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and thus its adaptive potential. For these approaches, large laboratory experiments per-
formed on successive generations are essential and usually the genetic component of the
physiological trait associated with tolerance (i.e., its heritability) has to be studied (see
Section 14.4.2 for examples).
The second family of markers associated with genetic diversity concerns molecular
markers. Allozymes (enzymatic proteins) were the first markers to be used in populations
facing environmental stress. Allozyme loci may have several allelic variants with different
biochemical characteristics, in particular, molecular variations that induce variations in
charge or structure, leading therefore to differences in migration speed during electro-
phoresis. Considering several individuals and several allozyme loci, the number and the
frequency of the different alleles of the whole population may be estimated and its genetic
diversity characterized. Enzymatic polymorphism reveals about 30% of the DNA genetic
variability since many molecular mutations do not lead to variations in charge or struc-
ture of the subsequent protein. The “tolerance to contaminants/genotype” association of
allozyme loci has been encountered in many aquatic organisms since the 1980s, both in
freshwater and marine organisms. For some authors, allelic variants are important for
the determination of tolerance (Schluter 2000). For others, heterozygosity of particular loci
(Benton and Gutman 1992) or multiloci heterozygosity (Peles et al. 2003) are determinant
characters for tolerance. Allozyme markers can still be considered pertinent tools for the
evaluation of genetic variability in ecotoxicology (see Section 14.4.1 for examples), but more
often, current research considers genetic variation directly at the DNA level (genomic
approaches).
At the DNA level, two types of markers—neutral markers and markers under selection—
are important to consider in ecotoxicology. Neutral markers are loci not evolving directly
in response to selection, the dynamics of which are controlled mainly by mutation, genetic
drift, and migration. These markers are usually noncoding regions of the nuclear DNA (such
as microsatellites, introns, and intergenic regions) and mitochondrial loci (the control region
called the D-loop, the different subunits of NADH, etc.) and are useful in ecotoxicology:
• For identifying genetic structure in natural populations and, in particular, poten-
tial gene flow between chronically contaminated populations and reference
populations
• For exploring the genetic history of populations in different environmental
contexts
• For detecting the impact of drastic reduction in population size that can lead to the
loss of genetic variation through genetic drift
Their level of variability may, however, be affected by mutagen toxicants (see Section
14.4.1.1 for examples) or influenced by selection on nearby (linked) loci (through the phe-
nomenon of hitchhiking).
In an ecotoxicological context, markers under selection are fragments of gene that code
for a protein conferring a selective advantage or disadvantage on the individual subjected
to a chemical stress. Identifying the genes that potentially affect particular traits is the
major difficulty. Studies were initially focused on individual candidate genes such as the
gene coding for xenobiotic metabolizing enzymes (i.e., cytochrome P450s, glutathione-
S
-
transferases), for a transcription factor AhR (aryl hydrocarbon receptor), a well-known
chemical sensor, or for proteins involved in energy production (Hahn 1998; Hulla et al.
1999; Burnett et al. 2007). The identification of their polymorphism relied in particular on
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