Behavioral Genetics and Animal Domestication - Genetics and the Behavior of Domestic Animals - page 49

percentage of the matings. This is especially true when selection indices
(BLUP and the like) are used to calculate the most desirable individuals to
breed from, and therefore tend to increase the rate of inbreeding ( Belonsky
and Kennedy, 1988; Bijma and Woolliams, 2000 ; reviewed in Kristensen
and Sørensen, 2005 ). This can be a greater problem in certain domestic
breeds than others.
For instance, the level of inbreeding in dogs has been shown to be
extremely high ( Calboli et al., 2008 ), whereas chicken breeds remain
remarkably diverse, even in domestic breeds ( Rubin et al., 2010 ). The gen-
eral effects of inbreeding are commonly agreed to lead to inbreeding depres-
sion. In many appearances the opposite of heterosis (or outbreeding vigor),
this is manifest in a decrease in general fitness ( Lynch and Walsh, 1998 ).
Although many such examples exist for a variety of animal models with
rather variable results, as concerning domestication, sheep ( Wiener et al.,
1992, 1994 ) and dairy cows ( Miglior et al., 1995; Smith et al., 1998 ) have
been shown to exhibit effects in primary fitness. Inbreeding can also have
negative effects on behavior. Courtship behavior in guppies ( Mariette et al.,
2006 ) and Drosophila ( Sharp, 1984 ) have both been shown to be negatively
affected by inbreeding. Parental behavior has also been found to be affected
in mice ( Margulis, 1998 ). The detrimental effects of inbreeding can also be
seen in the numerous mechanisms to avoid inbreeding (for example non-selfing
mechanisms in plants ( Charlesworth and Charlesworth, 1979 ) and behavioral
mechanisms in animals ( Pusey and Wolf, 1996 )).
The mechanism through which the negative effects of inbreeding act
have traditionally been thought of as being due to the effects of dominance
between loci ( Lynch and Walsh, 1998 ). If the effects were purely additive,
they would be independent of the genetic background and there should be
no effect of increased homozygosity. There are three general hypotheses
for describing inbreeding depression: the (erroneously named) dominance
hypothesis, the over-dominance hypothesis, and the breakdown of epistasis
hypothesis. The dominance hypothesis is in fact nothing to do with domi-
nance, but is related to the expression of deleterious recessives that are more
likely to arise due to identity by descent ( Bruce, 1910; Davenport, 1908 ).
The overdominance hypothesis relies on the fact that heterosis causes an
increase in fitness, therefore a reduction in heterozygosity (as occurs during
inbreeding) will lead to a reduction in fitness ( Bruce, 1910; Davenport,
1908 ). This hypothesis means that variation is maintained in the absence
of mutation, whereas with the dominance hypothesis a flux of deleterious
mutations arise and then become fixed in the inbred individuals. The break-
down of epistasis is a more recent theory and relates to the web of epistatic
interactions that potentially exist between alleles ( Templeton and Read,
1994 ). As these are disrupted in inbred individuals (especially if heterozy-
gosity is required for these epistatic interactions) this could explain both
heterosis (hybrid vigor) and inbreeding depression.