Biology Reference
In-Depth Information
D
ISCUSSION
In the present study, 16 different alleles were identified from the theoretical translation of
the exon 2
MHC DQB
locus in 60
Inia
dolphins. The sequences do not show in/del mutation
or stop codons. Thus, they might be functional ―in vivo‖. Additional evidence of functionality
is the existence of high mutation spots on PBR creating variation in the binding site (Edwards
& Hedrick 1998) that may be the result of environmental selective pathogenic pressure (Klein
et al., 1993; Hedrick 1994; Bergstrom & Gyllensten, 1995).
Ten out of 17 polymorphic sites, 10 sites (5, 9, 18, 36, 37, 39, 43, 49, 50 and 51) were
non-conservatives due the physicochemical properties of the residues, which would indicate
changes in the recognition of foreign peptides by the PBR. These hotspots show the effect of
balancing natural selection through over-dominance or frequency-dependent selection
(Hughes & Nei, 1989; Garrigan & Hedrick, 2003)
Site 49 changed from a long acid hydrophilic residue in group 01 to a basic residue on
group 02. In position 43, a small hydrophobic residue, characteristic of group 03, was
replaced by a basic one in groups 01 and 02. And site 37 changed from a small hydrophilic
residue on group 03 to an aromatic one on groups 01 and 02. These changes could be
evidence of distinct antigen recognition between the allele groups.
It is important to note that, although there were many private (unique alleles found in
only one population of that studied) alleles, most of the amplified products (60.8%) were the
four most frequent alleles:
Inge
DQB -0101, -0201, -0203 y -0204. These alleles could give
some kind of adaptive advantage to animals that carry them or reflect trans-specific
polymorphism of MHC genes (Munguia-Vega et al., 2007; Xu et al., 2008).
The results of the exon 2
MHC DQB
locus revealed basic aspects of the evolutionary and
demographic history which agree quite well with that revealed by other molecular markers.
The Bolivian populations showed the lowest polymorphic level of the three basin populations
studied. By using two mtDNA genes (Banguera-Hinestroza et al., 2002; Ruiz-García et al.,
2009a), nuclear DNA microsatellites (Ruiz-García, 2007, 2010; Ruiz-García et al., 2010b),
autosome and Y chromosome intron sequences (Ruiz-García et al., 2008) and RAPDs (Ruiz-
García et al., 2007), we demonstrated that the Bolivian populations was also the population
with the lowest gene diversity. In parallel, the Bolivian basin individuals yielded the highest
percentage of private alleles (33%) when compared with the number of private alleles found
in the upper Amazon (18%) and Orinoco basins (22%). These results show that the Bolivian
population came from a strong founder event that implied a bottleneck and that these original
animals proceeded from the Amazon basin. The existence of the highest level of private
alleles in the Bolivian rivers could show that the physical-chemical water characteristics and
the number and types of pathogens of the Beni-Mamoré waters are note worthily different
from those of the Amazon. Other interesting points revealed by molecular markers, were the
highest levels of polymorphism and internal genetic differences found within the Orinoco
basin. Ruiz-García (2010) and Ruiz-García et al., (2010a) showed that there are two different
mitochondrial lineages in the same rivers of the Orinoco basin and that proceeded from the
Amazon in two different epochs.
This could explain why in the Orinoco basin, we found the highest polymorphic level and
the highest internal divergence among the individuals. The existence of a lower percentage of
private alleles in the Orinoco than in the Bolivian rivers could be because the separation of
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