Parasite-mediated selection against inbred Soay sheep in a free-living, island population

Parasites are thought to provide a selective force capable of promoting gen etic variation in natural populations. One rarely considered pathway for th is action is via parasite-mediated selection against inbreeding. If parasit es impose a fitness cost on their host and the offspring of close relatives have greater susceptibility to parasites due to the increased homozygosity that results from inbreeding, then parasite-mediated mortality may select against inbred individuals. This hypothesis has not yet been tested within a natural vertebrate population. Here we show that relatively inbred Soay s heep (Ovis aries), as assessed by microsatellite heterozygosity, are more s usceptible to parasitism by gastrointestinal nematodes, with interactions i ndicating,greatest susceptibility among adult sheep at high population dens ity. During periods of high overwinter mortality on the island of Hirta, St . Kilda, Scotland, highly parasitised individuals were less likely to survi ve. More inbred individuals were also less likely to survive, which is due to their increased susceptibility to parasitism, because survival was rando m with respect to inbreeding among sheep that were experimentally cleared o f their gastrointestinal parasite burden by anthelminthic treatment. As a c onsequence of this selection, average microsatellite heterozygosity increas es with age in St. Kildan Soay sheep. We suggest that parasite-mediated sel ection acts to maintain genetic variation in this small island population b y removing less heterozygous individuals.