Toxicity and Sexual Reproduction in Rotifers: Reduced Resting Egg Production and Heterozygosity Loss

Abstract

The effect of toxicants on the genetic structure of populations is not well understood. Toxicant exposures often cause strong directional selection and concomitant increased genetic drift at nonselected loci that could alter a population’s probability of extinction. Current toxicity tests with laboratory or natural populations do not consider such effects. The main long-term impacts of toxicity on sexual reproduction in rotifer zooplankters are reduced resting egg production and loss of heterozygosity. Using computer simulation of rotifer population dynamics, we explored how toxicity alters genetic structure through reductions in population growth rate, mictic ratio, and fertilization rate, and how these changes affect loss of heterozygosity. The relationship between resting egg production and toxicant-caused reductions in population growth rate (r) is a negative exponential, so small reductions in r lead to large reductions in resting egg production. Reductions in the mictic ratio, the proportion of daughters produced by a female that are mictic, also caused exponential decreases in resting egg production. The rate of heterozygosity loss is directly related to effective population size. In rotifers, effective population size (Ne ) is determined by the volume from which a mictic female obtains her mate, because this determines the number of randomly-mating males and females participating in resting egg production. For mating volumes greater than 100 liters, Ne is so large that heterozygosity loss is not accelerated by reductions in r, mictic ratio, or any other parameter investigated. Only when mating volume is less than 10 liters (Ne=18) do toxicant-induced reductions in these parameters accelerate heterozygosity loss. However, under all conditions tested, heterozygosity loss due to toxicant-caused reductions in population growth rate are small. What is currently being measured by standard toxicity tests may not be very relevant for predicting the long-term survival of zooplankton populations because resting egg production is not considered. Yet resting eggs are essential for the long-term persistence of rotifer populations in most environments. By emphasizing the development of convenient, rapid toxicity tests, ecotoxicologists may have overlooked variables that have the greatest impact on the evolutionary fate of zooplankton populations.