Gene‑Environment Interaction and the Emergence of Individual Differences
Environmental conditions experienced during the ontogeny of an individual successively reduce phenotypic plasticity and thereby the individual adapts to the specific environmental demands. In mammalian species, phenotypic options are cut down gradually as the fetus grows, depending on the environment in the uterus, which in turn is largely dependent upon the outer environment, the mother experiences during pregnancy. This early shaping of the phenotype of the unborn offspring has been discussed in terms of an adaptive process by which the mother programs her progeny to suit the environment it is born into. After birth, a complex interaction between innate constitution and environmental conditions shapes and reduces plasticity, bringing about diversity among individual subjects.
In experimental biomedical science the behavior of rodents is analyzed by focusing on effects of specific experimental treatments. Any non-experimental variation in the subjects is undesirable and thus tried to be reduced through standardization. Fundamentally, the genetic variability of mice has been minimized by inbreeding for many generations. Since the first inbred line of mice was created as early as in 1910, several hundreds of inbred mouse strains were bred and are available by now. There are many advantages of using these inbred strains with defined genetic compositions for biomedical research. However, with regard to a reduction of variability, the effect of inbreeding appears to be well below expectations. Although there is a traceable reduction of variability in inbred mice, still a considerable amount of variability seems to resist the genetic standardization.
Common protocols of standardization include for example housing and breeding conditions, social composition of animal groups, nutrition, and experimental procedures. Nevertheless, the outcome of behavioral phenotyping still can be dramatically influenced by additional environmental factors that seem to escape standardization protocols. Hence, although the genetic background of individuals might be highly standardized, it is virtually impossible to control for all possible ontogenetic impacts on their phenotype. In this regard, it may be explicable that variability between different labs is frequently found despite all efforts of standardization.
In recent studies we posed the question whether the observed phenotypic variation is merely random and irrelevant or even disruptive for behavioral research or whether it might involve individual specific behavioral patterns consistent over time and contexts. Such consistent individual differences fit the definition of 'animal personalities' and thus may actually have a relevant and potentially important adaptive biological function. If true, such an explanation might also have wide-ranging implications for biomedical research as it points to the fact that individual differences in behavior over time and across contexts will be possibly resistant to all standardization and thus individuality has to be taken into account in future research.
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