Patterns of immunogenetic and malarial parasite diversity in three species of Vietnamese passerines

Understanding how genetic diversity in host immune genes affects susceptibility to pathogen infections is crucial for deciphering host-pathogen coevolutionary dynamics. In this study, we investigated the genetic diversity of mitochondrial cytochrome b (cyt b) and key immune genes including Toll-like receptors (TLR3 exon 4 and TLR4 exon 3) and the major histocompatibility complex (MHC class I exon 3) in three sympatric passerine bird species (Arachnothera longirostra, Copsychus malabaricus, and Pitta moluccensis) from Southeast Asia, includinh their associations with Plasmodium infections. Our cyt b analyses indicated that A. longirostra experienced past population expansions, evidenced by significantly low Tajima's D and Fu's Fs values, whereas C. malabaricus and P. moluccensis maintained relatively constant effective population sizes. Immune gene diversity revealed unique profiles across the three species: MHC class I genes displayed positive Tajima's D values, suggesting balancing selection and possible historical bottlenecks, particularly in A. longirostra and P. moluccensis. Selection analyses showed that MHC genes are primarily under purifying selection with species-specific codons under diversifying selection. TLR genes were predominantly governed by purifying selection, with minor instances of diversifying selection noted. Generalized linear models demonstrated that the total number of MHC supertypes did not significantly influence Plasmodium infection status across passerine species, providing no support for the heterozygote advantage hypothesis. However, in P. moluccensis, specific MHC supertypes were significantly associated with Plasmodium operational taxonomic units (OTUs), indicating a possible rare-allele advantage in MHC-mediated malaria resistance. These findings highlight the complex interplay between host genetic diversity and parasite infection, suggesting that balancing selection on immune genes may vary even among sympatric species facing similar pathogen pressures.