology for a Butterfly Pathogen

Abstract

Monarch butterflies (Danaus plexippus) throughout the world are commonly infected by the specialist pathogen Ophryocystis elektroscirrha (OE). This protozoan is transmitted when larvae ingest infectious stages (spores) scattered onto host plant leaves by infected adults. Parasites replicate internally during larval and pupal stages, and adult monarchs emerge covered with millions of dormant spores on the outsides of their bodies. Across multiple monarch populations, OE varies in prevalence and virulence. Here, we examined geographic and genetic variation in OE spore morphology using clonal parasite lineages derived from each of four host populations (eastern and western North America, South Florida and Hawaii). Spores were harvested from experimentally inoculated, captive-reared adult monarchs. Using light microscopy and digital image analysis, we measured the size, shape and color of 30 replicate spores per host. Analyses examined predictors of spore morphology, including parasite source population and clone, parasite load, and the following host traits: family line, sex, wing area, and wing color (orange and black pigmentation). Results showed significant differences in spore size and shape among parasite clones, suggesting genetic determinants of morphological variation. Spore size also increased with monarch wing size, and monarchs with larger and darker orange wings tended to have darker colored spores, consistent with the idea that parasite development depends on variation in host quality and resources. We found no evidence for effects of source population on variation in spore morphology. Collectively, these results provide support for heritable variation in spore morphology and a role for host traits in affecting parasite development.

Host Diet Affects the Morphology of Monarch Butterfly Parasites

“…Then we calculated the area and aspect ratio (ratio between spore length and width) for each spore. Unlike the study by Sander et al (2013), we used calculations for spore areas because the contrast between parasite spores and the paper backgrounds used made it difficult to accurately outline spores for area measurements. However, because O. elektroscirrha spores are typically lemon-shaped (McLaughlin and Myers, 1970), we determined that these calculations are representative of the true spore areas.…”

“…For this experiment, a single parasite clone of O. elektroscirrha was used to minimize any morphological differences due to parasite genetics (Sander et al, 2013). The parasite clone was obtained from a wild-caught infected monarch butterfly and propagated by inoculating a lab-reared monarch larva with a single parasite spore.…”

“…Lastly, we incorporated both plant chemistry and plant species identity as fixed factors to assess whether significant effects of plant species on spore morphology could be explained entirely by plant chemistry. Butterfly mass was included as a co-variate to eliminate indirect effects of host plant on parasite morphology (Sander et al, 2013).…”

“…Due to host constraints, males of this isopod normally have reduced body size, which allows females to grow larger and to increase clutch size (Tsai et al, 2001). Aside from mass, spore shape is also important in determining parasite fitness (Sander et al, 2013). Overall, variation in spore size and shape is related to both transmission and specificity to hosts (Monis et al, 2003;Roper et al, 2008;Wang and Lin, 2012).…”

“…These previous studies have shown reductions in parasite number and increases in monarch lifespan, with increasing cardenolide concentrations, but have not investigated effects of cardenolides on spore morphology. In a previous study, Sander et al (2013) found that parasite genetics significantly affected parasite spore size. This study also found that parasite size and color were positively correlated with monarch wing size and color, demonstrating that both parasite and host traits play a vital role in parasite morphology.…”