Influences of maternal exposure on the tolerance and physiological performance of Daphnia magna under mercury stress

We examined the tolerance development to mercury (Hg) by a population of freshwater zooplankton (Daphnia magna) with different pre-exposure histories to Hg. The growth and reproductive performance of the F, offspring as affected by the maternal (F,) and offspring (F,) exposures was quantified. The F-0 daphnids exposed to 2.5 and 25 nM of Hg for 4 d and followed by 4 d of depuration had elevated levels of Mg and metal lothionein-like proteins (MTLPs), as well as higher tolerance to Hg toxicity than the control daphnids. The higher Hg tolerance may be attributed to the higher proportion of Hg partitioned to the MTLPs. Moreover, significant enhancement of Hg tolerance also was found in the F, offspring originating from the F0 mothers exposed to 25 nM of Hg, but there was no significant induction of MTLPs in these F, offspring when compared to the offspring from the control mothers. The Hg tissue concentrations in the F-1 neonates were approximately 25\% of those in the F0 adults. However, there was similar Hg tolerance in the F-2 offspring originating from both the control and Hg-exposed F-0 mothers, indicating that the Hg tolerance in the daphnids disappeared two generations after Hg contamination. Further exposure of the F, offspring to different Hg concentrations (1.5 and 15 nM for 28 d) indicated that maternal exposure history did not affect their growth and reproductive performance, which solely were influenced by the offspring exposure. Unexpectedly, the F-1 offspring exposed to Hg had significantly higher final wet weights and reproductive rates than the control groups, suggesting the possibility of Hg hormesis. Furthermore, the maternal exposure had no effect on the Hg accumulation and the MTLP concentrations in the F-1 offspring. Therefore, we concluded that the Hg tolerance might disappear quickly once the Hg contamination was removed and the maternal exposure history was not important in determining the physiological performance and Hg accumulation of the subsequent generations