Part III: Conservation and Management382
(Frantz et al. 2015a). The combined use of whole-genome
sequencing data plus the genotypes of 600 pigs and wild boar
typed with the Illumina 60K SNP BeadChip also evidenced that
gene flow was strongly asymmetric, and that the preferential
direction in Europe was from wild boar to pigs (Frantz et al.
2015a). Multiple wild boar populations were probably involved
in these introgression events, demonstrating that pig domesti-
cation was much more complex than previously thought.
Recurrent hybridization between wild boar and pigs would
imply the progressive loss of anatomical and behavioural fea-
tures fixed in porcine populations by artificial selection, but
this has never been observed in the field. Frantz et al. (2015a)
explained this paradoxical outcome by assuming that migration
was counteracted by artificial selection. Moreover, they hypoth-
esized about the existence of genomic regions associated with
domestication (islands of domestication) that would be less sus-
ceptible to the homogenizing effects of gene flow.
Genetic exchanges between pigs and wild boar could have
significant repercussions because hybrid individuals might be
more fertile and larger than their wild counterparts (pigs have
a higher prolificacy and growth rate than wild boar), although
they may also differ in their susceptibility to specific infectious
diseases (Goedbloed et al. 2015). The presence of hybrids in
the wild may be explained by multiple factors. For instance,
Japanese wild boar populations are sometimes restocked with
wild boar–pig hybrids, the so-called inobuta, which are much
easier to hunt than their purebred wild counterparts (Knight
2003). The escape or intentional release of hybrid individu-
als from farms, where they are bred to produce a luxury meat,
e.g. Iron Age pigs in the UK (Booth 1995), could be another
source of introgression. The mating of abandoned pot-bellied
Vietnamese pigs (Figure 34.6), often used as pets until they
reach their adult size, with wild boar has also been reported in
Spain (Delibes-Mateos & Delibes 2013).
The consequences of these introgressions could be very
negative because they may contribute to the uncontrolled
demographic growth of wild boar populations. Wild boar candamage crops, provoke traffic accidents and transmit infectious
diseases, such as swine fever, pseudorabies, and brucellosis, to
pigs (Cahill et al. 2003). Besides, they can prey on the eggs of
ground-nesting birds, amphibians and reptiles, and, by feeding
on seeds or seedlings, they can also hinder the regeneration of
certain tree species (Cahill et al. 2003; Massei & Genov 2004).
Rooting could also cause the destruction of woodland plants
and grasslands (Massei & Genov 2004). Measures impeding the
accidental or intentional release of pigs in the wild, combined
with controlled hunting and other preventive policies, would
be an important contribution towards the sustainable manage-
ment of wild boar populations in areas where the density of this
species has significantly increased.The Role of Selection in Shaping the Genetic
Variation of Wild Boar
Encompassing Eurasia and North Africa, the geographic
distribution of wild boar is one of the widest of all terrestrial
mammalian species. Thanks to their extraordinary adaptive
capacity, wild boar have been able to thrive in a broad variety
of environments differing in temperature, altitude, topogra-
phy, humidity, pathogen and predator pressures, vegetation,
and food availability. Ecological and biological factors shape
genetic variation by favouring the preferential transmission of
certain alleles associated with an increased chance of surviv-
ing and mating (Vitti et al. 2013). This type of selection, called
positive selection, can be detected by measuring, at a genome-
wide scale, certain informative parameters (Vitti et al. 2013), i.e.
(i) the amount of genetic differentiation among populations
(allele frequencies of a given locus can be very different in
populations under selection vs those unselected); (ii) the quo-
tient of the rate of non- synonymous substitutions per non-
synonymous site (dN) vs the rate of synonymous substitutions
per synonymous site (dS), where a dN/dS above 1 may indicate posi-
tive selection; (iii) the site frequency spectrum (selection causes a
population-wide decrease of variation around the selected locus);Figure 34.6 Potential hybrids of
Vietnamese pot-bellied pig together
with a young wild boar (at the right
side of the larger picture) killed in
a hunting day. The morphological
aspect of the potential hybrids differs
notably from the young wild boar
and from a docile pig (captured by
a hunter in the same region; see
small picture at the upper right side).
These two photographs have been
extracted from Delibes-Mateos and
Delibes (2013) with permission of the
editors and authors..03612:55:56