Interactions between monarch butterflies and the protozoan parasite, Ophryocystis
elektroscirrha
Sonia M. Altizer
Department of Ecology, Evolution, and Behavior
University of Minnesota
St. Paul, MN
Abstract
Monarch 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 Altizer
Present address:
Dept. of Ecology and Evolutionary Biology
Princeton University
Princeton, NJ 08544-1003
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.
Sonia's Research Questions
- How many monarchs in natural populations are infected with
O. elektroscirrha?
- What effects does this parasite have on monarch survival and reproduction?
- How is O. elektroscirrha transmitted among captive and wild monarchs?
- Are parasites and hosts from different populations genetically distinct?