Effects of freshwater pollution on the genetics of zebra mussels (Dreissena polymorpha) at the molecular and population level

Abstract

Revealing long-term effects of contaminants on the genetic structure of organisms inhabiting polluted environments should encompass analyses at the population, molecular, and cellular level. Following this concept, we studied the genetic constitution of zebra mussel populations from a polluted (Dp) and reference sites (Cl) at the river Drava, Croatia, and applied microsatellite and DNA damage analyses (Comet assay, micronucleus test (MNT)). Additionally, mussels from both populations were exposed to polluted wastewater in the laboratory for three days, and DNA damage was analyzed to evaluate acclimatization and genetic adaptation of the investigated populations to the polluted environment. The two populations differed in their genetic constitution. Microsatellite analysis suggested that Dp had undergone a genetic bottleneck. Comet assay did not indicate any difference in DNA damage between the two populations, but MNT revealed that Dp had an increased percentage of micronuclei in hemocytes in comparison to Cl. The laboratory experiment revealed that Dp had a lower percentage of tail DNA and a higher percentage of micronuclei than Cl. These differences between populations were possibly caused by an overall decreased fitness of Dp due to genetic drift and by an enhanced DNA repair mechanism due to acclimatization to pollution in the source habitat.

Figure 1

Location of the sampling sites in Croatia. Cl: Čakovec lake (control population); Cl2: reference site for water analyses and reference sampling site of mussels used in the laboratory experiment; Dp: contaminated population located downstream from the mouth of the wastewater treatment plant effluent channel (W.C.). For sediment analyses a sample was taken in the W.C.

Figure 2

DNA damage measured by the Comet assay in the hemocytes of zebra mussels of the Cl and Dp populations, river Drava. Light grey bars correspond to field populations, dark grey bars to population samples exposed to dechlorinated tap water in the laboratory experiment, and black bars to population samples exposed to wastewater in the laboratory experiment. Different letters indicate statistically different DNA damage (P < 0.05).

Figure 3

Number (per mill) of micronuclei (MN) in hemocytes of zebra mussels of the Cl and Dp populations, river Drava. Light grey bars indicate field populations, dark grey bars population samples exposed to dechlorinated tap water in the laboratory experiment, and black bars population samples exposed to wastewater in the laboratory experiment. Different letters indicate statistically different numbers of micronuclei (P < 0.05).