1. ¹Department of Virology, Erasmus Medical Center, Erasmus University Rotterdam, Rotterdam, The Netherlands
2. ²Dutch Wildlife Health Center, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands
3. ³Department of Zoology, University of Cambridge, Cambridge, UK

Correspondence: Dr W Amos, Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK. E-mail: w.amos@cam.ac.uk

Received 10 October 2007; Revised 1 February 2008; Accepted 8 February 2008; Published online 9 April 2008.

Abstract

In several studies, heterozygosity measured at around 10 microsatellite markers correlates with parasite load. Usually the effect size is small, but while this may reflect reality, it may also be possible that too few markers are used or the measure of fitness contains too much error to reveal what is actually a much stronger underlying effect. Here, we analysed over 200 stranded harbour seals (Phoca vitulina) for an association between lungworm burden and heterozygosity, conducting thorough necropsies on the seals and genotyping the samples obtained for 27 microsatellites. We found that homozygosity predicts higher worm burdens, but only in young animals, where the worms have the greatest impact on fitness. Testing each locus separately, we found that a significant majority reveal a weak but similar trend for heterozygosity to be protective against high lungworm burden, suggesting a genome-wide effect, that is, inbreeding. This conclusion is supported by the fact that heterozygosity is correlated among markers in young animals but not in otherwise equivalent older ones. Taken as a whole, our results support the notion that homozygosity increases susceptibility to parasitic infection and suggest that parasites can be effective in removing inbred individuals from the population.