Behavioral Genetics and Animal Domestication - Genetics and the Behavior of Domestic Animals - page 65

have been experimentally connected to variations in social behavior both
between and within species. For example, the expression of the vasopressin
receptor gene AVPR1a is highly correlated with the degree of gregariousness
across different finch species and with the degree of mate attachment in voles
( Donaldson and Young, 2008 ). The expression differences appear to be largely
connected to variations in the length of a highly polymorphic microsatellite
(a stretch of repetitive DNA-sequences) located at the 5 0 flanking region of
the gene. It seems likely that the length of this microsatellite may affect the
transcription level of the gene. Correlations between the length of the microsat-
ellite allele and sociality have been demonstrated in birds, voles, and humans.
The other of the two closely related nine-amino-acid neuropeptides,
oxytocin, has received a large interest for its role in social bonding and nursing
and nurturing in humans. It exerts basically the same effects across different
mammalian species, where increased levels of circulation are associated with
a stronger bonding and nurturing tendency ( Donaldson and Young, 2008 ).
It appears that oxytocin mainly exerts its effects on females while vasopressin
chiefly is the male effective hormone.
Selection for favorable variants of the receptor genes of these neuropep-
tides would hypothetically be a plausible pathway for causing the domesti-
cated social behavior, which seems to be characterized by increased social
tolerance and an increased ability to bond with humans. However, there has
been little direct research into this possibility. Meanwhile, some suggestive
observations lend a bit of support to the hypothesis.
In a large-scale QTL-analysis of a cross between Red Junglefowl and
domesticated White Leghorn layers, a major QTL affecting growth and several
other domestication-related traits, was discovered on chromosome 1 ( Kerje
et al., 2003; Sch¨tz et al., 2002 ). The QTL was named Growth 1, in recogni-
tion of its immense effect on growth rates—the genotype at this locus explains
more than 20% of the difference in adult body weight between the breeds.
In addition, Growth 1 also affects several other domestication related traits,
such as egg production and fearfulness, as measured in open-field tests.
A closer analysis of the gene content in the region showed that AVPR1a
is located centrally in the QTL. Also another vasopressin receptor AVPR2,
as well as the oxytocin gene, are well within the confidence interval of
Growth 1. A plausible hypothesis would therefore be that social behavior
could be modified by these loci as a correlated response to selection for
increased growth.
Wir ´ n and colleagues bred birds from an advanced inter-cross line
between Red Junglefowl and White Leghorns to be homozygous for alterna-
tive alleles at a microsatellite marker in Growth 1, closely linked to AVPR1a
( Wir ´ n and Jensen, 2011 ). These birds, and birds from the pure lines, were
tested in different social setups. Red Junglefowl, as expected, were more
aggressive towards intruders and less inclined to interact with strangers.
In the inter-cross birds, the difference in genotype on the selected marker