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functional genomic techniques are becoming available for a growing number of aquatic
species (Pierron et al. 2009; Hook 2010; Scown et al. 2010; Pierron et al. 2011; Xuereb et al.
2011; Martini et al. 2012).
Overall, two approaches can be defined to explore the alteration of gene expression by
pollutants: an
a priori
approach and a global one.
14.5.1
A Priori
Approach
When genes linked to the response of an organism to contaminants are identified and
their sequences known, their expression levels can be assessed. This approach (called
a
priori
) must anticipate variations in the transcripts in order to be effective, and has been
mainly developed on species that are of economic interest and/or have been subjected to
many prior biochemical and genetic studies. In this frame, the semiquantitative estimation
of gene expression can be conducted first by the assessment of band intensities of reverse
transcription RT-PCR products on electrophoresis gels. This approach was carried out
in the case of glutathione-
S
-transferase (GST) genes, a family encoding multifunctional
proteins that catalyze the conjugation of glutathione to a wide variety of hydrophobic com-
pounds leading to their elimination. Among GST studies, Boutet et al. (2004) evaluated the
level of the transcription of GST genes in the oyster
Crassostrea gigas
; this work showed that
two classes of GST (mu and omega) should be useful biomarkers of exposure to hydrocar-
bons, whereas two other classes (pi and sigma) should be relevant as biomarkers of expo-
sure to pesticides. Li et al. (2012) showed that two genes encoding different GST proteins
(sigma and pi) present different expression patterns in the case of exposure of the clam
Venerupis philippinarum
to bacterial stress, confirming different roles for these two families
of proteins.
Among
a priori
methods, quantitative measurement of transcription processes is widely
carried out using quantitative PCR or real-time PCR. Thus, Zhou et al. (2011) measured the
impact of bisphenol A on the expression of three genes involved in the larval development
of the gastropod
Haliotis diversicolor
, demonstrating an embryotoxic effect at cellular and
molecular levels. In another study on the oyster
C
.
gigas
for which previous knowledge
was available on the effect of cadmium on energy metabolism, Achard-Joris et al. (2006)
assessed the transcription level of the mitochondrial gene encoding the first subunit of
the cytochrome C oxidase protein complex CoxI, involved in electron transfer, in oysters
exposed experimentally to cadmium contamination. They showed an induction of the
transcription of CoxI, but no relationship was detected between the level of gene expres-
sion and the concentration of metal in the tissues. This induction was maintained but at
a lower level in the case of contamination with both cadmium and zinc, but disappeared
in the case of zinc contamination alone; these results confirmed that zinc could limit cad-
mium toxicity. In this experiment, the processes of gene overexpression could be consid-
ered compensation mechanisms to restore the level of metabolism activities necessary to
cope with cadmium contamination.
14.5.2 “Global” Approach (Open System)
When no data are available on the nature of pollutants or on the genes involved in the
responses to stress, a “global” approach can be carried out, exploring the patterns of gene
expression over several hundred or thousand genes, by different genomic tools (Ghosh et
a l. 2011).
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