Behavioral Genetics and Animal Domestication - Genetics and the Behavior of Domestic Animals - page 68

A large proportion of the epigenetic variation is constitutional—it varies
vastly between cell types and organs, but within a particular cell type, varia-
tion is much smaller between individuals ( Richards, 2006 ). This constitutional
variation depends on genetic differences, either in the promoter regions or
in translocated loci. However, some of the epigenetic variation is dynamic and
modifiable, so loci involved in this may be highly responsive to environmen-
tally mediated input—hence, the epigenome can be thought of as a bridge
between nature and nurture.

Epigenetic Changes and Long-Time Behavior Modifications
In a pioneering series of experiments, Meaney and co-workers studied the long-
term effects of early experience on later behavior, and particularly investigated
the epigenetic mechanisms involved ( Weaver et al., 2004 ). In a cross-fostering
experimental design, rats were raised either by attentive and caring mothers
(which licked their pups a lot and performed much arched-back nursing) or by
less careful females. Offspring of the more caring mothers were more resistant
to restraint stress when they were adults, and displayed a more attentive and
careful maternal behavior when they themselves gave birth. This was found
to be due to an induced difference in methylation status of a promoter region
of the glucocorticoidreceptor gene (GR), particularly the binding site associated
with the transcription factor NGFI-A,alsoknownasEGR1. The methylation
differences between differently nurtured rats also caused histone modifications
in the same region, leading to a modified expression of GR.
This experiment was one of the first clearly demonstrating that experiences
gained during ontogeny can alter the epigenetic state of the genome, and subse-
quently give rise to behavioral differences. A number of other experiments
have demonstrated similar effects after this. For example, maternal care has
been shown to affect methylation states of estrogen receptor alpha in rodents
( Champagne and Curley, 2008 ), and epigenetic factors affect cognitive and behav-
ioral traits in a variety of species, including humans ( Gr¨ff and Mansuy, 2008 ).
Stress plays a crucial role in the inter-relationship between environmental
factors and epigenetic modifications. Although different definitions of stress
are ubiquitous, there is a general consensus that steroid hormones, such as
cortisol (or corticosterone), testosterone, and estrogens, are usually elevated
under conditions perceived by an individual as stressful. Steroids act as DNA
transcription modifiers when the steroid

receptor complex binds with binding
proteins to specific DNA-regions, and this provides an open gate from stress
experiences to modifications of gene expression ( Murgatroyd et al., 2009 ).

Epigenetic Changes in Domestication
Long-term effects of epigenetic modifications can also be transferred across
generations, by changes in the germ-line epigenomes. Skinner and coworkers