Genetic diversity predicts pathogen resistance and cell-mediated immunocompetence in house finches

Abstract

Evidence is accumulating that genetic variation within individual hosts can influence their susceptibility to pathogens. However, there have been few opportunities to experimentally test this relationship, particularly within outbred populations of non-domestic vertebrates. We performed a standardized pathogen challenge in house finches (Carpodacus mexicanus) to test whether multilocus heterozygosity across 12 microsatellite loci predicts resistance to a recently emerged strain of the bacterial pathogen, Mycoplasma gallisepticum (MG). We simultaneously tested whether the relationship between heterozygosity and pathogen susceptibility is mediated by differences in cell-mediated or humoral immunocompetence. We inoculated 40 house finches with MG under identical conditions and assayed both humoral and cell-mediated components of the immune response. Heterozygous house finches developed less severe disease when infected with MG, and they mounted stronger cell-mediated immune responses to phytohaemagglutinin. Differences in cell-mediated immunocompetence may, therefore, partly explain why more heterozygous house finches show greater resistance to MG. Overall, our results underscore the importance of multilocus heterozygosity for individual pathogen resistance and immunity.

Figure 1

Multilocus heterozygosity predicts disease severity in response to infection with Mycoplasma gallisepticum. We scored the severity of inflammatory symptoms for 70 days post-inoculation and summed these scores as a measure of disease severity. Disease severity was square-root transformed prior to analysis.

Figure 2

Multilocus heterozygosity correlates with T-lymphocyte proliferation in response to phytohaemagglutinin (PHA) injection. We calculated PHA response as the ratio of post-injection (48 h) patagial width to pre-injection width.