Genetic Variation Of The MHC DQB Locus In The Finless Porpoise (Neophocaena Phocaenoides Sunameri) In Bohai Sea And Northern Yellow Sea

The major histocompatibility complex (MHC) is a large multigene coding for glycoproteins that play a key role in the initiation of immune responses in vertebrates. In the present study, MHC class II DQB locus was analyzed (including part of the putative peptide-binding region, PBR) in 170 finless porpoises (Neophocaena phocaenoides sunameri) in Bohai Sea and Northern Yellow Sea. Exon 2 of DQB locus was amplified via the polymerase chain reaction, followed by single-stranded conformation polymorphism analysis (SSCP). SSCP silver staining with standard conditions produced complex, variable profiles that did not show a simple correspondence to alleles inferred from direct sequencing.Therefore, to evaluate potential amplification of duplicate loci, PCR products from 52 finless porpoise were cloned and sequenced. Among the 172bp nucleotide sequences of more than 400 clones, 22 variable sites were determined and 23 unique alleles were identified, with most of the individuals holding more than 3 alleles, suggesting duplication of DQB gene. Except for one delete found in one allele, others were in continuous open reading frames, with no start or stop codons observed. Duplication can be created by unequal crossing over, and insertion/deletion was considered to be the result of this mechanism.Compared with the low MHC variability detected in most other marine mammals, the variability of 12.8% (22/172) at the nucleotide level resulted in 26.3% (15/57) variability at the amino acid level, showing a relatively high MHC diversity.High rate of non-synonymous vs synonymous (d_N/d_S) substitution especially in the peptide-binding region (PBR) suggested strong local positive selection or balancing selection for maintaining high MHC diversity at the DQB loci.Phylogenetic reconstruction of DQB gene of the finless porpoise with other whales suggested trans-species evolution in the cetacean MHC gene. This also indicated that the DQB variation was maintained by balancing selection.Through the analysis of genetic variation in DQB locus, this study provided some valuable suggestions for the conservation and management of the finless porpoise.