HIGH LEVEL OF MHCPOLYMORPHISM IN THE BAIJI AND FINLESS PORPOISE, WITH SPECIAL REFERENCE TO POSSIBLE CONVERGENT ADAPTATION TO THE FRESHWATER YANGTZE RIVER - Biology, Evolution and Conservation of River Dolphins Within South America and Asia - page 395

Chapter 21
H IGH L EVEL OF MHC P OLYMORPHISM IN THE B AIJI
AND F INLESS P ORPOISE , WITH S PECIAL R EFERENCE
TO P OSSIBLE C ONVERGENT A DAPTATION TO THE
F RESHWATER Y ANGTZE R IVER
Shixia Xu, Wenhua Ren, Kaiya Zhou and Guang Yang 1
Jiangsu Key Laboratory for Biodiversity and Biotechnology, College of Life Sciences,
Nanjing Normal University, Nanjing, China
We surveyed the sequence variability at exon 2 of the MHC class I and class II ( DRA and
DQB ) genes in the baiji ( Lipotes vexillifer ) and finless porpoise ( Neophocaena
phocaenoides ). Little sequence variation was detected at the DRA locus whereas considerable
variation was found at DQB and MHC-I. Three exon 2 MHC loci of the baiji revealed striking
similarity with those of the finless porpoise. Some identical alleles shared by both species at
the MHC-I and DQB loci suggest that convergent evolution as a consequence of common
adaptive solutions to similar environmental pressures in the Yangtze River. As for the DRA
locus, the identical alleles were shared not only by baiji and finless porpoise but also by some
other cetacean species of the families Phocoenidae and Delphinidae, suggesting trans-species
evolution of this gene.
Keywords : Lipotes vexillifer ; Neophocaena phocaenoides ; MHC; trans-species evolution;
convergent evolution
I NTRODUCTION
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
(Klein, 1986). Two major groups of MHC genes can be distinguished, i.e., Class I and II. The
fundamental role of class I genes is to recognize antigens from intracellular proteins,
including those from viruses. The primary role of class II genes is to recognize antigens from