PARENT SESSION
Tuesday, August 8, 8:00-11:30 am
COS 26 - Disease ecology I: parasites
L-13, Lobby Level, Cook Convention Center
Presiders: E Lehmer and S Altizer

Virulence evolution in monarch butterfly parasites.

de Roode, Jacobus^*,1, Altizer, Sonia^*,1, ¹ University of Georgia, Athens, GA

ABSTRACT- Explaining parasite virulence remains a challenge for evolutionary biologists. Intuitively, parasites that depend on their hosts for survival should be benign, yet many parasites cause harm. Theory has shown that parasites could evolve virulence (i.e. parasite-induced host mortality) as a consequence of selection maximizing fitness: in order to transmit between hosts, parasites must exploit their hosts, simultaneously damaging host tissues and increasing the chances of killing their hosts. Thus, higher transmission rates should come at the cost of higher mortality rates. Although this 'trade-off' model has been studied extensively by theoreticians, empirical support is still critically lacking. We studied the protozoan parasite Ophryocystis elektroscirrha in its natural host, the monarch butterfly (Danaus plexippus). We infected monarchs with cloned parasite genotypes obtained from wild populations, and assessed parasite transmission potential and host longevity. We found that genotypes producing more transmission stages also caused higher virulence, thus supporting theoretical assumptions. However, the relationship between transmission potential and virulence differed between parasites from western (Hawaii, California) and eastern (eastern US, Florida) populations, such that transmission stage production caused higher virulence in western rather than eastern parasites. Our results imply that selection on transmission rates is a major factor driving virulence evolution, and can explain why parasites harm their hosts. Moreover, population-specific ecological conditions apparently dictate the exact shape of the virulence-transmission relationship, and with that the adaptive level of parasite virulence in a population.

Key words: parasitism, disease ecology, insects