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250 individuals from pond A, so that any differences between the two phylogenetically
related populations A and C could be explained by environmental factors.
The study, which used RAPD and allozyme analyses of the genomic structure of the
four populations, showed:
• Identical patterns between the two noncontaminated populations A and B, which
differed from that of the radionuclide-contaminated populations.
• An increased genetic diversity with a higher percentage of polymorphism and
heterozygosity in the radionuclide-exposed populations C and D.
• Patterns of population C were much closer to those of population D than to pat-
terns of population A, from which it drifted phylogenetically. Observed changes
in banding patterns (lower or higher frequencies) conferred selective advantages
to the contaminated populations.
The influence of radionuclides on the genetic structure of the population was clearly
demonstrated. Site contamination resulted in severe stress affecting the viability and repro-
ductive success of transplanted individuals, which exhibited a high number of embryonic
abnormalities and a lower fertility than that observed at reference sites (Theodorakis 2003).
Contaminant-indicative bands (CIBs) were present at higher frequencies in the descen-
dants of the exposed populations and were also observed in control mosquito fish that
had been encaged
in situ
in pond C (Theodorakis et al. 1999). These changes in banding
patterns of contaminated fish were associated with increased fertility, and a decrease in
embryonic abnormalities and in DNA damage.
The hypothesis of the selection of resistant populations from surviving individuals has
been advanced to explain the observed changes in genetic structure. Thus, the contami-
nant-indicative RAPD markers could be generated from, or linked to, loci involved in the
response to radiation, or even a mechanism of radioresistance. The induction of an iden-
tified band was associated with that of a locus (NP nucleoside phosphorylase) involved
in nucleoside synthesis and DNA repair. The authors suggested two links, between the
induction of this NP locus and DNA breaks, and between increased genetic diversity and
a mechanism of adaptation or radioresistance. The characterization of other contaminant-
indicative RAPD markers and the identification of the function of the corresponding genes
have yet to be carried out (Theodorakis and Bickham 2004). Very similar results were sub-
sequently obtained by the same authors with redbreast sunfishes (
Lepomis auritus
), col-
lected along a contamination gradient in a pulp mill-contaminated river (the Pigeon River
located in North Carolina and Tennessee, USA) (Theodorakis et al. 2006).
These studies support the existence of a relationship between genotoxicity and popula-
tion effects.
One putative CIB (perhaps related to a locus involved in metal tolerance) was also found
in AFLP patterns (1) of a historically metal-exposed population and (2) of the more cop-
per-tolerant individuals from populations of
D. longispina
studied by Martins et al. (2009).
These authors studied
D. longispina
collected in or near the aquatic system of the aban-
doned cupric-pyrite mine of Sao Domingos (Portugal). Three populations were sampled
[two from control sites and one from a historically acid mine drainage (AMD) impacted
station]. The authors expected a loss of genetic diversity as a result of increased resis-
tance in the impacted population. If they could discriminate (genetically) the exposed
AMD population from the reference populations based on the AFLP patterns, they could
not conclude to a reduction of genetic diversity in this AMD-impacted population. They
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