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of the contamination with the genotype PGM-BB increasing significantly, suggesting a
higher tolerance to radionuclides for individuals carrying this genotype compared to the
others. Phosphoglucomutase plays a critical role in glycolysis and gluconeogenesis, and
the efficiency of this enzyme to produce energy may potentially constitute a major target
for selective pressures induced by stress, either natural or anthropogenic. These selec-
tive pressures may lead to changes in the distribution of allelic and genotypic frequen-
cies in exposed populations. Benton et al. (1994) have also highlighted distinct genotypic
frequencies between populations of caddisfly larvae
Nectopsyche albida
from sites highly
contaminated by trace metals (arsenic, cadmium, chromium, copper, and zinc) and cad-
disflies from reference sites. Several other studies have found selective pressures acting on
allozyme markers in natural populations exposed to complex mixtures of contaminants
(Heithaus and Laushman 1997; Mulvey et al. 2002; Marchand et al. 2003; Virgilio et al.
2003). Allozyme markers studied were particularly enzymes involved in energy produc-
tion and/or general metabolism and for which the polymorphism detected may lead to
differential physiological performances for organisms exposed to stress (Lawrence et al.
2003). In a recent study, Yap et al. (2011) found correlations between the genetic diversity of
allozymes and the levels of heavy metals in the horseshoe crab
Carcinoscorpius rotundicauda
populations in polluted and unpolluted intertidal areas in Peninsular Malaysia, higher
heterozygosities being detected in the contaminated populations.
At the DNA level, strong modifications of allelic frequencies at the oncogene c-abl
(Wirgin et al. 1990) and at the cytochrome P4501A1 (Roy et al. 1995) loci were detected
in populations of fish (
Microgadus tomcod
) from the Hudson River compared to reference
sites. Kim et al. (2003) found that a particular haplotype at the mitochondrial ND6 locus
in the intertidal gastropod
Littorina brevicula
had a weaker frequency in contaminated sites
compared to several reference sites. Timmermans et al. (2007) suggested that the metallo-
thionein gene (which codes for a metal-binding protein) and its surrounding DNA region
were affected by environmental metal contamination in the collembolan
Orchesella cincta
.
Nadig et al. (1998) and Krane et al. (1999) have used a more general approach for iden-
tifying modifications: they worked on RAPD (Random Amplified Polymorphic DNA)
markers in perch (
Lepomis auritius
) and crayfish (
Orconectes rusticus
) exposed to complex
mixtures of contaminants (HAP, PCB, trace metals). They both found higher genetic simi-
larities between individuals from exposed populations than between individuals from
exposed and reference populations. Nadig et al. (1998) have also highlighted a significant
correlation between a particular genotype frequency in redbreast sunfish
L
.
auritus
and a
pollution gradient. The analysis of persistent fish populations living in chronically con-
taminated estuaries for more than 60 years in the United States and Europe, compared to
populations living in “pristine” estuaries, have provided new insights into the identifica-
tion of candidate genes potentially involved in the response of fish to chemical stress in
the field. The polymorphism of AhR 1 and AhR 2 loci in the killifish
Fundulus heteroclitus
(Hahn et al. 2004, 2005), and the polymorphism of p53, BHMT3, and PGDS in the European
flounder
Platichthys flesus
(Marchand et al. 2010), could be related to the possible tolerance
of populations faced with complex mixtures of pollutants. Hemmer-Hansen et al. (2007)
confirmed the great capacity of the European flounder to adapt to contrasted environ-
ments in estuaries; they highlighted the genetic basis of the adaptation of this species to
the particular environment of the Baltic Sea compared to the North Sea (gradient of salin-
ity, chemical stress, hypoxia, etc.), considering a candidate gene (
Hsc 70
), and neutral loci
(microsatellites).
However, the examples cited above, either with allozymes or DNA markers, observed
in different contexts (simple contamination or mixtures) showed only simple correlations
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