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 396

extracellular proteins, including those from bacteria and other pathogens and parasites (Klein
& Horejsi, 1997; Dengjel et al., 2005). The ability of both class I and II genes to face various
pathogens is believed to be mainly associated with sequence variation among MHC alleles in
the functionally important peptide-binding region or PBR which is responsible for antigen
recognition (Ohta, 1998; Hughes & Yeager, 1998). Variation within the PBR suggests that
there has been evolutionary pressure for organisms to combat a wide range of immunological
challenges (Abbott et al., 2006). 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. Certain MHC loci exhibit an
extensive genetic polymorphism in most vertebrate species studied so far (Parham & Ohta,
1996; Babik et al., 2005; Sachdev et al., 2005). Despite the extensive polymorphism within
species, a remarkable sharing of polymorphic sequence motifs even identical alleles have
been observed between different mammalian species (Gustafsson & Andersson, 1994;
Kriener et al., 2000; Otting et al., 2002; Suárez et al., 2006; Huchard et al., 2006). Three
possible mechanisms have been put forward to explain this phenomenon. First, the similarity
of alleles between related species can be explained by their common ancestry — the
persistence of allelic lineages through speciation and their passage from species to species
(Klein, 1987). This ―trans-species polymorphism‖ has been documented to occur in primate
(Otting et al., 2002; Huchard et al., 2006), artiodactyl (Sena et al., 2003), and cetacea (e.g.,
Hayashi et al., 2003). The second mechanism for convergent evolution, the occurrence of
convergent evolution at the amino acid sequence level, has been controversial (Doolittle,
1994). Convergent evolution is the emergence of biological structures or species that exhibit
similar function and appearance but that evolved through widely divergent evolutionary
pathways (Gustafsson & Andersson, 1994; Hughes, 1999). The similarities that are shared in
the case of convergent evolution are not the result of evolution from a common ancestor
sharing those similarities. Instead, the similarities are typically explained as the result of
common adaptive solutions to similar environmental pressures (Kriener et al., 2000).
However, evidence for molecular convergence is either lacking or disputed (Doolittle, 1994).
A third possibility is that the similarity has arisen by chance (Kriener enmjt al., 2000).
The baiji or Yangtze River dolphin ( Lipotes vexillifer ) is endemic to the Yangtze River of
China, and is probably the most threatened cetacean in the world (Reeves et al., 2003). It has
become a flagship species for the conservation of endangered aquatic animals and the entire
aquatic ecosystem. The baiji is a relict species and the only living representative of the family
Lipotidae (Rice, 1998). This species was listed as critically endangered in the International
Union for Conservation of Nature (IUCN) Red List of Threatened Species due to its very
low abundance and projected continuing decline (Reeves et al., 2003). The finless porpoise
( Neophocaena phocaenoides ) is a small cetacean widely distributed along the coast waters of
Indo-Pacific Oceans and the Yangtze River (Reeves et al., 1997). The Yangtze finless
porpoise, a sole freshwater population, is sympatric with the baiji in the middle and lower
reaches of the Yangtze River. Due to its unique and limited distribution in freshwater, its
small and rapidly declining population size and highly endangered status, and its special
adaptation to the freshwater environment, the Yangtze population has been categorized as
endangered in the IUCN Red List (Reeves et al., 2003). In addition, a systematic survey
recently conducted by a team of scientists from China, USA, and four other countries could
not find a single baiji during a 6-week search, which suggested that this species might have
gone extinct in the wild. Meanwhile, the abundance of the Yangtze finless porpoise was