Interactions between monarch butterflies and the protozoan parasite, Ophryocystis elektroscirrha
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Sonia M. Altizer Abstract | Meet the Scientist | Research Questions | Other Research Topics AbstractMonarch butterflies are susceptible to infection by the obligate protozoan parasite, Ophryocystis elektroscirrha. Because monarchs form resident and migratory populations worldwide, they present an opportunity to examine how variation in parasite prevalence relates to seasonal host migration. Geographic variation in parasite prevalence suggests that prevalence is associated negatively with host dispersal distance. Among North American monarchs, 70% of a resident population in southern Florida are heavily infected. A western population that migrates moderate distances to overwintering sites has intermediate prevalence, and the eastern migratory population, which travels the longest distance, has exhibited less than 8% infection throughout the past 30 years. In addition, prevalence within a migratory population was lower among monarchs breeding close to overwintering sites versus those breeding farther away. The effect of O. elektroscirrha on monarch survival and reproduction depends on parasite dose and the larval stage at the time infection. Monarchs inoculated with the highest parasite dose had decreased survival to eclosion, and were smaller and shorter-lived as adults. Effects on host survival were more severe when larvae were inoculated at an earlier stage. Maternal and paternal parasite transmission was high in captive monarchs; up to 90% of the offspring of infected females emerged heavily infected, and 75% of the offspring of infected males became infected. Horizontal parasite transmission (from spores accumulating on milkweed plants) varied among different populations, and was highest for plants collected in a non-migratory population in southern Florida. Simulation models of host-parasite interactions demonstrated that small changes in parasite transmission rates can generate large differences in parasite prevalence between populations. By affecting parasite transmission or the survival of infected hosts, seasonal migration may also influence the evolution of host resistance or parasite virulence. To explore genetic variation among populations, I performed a reciprocal cross-inoculation experiment using hosts and parasites from the three North American populations described above. Results showed that host resistance was highest, and parasite virulence lowest, in the population that migrates the farthest distance. These results are consistent with the hypothesis that seasonal, long-distance migration affects both parasite prevalence and host-parasite coevolution in natural populations of monarch butterflies. Meet the Scientist:Sonia AltizerPresent address:
Sonia's Research Questions1. How many monarchs in natural populations are infected with O. elektroscirrha? 2. What effects does this parasite have on monarch survival and reproduction? 3. How is O. elektroscirrha transmitted among captive and wild monarchs? 4. Are parasites and hosts from different populations genetically distinct?
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I have been excited by biology and scientific research since I was
a middle school student in York, Pennsylvania, when my family bought
me a microscope and a grow-your-own butterflies kit for my twelfth
birthday. After almost majoring in electrical engineering at Duke
University, I took a course in animal diversity and changed my major
to biology before my senior year. Following graduation, I worked
in a laboratory at Duke and spent the summer at a biological station
in western Virginia. During this time I developed a keen interest
in studying the ecological and evolutionary interactions between
parasites and their hosts. After starting graduate school
at the University of Minnesota in 1993, I met Drs. Karen Oberhauser
and Don Alstad, who cultivated my interest in insect ecology and
monarch butterflies. I finished my doctoral degree in December
of 1998, and am currently teaching and doing research at Princeton
University. In addition to looking at butterfly parasites,
tromping through milkweed and analyzing data on my computer, I enjoy
reading magazines, riding horses, painting, sewing, and writing
poetry.