Ophryocystis elektroscirrha is an obligate, neogregarine protozoan that infects Monarch (Danaus plexippus) and Queen (Danaus gilippus) Butterflies. There are no other known hosts. The species was first discovered in Florida, around the late 1960s. Since then, it has been found in every Monarch population examined to date, including Monarchs sampled in North America, Hawaii, Australia, Cuba, and Central and South America.
Ophryocystis elektroscirrha is usually transmitted from females to their offspring when the females scatter spores on their egg chorions and surfaces of milkweed leaves, the host plant of Monarchs and Queens, during oviposition (egg laying). Milkweeds contains toxins that partially protect Monarch larvae. The spores are ingested by the larvae. After they enter a host's gut, the spores open and emerging sporozoites penetrate a larva's gut wall and migrate to its hypoderm (the layer of cells that secrete the larva's cuticle), where they undergo two phases of vegetative reproduction. After the caterpillar pupates, O. elektroscirrha starts reproducing sexually. Three days before adults emerge, developing parasite spores can be seen through their pupal integuments. The adult butterflies emerge covered with spores, mostly on their abdomens. Parasites do not continue to replicate on adult butterflies and spores must be eaten by larvae before they can cause new infections.
Effect on individual butterflies
Because O. elektroscirrha is geographically widespread, it may have a long history of occurrence with Monarch Butterflies. Nevertheless, infection by O. elektroscirrha can cause a range of lethal and sublethal effects, and parasitized Monarchs have lower survival before adulthood, smaller adult body size, and shorter adult lifespans. Studies designed to test the negative effects of O. elektroscirrha on Monarch fitness showed that this parasite's effects are more severe when Monarchs ingest a higher initial dose of spores, and further depend on the larval instar (age) at inoculation, with animals infected at earlier instars suffering more from their infections. Adult Monarchs that emerge with high spore densities have decreased eclosion (emergence from pupae) success, smaller wingspans, and lower body masses than uninfected adults. Although Monarch mating success decreases with higher parasite loads, females that survive long enough to mate and initiate egg production to not appear to experience a significant decline in lifetime fecundity, although they may be less active than uninfected females. Researchers have also shown that infected Monarchs have lower flight performance (shorter duration of active flight and lower flight speeds) than uninfected butterflies.
Population variation in prevalence
The prevalence (proportion of butterflies infected) with O. elektroscirrha is highly variable among wild Monarch populations and appears to vary inversely with host migration distances. Monarchs in southern Florida and Hawaii that breed year-round (i.e., non-migratory populations) bear the highest parasite loads (over 70% heavily infected). Approximately 30% of Monarchs from a migratory population in western North America are heavily infected. Less than 8% of Monarchs from the eastern migratory population (longest-distance migrants) are heavily infected. These differences among populations have persisted for many years, and suggest an association between parasite transmission and host migratory ecology.
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