The potential for arms race and Red Queen coevolution in a protist host-parasite system

Abstract

The dynamics and consequences of host-parasite coevolution depend on the nature of host genotype-by-parasite genotype interactions (G×G) for host and parasite fitness. G×G with crossing reaction norms can yield cyclic dynamics of allele frequencies (`Red Queen¿ dynamics) while G×G where the variance among host genotypes differs between parasite genotypes results in selective sweeps (`arms race¿ dynamics). Here we investigate the relative potential for arms race and Red Queen coevolution in a protist host-parasite system, the dinoflagellate Alexandrium minutum and its parasite Parvilucifera sinerae. We challenged 9 different clones of A. minutum with 10 clones of P. sinerae in a fully factorial design and measured infection success and host and parasite fitness. Each host genotype was successfully infected by four to ten of the parasite genotypes. In bottles where infection was successful, the fitness (growth rate) of the host was severely reduced compared to un-inoculated controls. In contrast, in bottles where infection was unsuccessful, host growth rate was higher than in uninoculated controls. There were strong G×Gs for infection success, as well as both host and parasite fitness. About 3/4 of the G×G variance components for host and parasite fitness were due to crossing reaction norms. There were no general costs of resistance or infectivity. We conclude that there is high potential for Red Queen dynamics, but also some scope for selective sweeps in this system