Major histocompatibility complex class II genes allele diversity in landlocked seals.

This article focuses on the allelic diversity of major histocompatibility complex class II (MHC-II) genes—specifically exon 2 of the DQB gene and exon 3 of the DRB gene—in three species of the genus *Pusa*: the landlocked Baikal seal (*Pusa sibirica*), Caspian seal (*Pusa caspica*), and the widespread Arctic ringed seal (*Pusa hispida*). The study found that while all three species exhibit high allelic diversity in exon 2, the Baikal seal shows a unique pattern characterized by fewer total alleles in the population but a higher number of alleles per individual and greater similarity among individual genotypes, likely reflecting the spatial and temporal homogeneity of Lake Baikal's environment. In exon 3, which encodes a more conserved β2 domain, the Baikal seal has the greatest number of amino acid sequences per individual, whereas the Caspian seal has the fewest variants but more species-specific sequences, suggesting a longer independent evolutionary history. These differences in MHC-II gene diversity among the seals are interpreted as adaptations to their distinct environmental conditions and pathogen loads, with implications for understanding their immune system evolution and population history.