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Table 3. Pairwise F ST (below diagonal) and ST (above diagonal) values for Control
Region between two riverine Sotalia population units (population units as indicated by
the SAMOVA analysis, Figure 1). Probability values based on 10,000 permutations
shown in italics. Significantly different values ( P < 0.05) in bold. Haplotype ( h ) and
nucleotide ( ) % standard deviation (SD) are shown on the diagonal for each
population unit.
Western
Amazon
Eastern
Amazon
ST
F ST
Western Amazon (n=13)
0.1288
(0.0468 )
h = 0.6026  0.08
 = 0.48  0.0033
Eastern Amazon (n=11)
0.0275
(0.1439)
h = 0.7636  0.08
 = 0.39  0.0026
T HOUGHTS AND C ONCLUSION
Population Structure
Less regional structure was found among the riverine population units compared to
coastal population units (Caballero, 2006). Although the Western Amazon and Eastern
Amazon population units share only two haplotypes, shorter genetic distances separate all CR
lineages. This could be due to the relatively shorter evolutionary history of Sotalia fluviatilis
when compared to the possibly longer evolutionary history of the coastal species (Caballero
et al., 2007). Higher levels of gene flow could also be expected between the Amazonian
population units due to the scattered distribution of small groups of individuals along the
main channels and tributaries of the Amazon River. Interestingly, in our study, significant
statistical differences were obtained at the  ST level between the two population units
considered in the AMOVA analysis (Table 3). This might be due to the presence of a few
very distinctive haplotypes with several nucleotide differences between these population
units.
The preliminary haplotype genealogy confirmed these findings, suggesting that
haplotypes X, S and T may be ancestral, considering that they are geographically widespread,
are connected to a higher number of other haplotypes, they have high haplotype frequencies,
and are located in a central position (Castelloe & Templeton, 1994). Also, it can be observed
that haplotypes EE and DD are more divergent. This is an interesting finding, since
haplotypes X, S and T were determined in samples collected along the main channel of the
Amazon River and also in some tributaries located centrally along the distribution of Sotalia
fluviatilis (Tefé, Puerto Nariño, Caquetá River) while haplotypes DD and EE were
determined in samples from locations located in the extremes of the distribution, for example
the Cuyabeno River (EE) and Santarém (DD). This result can be reflecting patterns of
connectivity among different Amazonian tributaries and channels with increasing haplotype
and population differentiation in more isolated tributaries. More sampling along other
Amazon River tributaries is required to describe with confidence, population units for Sotalia
fluviatilis as well as haplotype genealogies.
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