PARASITE-ASSOCIATED POLYMORPHISM IN A CYCLIC UNGULATE POPULATION

An unmanaged population of Soay sheep living on Hirta, St Kilda, Scotl and is persistently unstable, fluctuating between about 600 and 1600 i ndividuals. Population crashes occurring approximately every 3 years a re primarily due to winter food shortage. In this paper we show that s heep experimentally relieved of their gastrointestinal nematodes (pred ominantly Teladorsagia spp.) survived a crash better than matched cont rols, showing that nematode parasites contribute to the probability th at a sheep dies in a crash. We also show that over three successive cr ashes mortality was significantly different between individuals of the three different genotypes at the diallelic adenosine deaminase locus (Ada). FF animals were most likely to die, SS animals had an intermedi ate probability of dying, and FS animals were least likely to die. Fin ally, three independent lines of evidence suggest that nematode burden s differ between the three Ada genotypes. First, in August, heterozygo us females are less likely to have nematode eggs in their faeces than homozygous females. Second, at lambing, the periparturient rise in fae cal egg count was highest in homozygous FF individuals. Finally, durin g the Autumn mating season, heterozygous males has lower faecal egg co unts than homozgyotes, although this relation was complicated by inter actions with year and age of male. These results are consistent with t he idea that Ada allele frequencies are maintained in the sheep popula tion by parasite-associated selection.