Cost of resistance: relationship between reduced fertility and increased resistance in a snail-schistosome host-parasite system

Natural host populations often exhibit genetic variability in resistance to parasitism. One possible mechanism for maintaining such diversity is a tra de-off between fitness costs associated with resistance and fitness costs a ssociated with parasitism. However, little is known about the nature or mag nitude of these costs in animal populations. Using artificial selection exp eriments in a Biomphalaria glabrata-Schistosoma mansoni hose-parasite syste m, we demonstrated that resistance and susceptibility to infection are heri table. We then investigated whether resistance had any associated costs in terms of snail reproductive success. Susceptible-selected snail lines showe d significantly higher fertility (number of offspring produced) than resist ant-selected or unselected control snail lines, irrespective of current inf ection status. There were no consistent differences between snail lines in fecundity proportion of abnormal egg masses produced, or mean number of egg s per egg mass. Mortality rare was higher among infected than uninfected sn ails. These results are consistent with snails incurring costs of resistanc e to schistosome infection in the absence of the parasite.