Summary: Virulence determinants in a natural butterfly-parasite system
J. C. de ROODE*, L. R. GOLD and S. ALTIZER
Department of Environmental Studies, Emory University, Atlanta, GA 30322, USA
(Received 22 September 2006; revised 17 October 2006; accepted 18 October 2006)
Much evolutionary theory assumes that parasite virulence (i.e. parasite-induced host mortality) is determined by within-
host parasite reproduction and by the specific parasite genotypes causing infection. However, many other factors could
influence the level of virulence experienced by hosts. We studied the protozoan parasite Ophryocystis elektroscirrha in its
host, the monarch butterfly, Danaus plexippus. We exposed monarch larvae to wild-isolated parasites and assessed the
effects of within-host replication and parasite genotype on host fitness measures, including pre-adult development time and
adult weight and longevity. Per capita replication rates of parasites were high, and infection resulted in high parasite loads.
Of all host fitness traits, adult longevity showed the clearest relationship with infection status, and decreased continuously
with increasing parasite loads. Parasite genotypes differed in their virulence, and these differences were maintained across
ecologically relevant variables, including inoculation dose, host sex and host age at infection. Thus, virulence appears to be
a robust genetic parasite trait in this system. Although parasite loads and genotypes had strong effects on virulence,
inoculation dose, host sex and age at infection were also important. These results have implications for virulence evolution
and emphasize the need for a detailed understanding of specific host-parasite systems for addressing theory.
Key words: virulence evolution, host-pathogen interaction, infectious disease, Apicomplexa, neogregarine, horizontal
transmission, vertical transmission.