RNA-seq was used to genotype Plasmodium relictum infections and demonstrated the broad genetic diversity within the mitochondrial lineage and links to virulence.
The globally transmitted avian malaria parasite Plasmodium relictum (lineage SGS1) have been found to infects hundreds of different bird species with differences in infection outcomes ranging from more or less latent to potential mortal. In this study, we used two different isolates of SGS1, obtained in the wild from two different host species to investigate differences in mortality, parasitemia, and host blood parameters under experimental infections. The two isolated showed difference in mortality as well as in effects on the blood chemistry of its hosts. Simultaneously 258kb of the parasite genome was screened for genetic differences using parasite mRNA and compared between experimental groups.  Although previous studies using single genes have shown very limited within lineage genetic diversity in the European population of SGS1 we found, across 322 genes, 226 SNPs which clearly separating the two inoculation groups with a total of 23 SNPs that where fixed in either of the inoculation groups. Moreover, genetic variation was found within each inoculation group hinting towards that each avian malaria infection harbors standing genetic variation that might be selected upon during each infection episode.  Overall, these results highlight that there exists extensive genetic variation within the SGS1 population that are transferred into individual infections, thus adding to the complexity of the infection dynamics seen in these host-parasite interactions. Simultaneously the results open up for the possibility to understand how genetic variation within the parasite populations is linked to often observed variation in infection outcome observed during infection experiment and out in the wild.