The Major Histocompatibility Complex (MHC) is a suite of genes which first evolved in ancestral jawed vertebrates and now forms a key part of the adaptive immune system. This region exhibits high allelic polymorphism across many vertebrate taxa, and its high diversity and adaptability could make it a valuable marker for addressing both population-level patterns and broader ecological questions. However, MHC sequence resources remain limited across elasmobranchs, with characterization restricted to a relatively small number of species, limiting comparative understanding of MHC diversity in the group. We therefore expanded these resources by characterizing allele diversity and putative gene copy number of the MHC class II α and β subunits in four shark species for which MHC data have not previously been described: the coastal-pelagic tiger shark (Galeocerdo cuvier), shallow-water Atlantic sharpnose shark (Rhizoprionodon terraenovae), and the deep-sea Cuban dogfish shark (Squalus cubensis) and Genie's dogfish shark (Squalus clarkae). Our results indicate high levels of diversity in both MHC II α and β exon 2 regions across all four species that is equal to or greater than microsatellite loci, along with the presence of 1-3 putative gene copies in both subunits. Overall, the high genetic diversity observed in the functional MHC class II α and β genes provides a foundation for future comparative and evolutionary studies, while also supporting potential applications in conservation genetics.
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