Sequence Variability And Evolution Of MHC Genes In Cetaceans

The major histocompatibility complex (MHC) region of vertebrates is a highly polymorphic genomic region with an important role in the regulation of the immune system and in the recognition and differentiation of self from nonself. MHC variability reflects evolutionary relevant and adaptive processes within and between populations and is very suitable to investigate a wide range of open questions in evolutionary ecology and conservation. In the presented research, sequence variability at exon 2 of one MHC Class I gene and two Class II genes (DRA and DQB) including part of the putative peptide-binding region (PBR) were investigated in 13 cetacean species, including systemic information on sequence variation examined in 195 finless porpoises (Neophocaena phocaenoides) and 15 baiji (Lipotes vexillifer) samples, and 51 samples in other 11 cetacean species. It is expected to have an in-depth understanding on the behavior of these molecules, especially sequence variability possibly caused by selection pressure. Findings from this study will provide the first opportunity to observe evolutionary relationships among three MHC genes from members of 13 cetacean species at three MHC loci, and especially to provide basis for its conservation genetics and immunogenetics.Sequence variability at three MHC loci was investigated by PCR, SSCP, cloning, sequencing as well as sequence analyses in 13 cetacean species. Little sequence variation was examined at the DRA locus, but considerable sequence variation at the DQB and MHC-I loci. No more than two sequences were found in each individual at the DRA locus, strongly suggesting only one DRA locus was amplified in this study. One DQB locus was also examined in 11 cetacean species, however, the finless porpoise and baiji have at least three DQB loci as manifested with three to five distinct sequences from some individuals. While for the MHC-I gene, sequence representing at least three MHC-I loci were identified from all cetacean species examined in this study.Phylogenetic reconstruction revealed that the MHC sequences were not separated according to species, but were intermixed among different species. Sequence comparison showed some alleles were shared by different cetacean species at three MHC loci so they grouped together in the gene trees. At the MHC-I and DQB loci, some identical alleles were shared by closely related species such as species included in the same family, suggesting trans-species pattern of evolution in the cetaceans. However, some distantly related species, such as the finless porpoise (included in the family Phocoenidae) and the baiji (included in the family Lipotidae), also shared some identical allele at these two loci, which may be explained by the convergent evolution as a consequence of common adaptive solutions to similar environmental pressures such as the freshwater Yangtze River. It may be the forces of balancing selection act to maintain similar allele motifs or identical alleles in a trans-species or convergent manner as indicated by relatively high rates of nonsynonymous (d_N) vs. synonymous (d_S) substitution in the PBR at the MHC-I and DQB loci. As for the DRA locus, the identical alleles were shared not only by the baiji and finless porpoise but by some other cetacean species, e.g. 10 species shared one DRA sequences, suggesting DRA gene was very ancient and may be maintained by trans-species evolution. In addition, the ratio of d_N/d_S was less than unity, suggesting that there was no balancing selection on this gene.