WITHIN-POPULATION AND BETWEEN-POPULATION VARIATION FOR RESISTANCE OF DAPHNIA-MAGNA TO THE BACTERIAL ENDOPARASITE PASTEURIA-RAMOSA

Genetic variation among hosts for resistance to parasites is an import ant assumption underlying evolutionary theory of host and parasite evo lution. Using the castrating bacterial parasite Pasteuria ramosa and i ts cladoceran host Daphnia magna, we examined both within- and between -population genetic variation for resistance. First, we tested hosts f rom four populations for genetic variation for resistance to three par asite isolates. Allozyme analysis revealed significant host population divergence and that genetic distance corresponds to geographic distan ce. Host and parasite fitness components showed strong genetic differe nces between parasite isolates for host population by parasite interac tions and for clones within populations, whereas host population effec ts were significant for only a few traits. In a second experiment we t ested explicitly for within-population differences in variation for re sistance by challenging nine host clones from a single population with four different parasite spore doses. Strong clone and dose effects we re evident. More susceptible clones also suffered higher costs once in fected. The results indicate that within-population variation for resi stance is high relative to between-population variation. We speculate that P. ramosa adapts to individual host clones rather than to its hos t population.