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in subsequent generations coming from field-collected populations in comparatively pol-
luted and clean sites was recently reviewed by Johnston (in Amiard-Triquet et al. 2011).
Evidence is reported for copepods (exposed to metals, Co, Cr), daphnids (with Cd, Cu, or a
pesticide), chironomid larvae (Cd), bryozoans (Cu), gastropods (Cd, Pb, Zn), and fish (Cd,
PCB, pro-oxidant
t
-butyl hydroperoxide).
Within a given population, certain individuals have an inherent ability to cope better
with the presence of chemical contaminants in their environment. Studying microalgal
responses to a petroleum spill, Carrera-Martínez et al. (2010, 2011) have shown that crude
oil-resistant mutants had arisen through rare spontaneous mutations that had occurred
before crude oil exposure in the field or in the laboratory. Resistant mutants were enough to
assure the survival of microalgal species exposed to oil spills. In the crab
Carcinus maenas
,
Depledge et al. (1995) have shown that specimens with naturally low concentrations of
proteins in their hemolymph were more susceptible when exposed to copper. In shrimps
Palaemonetes pugio
exposed to chromium (VI) or to fluoranthene, individuals that were het-
erozygous for the glucose phosphate isomerase allozyme, involved in energy metabolism,
survived longer and had less overall mortality than the homozygous genotype (Harper-
Arabie et al. 2004). In eels
Anguilla anguilla
exposed to an herbicide thiocarbamate or to an
organophosphate insecticide, survival was improved for individuals able to adapt their
glutathione metabolism to respond to oxidative stress (Peña-Llopis et al. 2001, 2003).
Co-tolerance may occur when organisms that have been exposed to one toxicant, but
not to another one, become tolerant to both of them. Co-tolerance occurs most probably for
compounds that have similar chemical structures and activities and share common toler-
ance mechanisms. Co-tolerance may arise also because genes for resistance to, or transfor-
mation of, different contaminants are found on the same mobile genetic element such as
a plasmid or a transposon, thus eliciting co-tolerance to contaminants that are unrelated
structurally or functionally (Top and Springael 2003; Wright et al. 2008). Examples of co-
tolerance between toxicants have been provided in recent reviews for microbes includ-
ing bacteria, phytoplankton, and periphyton (Tlili and Montuelle; Amiard-Triquet and
Roméo, both in Amiard-Triquet et al. 2011). Other microalgal examples have been reported
involving different metals and also different organic compounds such as PCBs and DDT
(Cosper et al. 1987 and literature quoted therein; Takamura et al. 1989). Such studies are
scarce for animal species. However, Brown (1978) has shown the ability of copper-tolerant
freshwater isopods
Asellus meridianus
to detoxify lead by storing this metal in intracel-
lular structures involved in copper accumulation. Xie and Klerks (2003) have shown that
Heterandria formosa
(a fish species) that had acquired cadmium resistance in the course
of experimental exposure over six generations had also become tolerant to copper. More
frequent are studies dealing with cross resistance between pollutants and more natural
factors such as temperature, which is important in the context of global warming (http://
www.citeulike.org/user/dortsjennifer/tag/crossresistance). The induction of heat shock
proteins (HSPs) by environmental factors and cross-tolerance with metals and organics
have been recently reviewed (Mouneyrac and Roméo in Amiard-Triquet et al. 2011). The
estuarine fish
F. heteroclitus
resident in a harbor highly contaminated with PCBs, evolved
tolerance to these chemicals, possibly involving mechanisms that minimize the immuno-
suppressive effects of a bacterial pathogen
Vibrio harveyi
(Nacci et al. 2009). Likewise,
parasitized individuals of the freshwater bivalve
Pisidium amnicum
had an increased toler-
ance toward pentachlorophenol (Heinonen et al. 2001). Such phenomena may have great
ecological significance since most impacted sites are subjected to multiple pollutions.
Co-tolerance between different classes of toxicants or between toxicants and natural stress
factors can act as a confounding factor complicating the interpretation of biomarker data.
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