关键词: 平衡选择, 遗传变异, 中华白海豚, 跨种进化

Abstract: The major histocompatibility complex (MHC)consists of a group of closely linked genes that constitute the most important genetic component of the mammalian immune system. Exon 2 of three MHC loci,i. e. DQB and DRA of MHC class II,and class I,were chosen to preliminarily characterize the genetic variability of the Indo-Pacific humpback dolphin (Sousa chinensis),an endangered species found in coastal China. The DRA,DQB and MHC-I loci each contained two, two,and seven alleles,respectively. Little sequence variation was detected at the DRA locus,relatively higher at the DQB, but considerable sequence variation at the MHC-I. Relatively high rates of non-synonymous (dN )vs. synonymous (dS ) substitution in the antigen binding sites (ABS)suggested balancing selection for maintaining polymorphisms at the MHC-I and DQB loci,although this result was not supported by data from the DRA locus. Phylogenetic reconstruction suggested that three MHC loci exon 2 sequences are not separated according to species,but are intermixed with other species,which was consistent with the trans-species evolution model of the MHC.

Key words: Balancing selection, Genetic variation, Indo-Pacific humpback dolphin (Sousa chinensis), Trans-species evolution