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.
Highlights
Many parasites produce dormant transmission stages such as eggs or spores that persist outside of the host before encountering and infecting a new host [1]
We focused on a neogregarine sporozoan Ophryocystis elektroscirrha that commonly infects monarch butterflies, Danaus plexippus
All three spore trait measures examined here showed substantial variation among butterfly hosts and parasite clones: The mean surface area of Ophryocystis elektroscirrha (OE) spores ranged from 58.15 mm2 to
Summary
Many parasites produce dormant transmission stages such as eggs or spores that persist outside of the host before encountering and infecting a new host [1]. In some cases, these stages survive for only hours outside of their host, whereas other parasites produce thick-walled and longer-lived spores. Because parasite fitness depends on successful transmission, it is likely that the morphology of these dormant stages has been shaped by selection for dispersal propensity, and based on the ability of spores to withstand exposure to environmental stressors (e.g., desiccation, extreme temperatures, sunlight). Because parasites rely on host-derived resources for their own replication and development, parasite traits such as size and pigmentation could further depend on host traits, especially those related to energy acquisition and allocation, and on total parasite load [7±10]
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