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

Competition between co-infecting parasite strains alters within-host population dynamics in genetically diverse trypanosome infections.

Balmer, Oliver^*,1, ¹ Yale University, New Haven, CT

ABSTRACT- It is increasingly becoming evident that parasite infections often contain multiple, genetically distinct, conspecific strains. This has important implications for parasite population dynamics, host response, and parasite life-history evolution. This study tested if different strains of Trypanosoma brucei, a protozoan parasite causing African sleeping sickness, interact in mixed infections in mice and if this interaction leads to evolutionary changes in the involved strains. Fluorescence genes of differing color were introduced into two competing strains to visually track the population dynamics of individual strains. Live fluorescence furthermore enabled us to physically separate strains from mixed infections using a fluorescence activated cell sorter (FACS) to subsequently measure if competition has altered their life-histories. Both parasite strains exhibited significantly reduced growth rates and achieved lower densities in mixed infections compared to single infections. Unexpectedly, multiple infections also led to 18% increased survival times of mice (n=11, p<0.01) compared to single infection of the more virulent strain alone, probably due to reduced population growth of the more virulent strain. The combined total parasite densities in mixed infections remained below those of single infections. Reduction in parasite growth could thus not be attributed to simple numerical effects but must be due to direct interference between strains or indirect competition through host immune response. These results highlight the importance of genetic diversity within parasite populations for infection dynamics. Preliminary analysis of the evolutionary response data indicate that, contrary to common theory, the observed competition between strains did not lead to parasite virulence evolution over 60 parasite generations. This negative result is important because the experiment presented is the first direct empirical test of the long-standing hypothesis that within-host competition should lead to virulence changes in the strains involved.

Key words: competition, protozoa, fluorescence genes