Biology Reference
In-Depth Information
Genetic tools can be used particularly to analyze the differential capacity of organisms
(i.e., of genotypes) in a population to cope with a chemical stress, although the between-
individuals variability in biochemical or physiological responses is still being widely
ignored in ecotoxicology, or is considered background noise (Forbes and Forbes 1997).
Genetic investigations in ecotoxicology can actually lead to:
1. The assessment of within-population genetic diversity, a key factor allowing
species survival; any alteration of this diversity could lead to a possible loss of
adaptive capacity to meet any additional stress (one of the first principles of con-
servation genetics; Soulé 1987).
2. The estimation of the potential connectivity between populations by the analysis
of the between-populations genetic diversity. Are the populations more or less
isolated and thus more or less genetically differentiated? What is the level of gene
flow between populations? Does this level allow a possible recolonization of the
habitat by a species, after a massive demographic bottleneck?
3. The identification of relationships between genetic diversity and physiology by
searching for possible couplings between genotypes and phenotypes, considering
fitness-relevant biological traits.
4. The detection of modifications at gene expression level potentially leading to cel-
lular damage, or to physiological acclimation or even to resistance to chemical
stress.
Thus, we set out in this chapter to investigate the possible cause-and-effect relationships
between chemical stress and genetic diversity/gene expression/biological traits. Finally,
we will analyze the potential of genetic markers as diagnostic tools for a better knowledge
of the impact of contaminants on populations in the field.
14.2 Major Evolutionary Forces That Change Genetic Variability
Across the whole distribution area of a particular species, random mating between indi-
viduals is indeed impossible, the dispersion-migration capacity of the species being
limited. Thus, one observes generally in the field different local populations (or demes)
considered as reproduction units displaying spatial and temporal stability.
Within-population genetic variability is subjected to four major evolutionary forces
(mutation, genetic drift, migration, and selection) (Figure 14.1) that can lead to changes in
allele frequencies (Belfiore and Anderson 1998, 2001; Bagley et al. 2002).
14.2.1 Mutation
Mutation is a process that creates genetic diversity, producing a new allele different from
the ancestral allele. The different alleles observed for a particular gene are derived from
mutational modifications frequently linked to errors during DNA replication. Mutations
are rare events (single locus mutation rate per generation: 10 -4 to 10 -6 ); thus, they can change
allele frequencies in a population, but these changes are usually very slow. Mutations in
noncoding regions generally have no impact on the phenotypes and thus are not submitted
Search WWH ::




Custom Search