Does inbreeding and loss of genetic diversity decrease disease resistance?

Inbreeding and loss of genetic diversity are predicted to decrease the resistance of species to disease. However, this issue is controversial and there is limited rigorous scientific evidence available. To test whether inbreeding and loss of genetic diversity affect a host's resistance to disease, Drosophila melanogaster populations with different levels of inbreeding and genetic diversity were exposed separately to ( a) thuringiensin, an insecticidal toxin produced by some strains of Bacillus thuringiensis, and (b) live Serratia marcescens bacteria. Inbreeding and loss of genetic diversity significantly reduced resistance of D. melanogaster to both the thuringiensin toxin and live Serratia marcescens. For both, the best fitting relationships between resistance and inbreeding were curvilinear. As expected, there was wide variation among replicate inbred populations in disease resistance. Lowered resistances to both the toxin and the pathogen in inbred populations were due to specific resistance alleles, rather than generalized inbreeding effects, as correlations between resistance and population fitness were low or negative. Wildlife managers should strive to minimise inbreeding and loss of genetic diversity within threatened populations and to minimise exposure of inbred populations to disease.