3.4.3 Genotoxic Marker Response
In the same field study, primary DNA strand breaks were measured by the comet
assay (SCGE, single cell gel electrophoresis assay) in the haemocytes of caged zebra
mussels. This assay has gained importance in environmental genotoxicity studies
because it is a rapid, relatively inexpensive (compared to other methods) and
sensitive technique for highlighting a variety of DNA damage to individual cells,
i.e. single-strand breaks (SSBs), double-strand breaks (DSBs) or alkaline-sensitive
sites (ALSs) [55]. The results obtained in terms of levels of DNA strand breaks in
haemocytes and the sensitivity of zebra mussels to a genotoxic stress were (1) in
accordance with immune marker responses (see above) and (2) in keeping with
genotoxicity results obtained on other organs (i.e. gills, DG cells) of caged zebra
mussels during previous field studies carried out in the PIREN-Seine programme.
For example, the upstream–downstream effect related to the gradient of contamination between Marnay-sur-Seine and Triel-sur-Seine was evidenced with the levels of
DNA strand breaks in gill cells of zebra mussels. Levels of damage varied according
to the duration or the season of transplantation [56]. In the same study, the levels of
irreversible chromosomal damage such as micronuclei measured in gills were also
positively correlated with PAH bioaccumulation and related to the gradient of water
contamination between the three pilot sites. In another study [57], specific DNA
adduct patterns with levels varying between 3 and 18 adducts/10
6 nucleotides were
detected both in the gills and the DG of mussels, after 1 or 2 months of transplantation at the three sites on the Seine River. Conversely, this was not the case for
mussels in the control site. The DNA adduct formation could be correlated with the
modulation or the decrease of gene expression implicated in detoxification processes. Further work is necessary to fully understand the relationship between the
responses measured at different genomic scales (from DNA to chromosome), as well
as the incidence of structural and functional damage (phagocytosis, DNA repair,
apoptosis or necrosis, etc.) on cells and, more globally, on individual fitness.
Genotoxicity responses may then be regarded not only as biomarkers of exposure
but also as biomarkers of effects, which would reinforce their relevance as predictive
tools of ecological dysfunctions.
3.4.4 Conclusion
In situ experiments demonstrate the relevance of haemocytes as target cells for
immune and genotoxic biomarker measurements in D. polymorpha because they
reflect the contrasted levels of impact of the sites studied as well as the physiological
conditions of mussels in relation to the stage of sexual maturity. Research is
continuing for a better characterisation of the seasonal variability of biomarker
responses as well as the possible incidence of (a)biotic confounding factors. These
aspects are critical to define both thresholds and reference values useful for a robust
diagnosis of the environment’s quality. Because of their central functions on
258
M. Bonnard et al.
In the same field study, primary DNA strand breaks were measured by the comet
assay (SCGE, single cell gel electrophoresis assay) in the haemocytes of caged zebra
mussels. This assay has gained importance in environmental genotoxicity studies
because it is a rapid, relatively inexpensive (compared to other methods) and
sensitive technique for highlighting a variety of DNA damage to individual cells,
i.e. single-strand breaks (SSBs), double-strand breaks (DSBs) or alkaline-sensitive
sites (ALSs) [55]. The results obtained in terms of levels of DNA strand breaks in
haemocytes and the sensitivity of zebra mussels to a genotoxic stress were (1) in
accordance with immune marker responses (see above) and (2) in keeping with
genotoxicity results obtained on other organs (i.e. gills, DG cells) of caged zebra
mussels during previous field studies carried out in the PIREN-Seine programme.
For example, the upstream–downstream effect related to the gradient of contamination between Marnay-sur-Seine and Triel-sur-Seine was evidenced with the levels of
DNA strand breaks in gill cells of zebra mussels. Levels of damage varied according
to the duration or the season of transplantation [56]. In the same study, the levels of
irreversible chromosomal damage such as micronuclei measured in gills were also
positively correlated with PAH bioaccumulation and related to the gradient of water
contamination between the three pilot sites. In another study [57], specific DNA
adduct patterns with levels varying between 3 and 18 adducts/10
6 nucleotides were
detected both in the gills and the DG of mussels, after 1 or 2 months of transplantation at the three sites on the Seine River. Conversely, this was not the case for
mussels in the control site. The DNA adduct formation could be correlated with the
modulation or the decrease of gene expression implicated in detoxification processes. Further work is necessary to fully understand the relationship between the
responses measured at different genomic scales (from DNA to chromosome), as well
as the incidence of structural and functional damage (phagocytosis, DNA repair,
apoptosis or necrosis, etc.) on cells and, more globally, on individual fitness.
Genotoxicity responses may then be regarded not only as biomarkers of exposure
but also as biomarkers of effects, which would reinforce their relevance as predictive
tools of ecological dysfunctions.
3.4.4 Conclusion
In situ experiments demonstrate the relevance of haemocytes as target cells for
immune and genotoxic biomarker measurements in D. polymorpha because they
reflect the contrasted levels of impact of the sites studied as well as the physiological
conditions of mussels in relation to the stage of sexual maturity. Research is
continuing for a better characterisation of the seasonal variability of biomarker
responses as well as the possible incidence of (a)biotic confounding factors. These
aspects are critical to define both thresholds and reference values useful for a robust
diagnosis of the environment’s quality. Because of their central functions on
258
M. Bonnard et al.