INTRAPOPULATION VARIABILITY IN SUBLETHAL RESPONSE TO HEAVY-METAL STRESS IN SEXUAL AND ASEXUAL GASTROPOD POPULATIONS

1. Asexual species have been described by some to be more broadly tolerant of environmental stresses (i.e. to have general purpose genotypes) and by others to be more responsive to environmental stresses (i.e. to exhibit narrower tolerance distributions to environmental perturbations) in comparison to their sexual relatives. 2. We examined the sublethal response of asexual and sexual gastropod populations of the family Hydrobiidae to cadmium exposure to determine: whether populations rank consistently in mean and variance of growth rate when reared in different environments; whether the variance in growth rate within populations alters in response to stress; and whether growth rates within genetically uniform clones are less variable than growth rates within sexually outcrossed populations. 3. We found that, in addition to reducing mean growth rate in all populations, exposure to cadmium increased the variability in growth rate within populations and altered the ranking of both means and variances among populations. 4. The average intraclonal variability in growth in the parthenogenetic species Potamopyrgus antipodarum was comparable to the average intrapopulation variability in growth within the closely related but sexually reproducing Hydrobia species. Our results indicate that environmental toxicants, such as cadmium, can alter the relative performance of these potentially competing species. 5. Our results are of broad relevance to the examination of phenotype variance and its partitioning between genetic and environmental sources of variation. 6. Non-genetic variation was the most important component of the total phenotypic variance in growth rate within these gastropod populations. 7. In addition, the assumption frequently applied in the development of toxicity test protocols, that uniformity in phenotypic traits (i.e. response to toxicant exposure) is tightly coupled to genotypic uniformity, is not supported by our results.