Heritable variation in resistance to gastro-intestinal nematodes in an unmanaged mammal population

The impact of parasites on natural populations has received considerable at tention from evolutionary biologists in recent years. Central to a number o f theoretical developments during this period is the assumption of additive genetic variation in resistance to parasites. However, very few studies ha ve estimated the heritability of parasite resistance under field conditions , and those that have are mainly restricted to birds and their ectoparasite s. In this paper, to our knowledge, we show for the first time in a free-ra nging mammal population, Soay sheep (Ovis aries) living on the islands of S t Kilda, that there is significant heritable variation in resistance to gas trointestinal nematodes. This result is consistent with earlier studies on this population which have indicated locus-specific associations with paras ite resistance. We discuss our results in the context of current studies ex amining heritable resistance to parasites in domestic sheep and the possibl e mechanisms of selective maintenance of genetic variation for resistance t o gastrointestinal nematodes in the St Kilda Soay sheep population.