AMAZON RIVER DOLPHIN POLYMORPHISM AND POPULATION DIFFERENTIATION OF MHC CLASS II PEPTIDES - Biology, Evolution and Conservation of River Dolphins Within South America and Asia - page 119

1999; Carrington et al., 1999). This last concept has motivated studies on endangered
mammal species living in small populations, that due to their expected low MHC variation,
were also expected to be more susceptible to infectious disease, and thus to extinction
(Evermann et al., 1988; Lyles & Dobson 1993; Mikko et al., 1999).
These, and other recent studies, have helped to built the widely accepted, but somewhat
contentious perception, that MHC variation is related to disease resistance in wild vertebrates
(Hedrick & Miller 199; Gutierrez-Espeleta et al., 2001; Garrigan & Hedrick 2003; Aguilar et
al, . 2004; Hedrick 2004). A major consensus is that, with rare exceptions, MHC variation is
strongly influenced by selection, albeit it is more common to detect that selection if it has
operated at some point throughout evolutionary time rather than in the extant generations.
Most importantly, contentious perceptions point out that we know little on the evolution of
host parasite molecular interactions in wild vertebrates as a whole and less on the vertebrate-
sub-group of marine and freshwater cetaceans.
Early studies found low MHC polymorphism in fin and blue whales, which was attributed
to the weak pressure of marine pathogens (Trowsdale et al . 1989). However, more recent
studies found a considerable amount of MHC -II polymorphisms in Beluga whales
( Delphinapterus leucas ) , Narwhals ( Monodon monoceros ) and other cetaceans like the finless
porpoise ( Neophocaena phocaenoides ) and southern minke whales ( Balaenoptera
bonaerensis ) (Murray et al ., 1995; Murray & White 1998; Hayashi et al ., 2003).
On the other hand, the unique freshwater dolphin studied from this point of view showed
a high variability in the class II MHC DQB gene. This was the case of the recently extinct
Yangtze River dolphin Lipotes vexillifer (Yang et al., 2005). Eighteen animals were analyzed
and these authors found the highest level of polymorphism at this gene comparing with other
cetacean species. They proposed that the high polymorphism could be a consequence of the
adaptation to freshwater, although environmental studies are insufficient to conclude that the
freshwater systems have higher levels of pathogens than the marine ones (Yang et al., 2005).
Additionally, the MHC genes are related to sexual selection because female mammals
could detect males with adequate MHC genotypes, which could increase the fitness and
survival of their litters by augmenting their heterozygosity levels (and therefore an augment
of protection against external pathogens) at these MHC genes. This has been demonstrated for
rodents (Potts et al., 1991) as well as for humans (Wedeking et al., 1995; Wedeking &
Seebeck, 1996). In an identical sense, MHC genes are related to kin recognition (Beauchamp
et al., 1985; Manning et al., 1992; Potts & Wakeland, 1993; Brown & Eklund, 1994).
Furthermore, homozygosity in some MHC genes is correlated with lower fertility and higher
rates of spontaneous abortion (Weckstein et al., 1991; Ober et al., 1992; Ober, 1995; Balasch
et al., 1993; Ho et al., 1994).
The main goal of this study was to infer, using peptide sequence analyses, the
evolutionary and adaptative potential of river dolphins populations in each of the three
aforementioned Inia populations, because their different river environments suggest that they
have been submitted to different pathogen pressures and parasite virulences.