Genetic and molecular ecotoxicology: a research framework

Marine swimming-related illness: implications for monitoring and environmental policy

There is increasing evidence that environmental degradation may be contributing to an increase in marine-related diseases across a wide range of taxonomic groups. This includes a growing number of reports of both recreational and occupational users of marine waters developing gastrointestinal, respiratory, dermatologic, and ear, nose, and throat infections. The duration and type of exposure, concentration of pathogens, and host immunity determine the risk of infection. Public health authorities may not be able to accurately predict the risk of waterborne disease from marine waters due to the limitations of conventional monitoring, as well as erroneous perceptions of pathogen life span in marine systems. Pathogens undetectable by conventional methods may remain viable in marine waters, and both plankton and marine sediments may serve as reservoirs for pathogenic organisms, which can emerge to become infective when conditions are favorable. In this paper we address the environmental factors that may contribute to illness, the types of associated economic costs, the issues of water quality monitoring and the policy implications raised by the apparent rise in incidence of marine water-related illnesses.

Responses of chub (Leuciscus cephalus) populations to chemical stress, assessed by genetic markers, DNA damage and cytochrome P4501A induction

Indicators of effects at the population level (genetic variation using allozymes) and early indicators of pollution (EROD activity and DNA strand break formation) were analysed in chub (Leuciscus cephalus) living in weakly and heavily contaminated stations of the Rhône River watershed. The genetic erosion was mainly detected in a fish population living in a contaminated small river system, through modifications in allelic and genotypic frequencies for PGM-2 locus and could be linked to a genetic bottleneck and to the reduced gene flow from upstream unable to maintain or restore the genetic diversity. In a contaminated large river system, the genetic diversity for PGM-2 and other loci was maintained and was probably the consequence of a high gene flow from upstream, linked to a sustained drift of larvae and juveniles in the system. A convergent increase of the frequency of the 90 allele at PGM-2 was observed in two contaminated stations compared with the reference station, this trend being confirmed on a more extensive geographic scale over the Rhône River basin. A high level of EROD activity was detected in both contaminated sites but only the fish in the large river system showed a significant DNA damage level compared to the reference population. The low DNA damage level and high hepato-somatic ratio characterized the impacted population of the small river system and could be associated to a chronic high-level exposure of fish to pollutants which selected individuals exhibiting a high level of DNA damage repair. In the two contaminated systems, some genotypes at the PGM-2 and EST-2 loci showed a low level of DNA damage and/or a high EROD activity and may be considered as being tolerant to pollutants. A higher tolerance of the most heterozygous fish was also detected in the contaminated large system and confirmed that a high level of heterozygosity may be necessary for survival in such a system.

DNA fingerprint comparison of rainbow trout and RTG-2 cell line using random amplified polymorphic DNA

The detection of genotoxic effects using in vitro cell systems can be extremely useful in risk assessment procedures. However, care should be taken in the extrapolation of in vitro results since, amongst other factors, established cell lines may deviate from the genetic characteristics of their species. In this work, the genetic similarities between the RTG-2 cell line and rainbow trout individuals (Oncorhynchus mykiss) from several fish farms have been studied by the RAPD technique. Results show a significant analogy in the band patterns obtained for both systems, up to 73% of the bands composing the fingerprint of the RTG-2 cell line were found in all the individuals analysed. The inter-population similarity index (Lynch, 1990), considering the RTG-2 cell line as a population, gives a value of 0.931 between both systems. The dendrogram constructed from all the individuals, considering the RTG-2 cell line as just another individual of a single population, showed that the genetic structure of the cell line was not different from those of the other individuals tested. The strong genetic similarity of both systems, together with the previously proven capability of the RAPD technique to detect genetic alterations caused in vitro by genotoxic agents, can be very useful in genetic ecotoxicological studies.

Comparison of ultraviolet-induced genotoxicity detected by random amplified polymorphic DNA with chlorophyll fluorescence and growth in a marine macroalgae, Palmaria palmata

The random amplified polymorphic DNA (RAPD) technique was used to detect DNA damage in the sublittoral macroalgae Palmaria palmata (Rhodophyta) exposed to both ambient and elevated irradiances of UV-B (280-315 nm). To investigate the potential of this method in ecotoxicological assessments, the qualitative and quantitative modifications in RAPD profiles were compared with changes in a number of physiological and fitness parameters. RAPD detectable modifications in DNA profiles were observed in all UV exposed individuals compared with controls. Changes in chlorophyll fluorescence (F(v)/F(m) ratio), in vivo pigment absorptance, thallus growth and RAPD profiles, examined simultaneously, provided a sensitive measure of UV-induced toxicity. In conclusion, the application of the RAPD method in conjunction with other suitable physiological and fitness measurements, may prove to be a valuable tool for investigating the specific effects of genotoxic agents upon marine algal populations. Ultimately, this methodology may allow the ecotoxicological examination of the link between molecular alterations and measurable adverse effects at higher levels of biological organisation.

Molecular cloning and characterization of a metal responsive Aedes aegypti intestinal mucin cDNA

We have isolated a cDNA from Aedes aegypti that is transcribed in the larval midgut in response to metal exposure, and in the adult female midgut in response to iron or cadmium exposure, or a blood meal. The cDNA encodes a protein, designated Aedes aegypti intestinal mucin 1 (AEIMUC1), which has similarities with invertebrate intestinal mucins and peritrophins, and vertebrate mucins. Proline, serine and threonine comprise 30% of the amino acid composition of AEIMUC1, a characteristic of mucins. AEIMUC1 contains three cysteine-rich domains, two of which flank a proline/serine/threonine-rich domain, a feature shared by many mucin genes. This is the first report on the isolation of a metal-responsive gene from an aquatic insect.

Genotoxic, cytotoxic and developmental effects of tributyltin oxide (TBTO): an integrated approach to the evaluation of the relative sensitivities of two marine species

Adopting an integrated approach the potential genotoxic, cytotoxic and developmental effects of tributyltin oxide (TBTO), a known endocrine-disrupting agent for neogastropods, have been evaluated in two ecologically relevant invertebrates: Mytilus edulis (blue mussel) and Platynereis dumerilli (rag worm). Following determination of the maximum tolerated dose (MTD) in terms of developmental and survival effects, the embryo-larval stages of these organisms were exposed to a range of concentrations of TBTO, and analysed for cytotoxic (proliferation rate index) and genotoxic (sister chromatid exchanges and chromosomal aberrations) effects. The study suggested that: (1) TBTO is both toxic and genotoxic to embryo-larval stages of both species; (2) at comparable concentrations, for developmental and genotoxic effects, P. dumerilii (non-target species) is more sensitive compared to M. edulis (target species); and (3) genotoxic effects are more closely tied with the development and survival of the organisms. The study emphasises the need of the evaluation of genotoxic potential of other endocrine-disrupting agents in different taxonomic groups.

Effects of chemical contaminants on genetic diversity in natural populations: implications for biomonitoring and ecotoxicology

The conservation of genetic diversity has emerged as one of the central issues in conservation biology. Although researchers in the areas of evolutionary biology, population management, and conservation biology routinely investigate genetic variability in natural populations, only a handful of studies have addressed the effects of chemical contamination on population genetics. Chemical contamination can cause population reduction by the effects of somatic and heritable mutations, as well as non-genetic modes of toxicity. Stochastic processes in small populations, increased mutation load, and the phenomenon of mutational meltdown are compounding factors that cause reduced fitness and accelerate the process of population extirpation. Although the original damage caused by chemical contaminants is at the molecular level, there are emergent effects at the level of populations, such as the loss of genetic diversity, that are not predictable based solely on knowledge of the mechanism of toxicity of the chemical contaminants. Therefore, the study of evolutionary toxicology, which encompasses the population-genetic effects of environmental contaminants, should be an important focus of ecotoxicology. This paper reviews the issues surrounding the genetic effects of pollution, summarizes the technical approaches that can be used to address these issues, and provides examples of studies that have addressed some of them.

Detection of genotoxins in the marine environment: adoption and evaluation of an integrated approach using the embryo-larval stages of the marine mussel, Mytilus edulis

In genetic ecotoxicology or eco-genotoxicology, there is lack of well-validated systems which could demonstrate the utility of multiple endpoints in environmental quality assessment. For an evaluation of genotoxic potential of heterogeneous marine sediment samples collected from a small fishing harbour in the UK, an in vivo test system using embryo-larval stages of the common mussel, Mytilus edulis was validated against direct and indirect acting reference mutagens. The system appeared to be sensitive and reproducible for cytogenetic endpoints analysed (sister chromatid exchanges (SCEs) and chromosomal aberrations (CAbs)). Following validation and chemical characterisation of the environmental samples, multiple endpoints were measured. Determination of the maximum tolerated dose (MTD) was carried out as a measure to determine cytotoxic effects as a confounding factor for genotoxicity, based on developmental and cytotoxic (in terms of proliferative rate index or PRI) effects. Evaluation of the genotoxic potential of the samples gave a positive response for all the endpoints tested, linking different levels of biological organisation (i.e., chromosomal, cellular and organismal) for the observed effects. The study also emphasises the need for the assessment of the short and long-term impacts of dredge disposal on marine biota by including laboratory-based bioassays and incorporating an integrated approach which could yield as much useful information as possible in overall hazard and risk assessment for aquatic genotoxicity.

Qualitative assessment of genotoxicity using random amplified polymorphic DNA: Comparison of genomic template stability with key fitness parameters in Daphnia magna exposed to benzo[a]pyrene

A method of DNA profiling using the random amplified polymorphic DNA (RAPD) was used to assess toxicant-induced DNA effects in laboratory populations of Daphnia magna exposed to varying concentrations of the genotoxic hydrocarbon benzo[a]pyrene. These effects, represented by changes in the RAPD profiles, were compared with a number of key ecological fitness parameters (age-specific survival, age-specific fecundity, net reproductive rate, and intrinsic rate of population increase). Not only was the RAPD profiling method shown to be a rapid and reproducible assay of toxicant-induced DNA effects, but the qualitative measure of genomic template stability compared favorably with the traditional indices of fitness. The RAPD profiles, however, exhibited higher sensitivity in detecting toxic effects. The significance of these findings for future ecotoxicological studies is discussed.

Characterization of RTG-2 fish cell line by random amplified polymorphic DNA

The increasing presence of genotoxic chemicals in the aquatic environment has led to the development of both in vivo and in vitro assays for target species. The fish population represents an important level of aquatic ecosystems that can be threatened by increased environmental pollution. The authors have studied the DNA pattern of the RTG-2 fish cell line, a fibroblast-like cell line, derived from rainbow trout (Oncorhynchus mikyss), to use this cell line as an in vitro system to study genotoxicity by means of random amplified polymorphic DNA primers (RAPDs). A constant pattern in the DNA band is essential when an organism or cell line is used to detect DNA alterations produced by genotoxic environmental chemicals. DNA fingerprints with RAPDs were obtained for RTG-2 by testing 26 single and 70 pairwise combinations of primers. Different methods of DNA extraction (chelating resin, salting out, and phenolization), the influence of spectrometric measures at 320 nm in the 260/280 quotient to quantify DNA extracts, genomic DNA and primer concentrations, annealing temperatures, and cell line passage were studied in the cell line characterization. RAPD products were identified by agarose gel electrophoresis. The good results obtained should allow the use of this system as a possible tool for detection of the genotoxicity of aquatic pollutants.

The ecotoxicological significance of genotoxicity in marine invertebrates

Attention is drawn to the goals of genetic ecotoxicology, in particular, the need to relate genotoxicity in individuals to population and community level consequences. The evidence for pollutant-induced genotoxicity in marine invertebrates is reviewed. Neoplasia is apparently rare in marine invertebrates and only limited evidence is available to suggest that chemical genotoxins act as causative agents. It is unknown why marine invertebrates exhibit low tumour incidences and are much more tolerant of ionising radiation than their vertebrate counterparts. The importance of the genotoxic disease syndrome is highlighted. Disentangling phenotypic manifestations of genotoxic damage and that due to direct metabolic toxicity provides a major challenge for the future. Further work is required to assess the significance of interspecific and interindividual variability in susceptibility to genotoxicity, especially with regard to the evolution of resistant populations and communities of marine organisms at contaminated sites. Only by addressing the issues highlighted above can proper risk assessments of genotoxic agents be performed to minimise threats to human and ecosystem health.

Tissue dose, DNA adducts, oxidative DNA damage and CYP1A-immunopositive proteins in mussels exposed to waterborne benzo[a]pyrene

A collaborative study was performed on Mediterranean mussels (Mytilus galloprovincialis) exposed to a wide dose-range (0.5-1000 ppb) of benzo[a]pyrene (B[a]P). We selected this model polycyclic aromatic hydrocarbon in order to confirm the formation of a specific DNA adduct, previously detected in gill DNA, and to clarify the in vivo effects of this mutagenic chemical requiring host-metabolism in mussels. B[a]P concentration reached consistently higher values in the digestive gland than in other analyzed tissues of mussels exposed to B[a]P for 2 or 3 days. With the exception of some values at 1000 ppb of B[a]P. DNA adduct levels increased significantly with the dose in gills and digestive gland and ranged from 0.054 to 0.789 adducts per 10(8) nucleotides (mean values per dose-point). Conversely, more complex dose-response relationships were found by detecting in parallel the levels of an oxidative DNA lesion (8-OHdG) and of CYP1A-immunopositive proteins (the latter measured in the digestive gland only). Overall, the formation of DNA adducts, the evidence of oxidative DNA damage, and changes in CYP1A-immunopositive protein levels support the hypothesis that B[a]P can induce DNA damage in mussels through a number of different molecular mechanisms.

Evaluation of the genotoxicity of municipal sewage effluent using the marine worm Platynereis dumerilii (Polychaeta: Nereidae)

Samples of settled (primary) effluent were collected from a municipal sewage treatment works at Newton Abbot, Devon, UK, a site which discharges primary effluent via long sea pipeline into the English Channel (minimum of 200-fold initial dilution). Sewage samples were collected during the period February-April 1995 and were analysed for standard physico-chemical parameters (ammonia, chemical oxygen demand, conductivity, non-purgeable organic carbon and settled solids). Samples were also tested for cytotoxicity, genotoxicity, and for developmental effects in the embryo-larval stages of the marine worm, Platynereis dumerilii. Exposure to sewage concentrations of > or = 10% (v/v) in seawater at 20 +/- 1 degrees C led to a marked reduction in normal embryo-larval development (7 h EC50 values from 10% to 18% v/v, n = 5). There was also evidence of a simultaneous delay in the cell cycle progression (as determined by sister chromatid differential staining) following embryo-larval exposures to sewage concentrations of > or = 10% (v/v). Following the calculation of the Maximum Tolerated Dose (MTD), based on cytotoxic and developmental effects, cells from the same embryo-larvae were analysed for chromosomal aberrations (CAs). Results were consistent for all samples tested, demonstrating the absence of cytogenetic damage following the in vivo exposure of polychaete embryo-larvae to settled sewage.

Genetic susceptibility in ecosystems: the challenge for ecotoxicology

Environmental management is inevitably complicated by the large variation in susceptibility to chemical toxicity exhibited by the living components of ecosystems, a significant proportion of which is determined by genetic factors. This paper examines the concept of genetic susceptibility in ecosystems and suggests the existence of two distinct forms reflecting genetic changes at the level of the individual and at the level of population and community. The influence of genetic susceptibility on exposure-response curves is discussed and the consequent accuracy of data used for toxicity test-based risk assessments examined. The paper concludes by describing a possible biomarker-based approach to future studies of susceptibility in ecosystems, suggesting the use of modern molecular genetic methods.

Persistence of cadmium-induced adaptive response to genotoxicity of maleic hydrazide and methyl mercuric chloride in root meristem cells of Allium cepa L.: differential inhibition by cycloheximide and buthionine sulfoximine

With an objective to determine the period of persistence of the metal-induced adaptive response to chemical mutagens and heavy metals, growing root meristems of Allium cepa were conditioned by cadmium sulfate (CdSO4), 4 x 10(-7) and 4 x 10(-6) M for 1 h and subsequently challenged by maleic hydrazide (MH), 5 x 10(-3) M or methyl mercuric chloride (MMCl), 1.26 x 10(-6) M for 3 h at different time intervals ranging from a few minutes to several hours following the conditioning dose. Root meristems, fixed at regular intervals during recovery from 6 to 48 h, were cytologically analysed for cells with micronuclei (MNC). The adaptive responses to MH and MMCl were observed as early as 5 min after the Cd-conditioning that persisted for at least 48 h. Metabolic inhibitors, cycloheximide (CH). 10(-7) M and buthionine sulfoximine (BSO), 10(-4) M administered either prior to or simultaneous with Cd-conditioning effectively prevented the adaptive response to MH. Whereas BSO, an inhibitor of phytochelatin synthesis, prevented the adaptive responses from 15 min to 8 h after the conditioning dose, CH an inhibitor of cytoplasmic protein synthesis prevented the same from 6 to 48 h. The findings underscored the differential roles of phytochelatins and proteins underlying the foregone metallo-adaptive response.