Genetic differences in the interactions of a microsporidian parasite and four clones of its cyclically parthenogenetic host

SUMMARY Host–parasite interactions were studied for the microsporidiumPleistophora intestinalisand its host,Daphnia magna. Two host clones were established from the same population from which the parasites were taken (home-1 and 2), and two clones from two other ponds (neighbour and Munich clone). With increasing clutch number infected females from home-1 clone produced relatively smaller clutches than uninfected females. Age and body length at maturity were not affected by the infection, but body length of the sixth adult instar was reduced. In an experiment including all four host clones, the parasite reproduced well in the two home clones and in the neighbour clone, but poorly in the Munich clone. Juvenile growth and age at maturity was not affected in the two home clones, but for the neighbour and the Munich clone age was delayed by 2·2 days and 4·1 days, and juvenile growth reduced by 16 and 23%, respectively. Significant host-clone x parasite-treatment interactions were also found for size at maturity and clutch size. This pattern of host-parasite interactions suggests that there is no general positive relation between disease severity and parasite multiplication rate.