Behavioral Genetics and Animal Domestication - Genetics and the Behavior of Domestic Animals - page 67

behavioral responses were very similar. Both types of fish had a reduced
anti-predator behavior, explained by the authors as a response to a realloca-
tion of resources from vigilance and alertness to growth. Similarly, Wright
et al. (2006c) found correlations between growth and fear

avoidance behav-
ior in an inter-cross population between wild and laboratory zebrafish.

EPIGENETICS
So far, we have discussed how different mutations and variants in DNA-
sequence may be involved in causing the phenotypes associated with
domestication. According to current genetic and evolutionary theories,
spontaneous and randomly occurring mutations are the only substrate for
natural and artificial selection. As we have seen, many of the identified
mutations are mainly regulatory, and do not affect protein structures. So
the strongest driving forces behind evolutionary change appear to be related
to the timing of expression of protein coding genes, or in other words, how
the genome is orchestrated during ontogeny.
This has sparked an increased interest in understanding the mechanisms
involved in gene regulation. Some of these mechanisms are purely genetic
in their nature, as explained above when we discussed eQTL-studies, in
which case the expression profile of a particular gene is contingent on the
DNA-sequence on another locus. However, a substantial part of the expres-
sion variation is due to chemical modifications, which do not affect or relate
directly to DNA-sequence. These are the factors we refer to as epigenetic
( Richards, 2006 ), and they represent a recently discovered and as-yet poorly
understood bridge between the environment and the genome.
At the molecular level, mainly two chemical changes have attracted
research so far. The first consists of a modification of cytosine bases, where
a methyl group is added to the fifth carbon, changing C to 5 mC. Most methyl-
ation of cytosine occurs when this nitrogen base is positioned next to a gua-
nine base, a so called CpG-position, and throughout the genome, repetitive
sequences of this combination occur mainly in regulatory regions, in so called
CpG-islands. Increased methylation is usually associated with decreased
expression of the gene.
The second important group of epigenetic modification affects histones,
the proteins responsible for packing DNA in the nucleus. The histone tails
may receive additions of, for example, methyl- or acetyl-groups, which will
change the density with which DNA is packed. Consequently, the more
densely packed the DNA, the less transcription of the genes in the region.
It is important to remember that although both DNA-methylation and histone
modifications are important regulators of genome activity, they do not affect
the DNA-sequence itself. A novel term to capture the status of the genome
with respect to its epigenetic modifications is “epigenome”.