Effects on Fitness and Ecological Parameters

Environmentally induced DNA damage and resultant mutations may be pertinent for ecologically relevant organisms because they may affect organismal-level fitness components. This may be translated into effects on populations, and eventually communities and ecosystems. This is illustrated in Figure 8. First, because DNA damage and mutations can lead to cell death and cancer, this may affect survival. Because DNA damage enhances the rate of cell senescence, accumulation of unrepaired damage and mutations may affect longevity and population age structure. DNA damage and mutations have their greatest deleterious effect on rapidly dividing cells. Because gonadal germ cells are rapidly dividing, they are particularly susceptible to the effects of DNA damage and mutations. Growth may also be affected because of induced cell death, interference with DNA replication, or induced delay of cell division (DNA damage induces cell cycle delay, a phenomenon that halts the cell cycle to allow time for repair before DNA replication or mitosis). Immune cells, both mature white blood cells and white blood cell stem cells (which divide rapidly), are also particularly susceptible to the effects of genotoxins and mutagens. An inhibited immune system may in turn affect fitness of affected organisms. Genotoxic effects may also affect bioenergetics or organisms for two reasons. First, DNA repair is an energetically expensive process. Second, mitochondria contain their own DNA, and damage and mutations in mitochondrial genes may affect mitochondrial function and ATP production. Furthermore, DNA damage and mutagenesis may affect development, both by inducing teratogenic (causing developmental defects) mutations and by delaying development because of cell cycle delay and interference with DNA replication. Finally, DNA damage in nerve cells may result in acute neurological effects, neurodegeneration, or neurodevelopmental effects.

Mutations in germ cells may result in heritable mutations that can be passed on to future generations (transgenerational effects) or can spread through the

Population-level parameters

Figure 8 Possible mechanisms whereby DNA damage and/or mutations may affect fitness of organisms.

Population-level parameters

Figure 8 Possible mechanisms whereby DNA damage and/or mutations may affect fitness of organisms.

populations. Spontaneous and endogenous germ-line mutations are relevant in evolutionary terms. Missense, nonsense, and frameshift mutations are almost always harmful, or deleterious, but in some rare instances may produce a phenotype with an adaptive advantage (beneficial mutations) in certain environments. Chromosomal mutations are mostly deleterious (especially aneuploidy), although some may be neutral, and a few may provide an adaptive advantage. If they do provide such an advantage, they may increase in frequency in the population due to natural selection. If dominant mutations in germ cells (i.e., gamete stem cells) result in the death of the offspring, these are called dominant lethal mutations. Deleterious mutations may eventually be removed from the population, but this may take many generations, especially if they are only mildly deleterious. If the deleterious mutation is recessive rather than dominant, it may persist indefinitely in the population. Neutral, beneficial, recessive, and mildly deleterious mutations may persist in the populations and increase population genetic diversity over time.

Environmentally induced mutations (i.e., due to chemical exposure, ionizing radiation, or UV) can affect survival, metabolism, growth, reproduction, propensity to develop cancer, or behavior in offspring or other descendants at any life stage. If these mutations are expressed in a dominant fashion, their effects may be always apparent when a mutant allele is present. If they are recessive mutations, the mutant phenotype is apparent only in the homozygous state. Exposure to mutagenic agents may increase the mutation rate of populations, that is, the number of new mutations per generation.

The relative number of persistent deleterious mutations in the population is called the mutational load. The deleterious effects of mutational load may depend on population size, because smaller populations have a higher level of inbreeding (the mating of genetically similar individuals). This leads to increased number of homozygous loci in the population, which increases the chance that deleterious recessive mutations are expressed - a process known as inbreeding depression. Small populations may experience inbreeding depression, which leads to further reduction in population size due to decreased average fitness, which leads to further inbreeding depression, etc., such that these populations may spiral toward extinction in a phenomenon called muta-tional meltdown. Exposure to mutagenic agents may hasten this process. Thus, although exposure to muta-genic contaminants may increase population genetic diversity and thus hasten the rate of evolution, loss of fitness may also result in population bottlenecks and reduction of genetic diversity. If the relative loss of fitness is genotype dependent, this may lead to evolution ofmore mutagen-resistant populations.

See also: Air Quality Modeling; Evolutionary Ecology: Overview; Fitness; Pollution Indices; Population and Community Interactions; Radiation Balance and Solar Radiation Spectrum.

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