rDNA Copy Number Change and Sequence Variation in Mutation Accumulation Lines of Daphnia pulex

Abstract

Freshwater ecosystems are subject to a variety of stressors, including metal contamination. Lakes and ponds in southern Ontario are especially contaminated with metals, as mines in this region are among the top copper and nickel producers in the world. These metals can mutate DNA, although it is generally unknown how the freshwater microcrustacean Daphnia pulex is affected by chronic metal exposure. To determine how metal exposure affects D. pulex ribosomal (r)DNA, a highly conserved genetic compartment, I estimated rDNA copy number and sequence variation in mutation accumulation lines of D. pulex. I found that chronic metal exposure results in large fluctuations in rDNA copy number in D. pulex, including significant losses of rDNA copies. However, chronic metal stress does not influence the rate of new point mutations in D. pulex rDNA. These findings contribute to our understanding of how D. pulex rDNA evolves with chronic, sub-lethal exposure to metal.