Nevertheless, there are quite a few impressive studies that demonstrate single gene influences on behavior. Several of these even used wild-caught or wild-type individuals. In black-capped warblers, migratory routes in autumn differ among populations in Europe. Birds from western Europe, or central Germany, tend to migrate in a westerly direction and stay in southern Britain for the winter. Birds from southwestern Germany tend to migrate in a southwesterly direction via Spain to western Africa, birds from Austria migrate in a southeastern direction via the Balkan, Turkey, Israel, to eastern Africa. Breeding in laboratories could confirm that these migratory routes are indeed genetically fixed and that hybrids between southwestern
German with Austrian birds had a southern orientation, thus were indeed intermediate between those of the P-generation.
Another, rather well-studied example is of the feeding strategies of Drosophila melanogaster larvae. When put into a feeding tray, they show one of two strategies: they either move around quite a lot (up to 8 cm in 5 min, which is a lot for small larvae) or they just stay put. The first group is called 'rovers', the second one 'sitters'. Rovers and sitters in a wild population are found in a proportion of about 70:30. In a series of crossing experiments, it could be shown that rover is the dominant, sitter the recessive allele of a gene called 'foraging'. Only under conditions ofextreme food scarcity do sitters behave as rovers. In an experiment with artificially high larval density, it could be shown that over 70 generations' directional selection (see below) occurred and the proportion of rovers increased; under artificially low densities the other way round, sitters became more frequent.
In mice, a single gene called 'clock' is responsible for circadian rhythms. In its wild type, this gene programs mice for a rhythm ofabout 24 h length. In a mutant form, it causes first a rhythm of about 28 h, but without an external zeitgeber cue, this rhythm breaks down totally after a few repetitions.
In another strain of laboratory mice, a single gene influencing synthesis of monoaminooxidase A (MAOA) in their brains could be suppressed by a one-gene mutant. These MAOA-deficient mice had a severely increased aggressive potential, but also several other physiological and behavioral deficits. The effect of MAOA is to facilitate the catabolism of both serotonin and noradrenalin, and the same defect could be found in male human beings, also with similar behavioral defects ofincreased antisocial behavior. Another mouse mutant lacking the gene for synthesis of nitric oxide synthase (NOS) also reacts with an extreme increase of aggression and sexual behavior in males - they are not reacting to submissive behavior of their counterpart, but they do not show any elevation of testosterone levels. None of these effects could be found in females, and no histological differences between wild type and mutant have been found so far.
The biochemical consequences of these single gene differences have also been found already for the Drosophila sitter versus rover decision. In both forms, a certain cyclic guanosine monophosphate (cGMP)-dependent protein kinase is expressed differently in the olfactory part of their brain, and this obviously leads to a different sensory system. In male mice mutants lacking the gene for oxytocin synthesis there is a lack of recognition of their female partners and they always show the same amount ofsocial exploration toward a female, whenever they meet her again, whereas wild-type males show an increasing familiarity with each repetition. In another interesting study, the gene for a vasopressin receptor V1a, that, among others, controls the development of a pair bond, was targeted. This receptor is characteristic for some monogamous species of voles, and it is lacking in other, polygynous species. If the gene for Via is transferred to house mice, they suddenly become more attentive to their female partner!
Some aspects of behavioral genetics have even been confirmed in studies on humans. As already demonstrated in the case of MAOA, several mental disturbances can be related to single gene activities. Another example is a gene-related reading disability, or a certain dopamine receptor gene, lack of which leads to specific spatial disorientation. Most studies on human aspects however have been done on identical twins, and heritabilities of traits in twins that were raised apart. These studies, for example, found heritabilities for performance in IQ tests of up to 70%. However, high correlations also exist for unrelated adopted children raised in the same household, which again, together with many other studies, demonstrates the importance of the environment. Personality scores in human identical twins normally reach heritabilities of around 50% (thus slightly higher than in birds; see above), but still there is about 50% of the difference explained by environmental factors.
These same interactions between environmental factors and genetic preconditions have also repeatedly been demonstrated in other animals. Thus, for example, male mice can also be socialized into SAL types by rearing conditions, such as being the only male in a litter of female littermates, or being exposed to the smell of two fighting adult males. Lab mice, even from less active and 'dumb' strains, can improve performance in learning, exploration, and open-field tasks significantly by being reared in a complex, enriched environment, etc.
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