Does immunogenetic diversity warrant fewer parasites in European bluethroats?

The health and fitness of vertebrates are constantly challenged by environmental stresses that pose significant evolutionary pressures on immune genes. The bluethroat (Luscinia svecica) has been extensively studied for its haemosporidian parasites across Europe, showing high diversity and prevalence rate. The present study explored the genetic diversity and selection mechanisms of three immunity genes (MHC I exon 3, TLR3 and TLR4) in three European bluethroat populations. Specifically, we seek to (1) characterize the genetic diversity of these immunity genes, (2) detect selection signatures shaping their diversity, and (3) investigate the relationship between these immunity genes and the presence of haemosporidian parasites (Plasmodium and Leucocytozoon). Despite biogeographical differences, nucleotide and haplotype diversities were similar across all populations. Most frequent haplotypes were shared by both red-spotted (L. s. svecica) and white-spotted (L. s. cyanecula) bluethroat subspecies. The “insular” Krkonoše population, despite being biogeographically peripheral and experiencing minor inbreeding, did not show significantly reduced immunogenetic diversity. Selection analysis revealed a higher presence of purifying selection in TLR genes and a combination of purifying and diversifying selection in the MHC gene, reflecting their evolutionary constraints and functional importance, while haemosporidian parasite pressure was not a major driver of genetic diversity.