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 118

I NTRODUCTION
Inia , the Amazon pink river dolphin, is the largest river dolphin and has the most stable
and largest population of all the world's freshwater dolphins. Its distribution includes three
major basins in northern South America: the Beni-Mamoré, in the Bolivian Amazon, and the
Amazon and Orinoco. Traditionally, Inia was considered a monospecific genus with only one
species, Inia geoffrensis (de Blainville, 1817), and three subspecies, Inia geoffrensis
boliviensis , I. g. geoffrensis , and I. g. humboldtiana , distributed within each of the major
basins. Nevertheless, some molecular and morphological studies suggest that, at least, the
Bolivian morphotype could be a different and an allopatric species, separated by 400 km of
falls and rapids in the Madeira River in Bolivia and Brazil (Pilleri & Gihr 1977; Banguera-
Hinestroza et al., 2002; Ruiz-García, 2007, 2010; Ruiz-García et al., 2006, 2007, 2008).
The waters that flow throughout these rivers in the overall Amazon and Orinoco basins
have two different origins. The white rivers have their origin in the Andean mountains and
have high levels of suspended sediments and organic resources, while the black rivers
originate in the lowlands and have low productivity because of their high concentration of
tannins that produce low pH values and few organic resources, although some black waters,
such as those from the Negro River watershed in Brazil can have astonishing biological
richness (Goulding et al., 1988).
Traditionally, it was believed that Inia could not live in black rivers, but some studies in
Venezuela suggest that the dolphins even inhabit the Casiquiare Channel (Pilleri & Pilleri,
1982; Borobia 1990, Meade & Koehnken, 1991; Romero et al., 2001) that has been
considered as a barrier between Inia geoffrensis humboldtiana (Orinoco subspecies) and I. g.
geoffrensis (Amazon subspecies) due to the extreme low pH and low productivity. So, these
dolphins are exposed to different environments and, probably, the pathogen pressure and
parasite virulence in these habitats is different too, because the habitats are quite distinct
displaying their own seasonal flooding and specific ecological settings, specific to black or
white tributaries. Furthermore, the amount of white and black water for any of the basins is
different; therefore we expect that the different Inia forms have different and specific
adaptations to live in those basins.
To solve this question we may use different molecular markers, but neutral
polymorphisms (as mitochondrial DNA, nuclear microsatellites, and autosome or Y
chromosome introns) do not provide precise information on how selection operates on
individual interaction with the environment nor their ability to adapt to environmental change,
two relevant issues for conservation (Meyers & Bull 2002; van Tienderen et al., 2002;
Crandall et al., 2000). In such cases, it may turn profitable to analyze genes potentially
subjected to selection (Cohen 2002; Koskinen et al., 2002), such as those of the major
histocompatibility complex ( MHC ). MHC genes are the most variable genes among
vertebrates (Robinson et al., 2003) due to high non-synonymous substitution rates that change
the amino acid sequence and physiochemical traits of MHC proteins (Hughes & Nei, 1988).
Studies on MHC polymorphism and evolution in humans and certain primate populations
are numerous ( e.g. Klein et al., 1993; Bergström et al., 1999; Bontrop et al., 1999) and point
out the benefits of MHC variation in humans and macaques (Black & Hedrick 1997; Dorak et
al., 2002; Sauermann et al., 2001) and the importance of certain MHC alleles for eliciting
efficient immune responses ( e.g. Hill et al., 1991; Jepson et al., 1997; Hill 1999; Thursz et al.,