Adaptive landscape genetics and malaria across divergent island bird populations

Environmental conditions play a major role in shaping the spatial distributions of pathogens, which in turn can drive local adaptation and divergence in host genetic diversity. Haemosporidians, such as Plasmodium (malaria), are a strong selective force, impacting survival and fitness of hosts, with geographic distributions largely determined by habitat suitability for their insect vectors. Here, we have tested whether patterns of fine‐scale local adaptation to malaria are replicated across discrete, ecologically differing island populations of Berthelot's pipits Anthus berthelotii. We sequenced TLR4, an innate immunity gene that is potentially under positive selection in Berthelot's pipits, and two SNPs previously identified as being associated with malaria infection in a genome‐wide association study (GWAS) in Berthelot's pipits in the Canary Islands. We determined the environmental predictors of malaria infection, using these to estimate variation in malaria risk on Porto Santo, and found some congruence with previously identified environmental risk factors on Tenerife. We also found a negative association between malaria infection and a TLR4 variant in Tenerife. In contrast, one of the GWAS SNPs showed an association with malaria risk in Porto Santo, but in the opposite direction to that found in the Canary Islands GWAS. Together, these findings suggest that disease‐driven local adaptation may be an important factor in shaping variation among island populations.     

Comparative evidence for a positive correlation between haematozoan prevalence and mortality in waterfowl

Hamilton and Zuk (1982) suggested a new hypothesis to explain the evolution of secondary sexual characters, namely that male ornaments may represent honest signals of male capacity to resist parasites. Since then several authors have addressed the question of the role played by parasites in the evolution of host secondary sexual characters. Comparative analyses of Hamilton and Zuk's hypothesis have, however, failed to show the predicted positive association between parasitism levels and sexually-selected traits. One possible explanation for this discrepancy involves a low pathogenicity of the parasites used to test the hypothesis. Here we show that haemosporidians are positively correlated with mortality rates in a sample of waterfowl, and that the correlation between the two variables is of similar magnitude in both males and females. Contrary to the expectations based on the Hamilton and Zuk's hypothesis, we did not find any correlation between plumage brightness and haemosporidian prevalence. These results indicate that haemosporidians may have a pathogenic effect on their avian hosts and that plumage brightness is likely to have evolved in response to ecological factors other than parasitism infections.