| Abstract: | Marine mammals are often reported to possess reduced variation of majorhistocompatibility complex (MHC) genes compared with their terrestrial counterparts. Weevaluated diversity at two MHC class II B genes, DQB and DRB, in the NewZealand sea lion (Phocarctos hookeri, NZSL) a species that has suffered highmortality owing to bacterial epizootics, using Sanger sequencing and haplotypereconstruction, together with next-generation sequencing. Despite this species''prolonged history of small population size and highly restricted distribution, wedemonstrate extensive diversity at MHC DRB with 26 alleles, whereas MHCDQB is dimorphic. We identify four DRB codons, predicted to beinvolved in antigen binding, that are evolving under adaptive evolution. Our data suggestdiversity at DRB may be maintained by balancing selection, consistent with therole of this locus as an antigen-binding region and the species'' recent history ofmass mortality during a series of bacterial epizootics. Phylogenetic analyses ofDQB and DRB sequences from pinnipeds and other carnivores revealedsignificant allelic diversity, but little phylogenetic depth or structure among pinnipedalleles; thus, we could neither confirm nor refute the possibility of trans-speciespolymorphism in this group. The phylogenetic pattern observed however, suggests somesignificant evolutionary constraint on these loci in the recent past, with the patternconsistent with that expected following an epizootic event. These data may help furtherelucidate some of the genetic factors underlying the unusually high susceptibility tobacterial infection of the threatened NZSL, and help us to better understand the extentand pattern of MHC diversity in pinnipeds. |
