Royally Split: Morphological divergence of parasites in milkweed butterflies

Katie Yan, a student from Skidmore College, worked in Dr. Sonia Altizer‘s lab.

Abstract Ophryocystis elektroscirrha (OE) is a protozoan parasite found in Monarchs (Danaus plexippus). Same or similar OE-like parasites have been found in Queens (D. gilippus) and other Danaus butterflies. Experimental cross infection provided evidence of parasite specialization on natal host species via low infection rates on novel hosts, motivating in-depth analysis of parasite morphometric and genetic variation across host species from different locations. To examine OE and OE-like spore morphology across Danaus species, we looked at five host species from previously curated museum samples including: Jamaican Monarch (D. cleophile), Plain Tiger (D. chrysippus), Queen (D. gilippus), Common Monarch (D. plexippus), and Lesser Wanderer (D. petilia). Analysis of wild-collected museum spores showed that on average, Jamaican Monarchs and Common Monarchs had larger spores than other species. This relationship is consistent with the phylogenetic relationships of host similarity within the Danaus genus and further supports the hypothesis of parasite specialization on hosts. We then examined the influences of host and environmental factors on parasite morphology by analyzing spores from a cross-infection experiment involving Monarchs and Queens. Hosts in this experiment were fed either tropical milkweed (Asclepias curassavica) or swamp milkweed (A. incarnata). Monarchs and Queens were infected at a high rate by their natal parasites, some Monarchs were infected by Queen parasites, and no Queens were infected by Monarch parasites. Morphometric analysis of spores showed that Monarch parasites from Monarch hosts were largest, and that Queen parasites from Queen hosts were smallest; Queen parasites from Monarch hosts were intermediate in spore size. Other spore traits (shape, hue, density) were similar across treatments. Additionally, we found that spore size positively correlated with host wing area, suggesting that larger spores are found on larger butterflies. Milkweed species, sex, and final spore load did not predict variation in spore morphology. In summary, we found strong evidence for parasite specialization on different host species based on differences in spore size; further work should ask whether molecular genetic divergence of OE parasites across host groups matches differences in spore morphology and host phylogeny.