Mutation Load in Natural Populations of the Sensitive Fern Onoclea Sensibilis Exposed to Soil Mutagens

This study examined mutations expressed in haploid fern gametophytes derived from spores collected in the wild in order to assay for the presence of mutagenic compounds in natural environments. Gametophytes of the fern Onoclea sensibilis were cultured from spores collected in 14 natural populations around the region of Montreal, Quebec, Canada. Most of the populations grew in soils exposed to spring flooding of the St. Lawrence River or its tributaries. Relative variation in the rate of somatic mutation was calculated from the proportion of fern clones in each population segregating for mutations that disrupt gametophyte development. Estimated somatic mutation rates ranged over approximately one order of magnitude among populations, with most of the variation due to differences between populations located downstream from Montreal (higher mutation rates) vs. those upstream or outside the immediate vicinity of the river (lower mutation rates). Organic (dichloromethane) extracts of soil samples from 11 of these sites were examined for mutagenicity using the plate incorporation version of the Salmonella/mammalian microsome test (strains TA98, TA100, YG1041, YG1042). Salmonella mutation rates per milligram equivalent of dry soil (i.e., mutagenic potency) were roughly an order of magnitude greater for soils from downstream vs. upstream locations. TA98 (frameshift) mutagenicity of soil extracts is positively correlated with the estimated proportion of the fern population segregating for gametophytically expressed mutations. The pattern of Salmonella mutagenicity results suggests the presence of homocyclic polycyclic aromatic hydrocarbons (PAHs). Results are discussed with respect to population viability of the fern O. sensibilis, the ability of microbial assays to identify mutagenic hazards to natural populations, previous studies of genotoxic pollution in the St. Lawrence River system, and previous studies of soil mutagenicity.