Chapter 34: A genomic perspective about wild boar demography and evolution385
showed a combination of short and very long ROHs that are
indicative of ancient bottlenecks and recent inbreeding, respec-
tively (Iacolina et al. 2016).
The predicted consequences of the genetic bottlenecks
experienced by wild boar populations would be the following:
(i) a reduction in the additive genetic variance that at some
instances may compromise the evolutionary potential of this
species to adapt to a dynamic environment (Fisher 1930), and
(ii) the purging of strongly deleterious alleles (because they
usually segregate at low frequencies), although this feature
would not necessarily imply an increase in fitness because
homozygosity would augment concurrently (Balick et al.
2015). This purging process is also influenced by the fact that
low effective size decreases the efficiency of selection against
mildly deleterious alleles. In any case, the relative importance
of genetic variance reduction and purifying selection is modu-
lated not only by the effective size of the population but also
by the duration of the bottleneck. Wild boar are a polygynous
species with a strong reproductive skew, a feature that tends to
decrease effective population size. At the same time, wild boar
are very prolific and have a high growth rate potential, a cir-
cumstance that tends to decrease the duration of bottlenecks
(Hoelzel 1999). Although the genetic diversity of wild boar in
certain geographic areas is low, probably as a consequence of
the factors outlined above, it is clear that the viability of this
species is not compromised at all. In fact, Europe is witnessing
a demographic explosion of wild boar because global warm-
ing is associated with milder winters as well as with the over-
production of acorns and chestnuts. The reforestation of lands
dedicated to agriculture, the existence of protected areas, and
the establishment of vast rapeseed and maize monocultures to
produce animal food and biofuel have also favoured the growth
of wild boar populations around the world.
Finally, it is worth mentioning that the expansion of wild
boar in Europe during the past decades is often based on geo-
graphically isolated, and frequently marginalized, populations.
Local populations, in some cases, even show high admixture
with pigs, and in other regions they display signs of restocking
of wild boar based on populations from elsewhere in Europe.
Expansion of inbred or admixed populations is often pro-
moted by ecological corridors that are required by national and
European legislation. Therefore, such inbred or admixed popu-
lations can become the main source for building far larger and
more widespread populations, which will determine the future
genetic composition of wild boar in Europe (Goedbloed et al.
2013a,b, 2015).Figure 34.8 Runs of homozygosity (ROH) are regions of the genome that display homozygous genotypes for a number of consecutive markers because identical
haplotypes have been inherited from each parent. Homozygosity can be increased by inbreeding, bottlenecks and founder effects. In this picture, the average
number of ROH is plotted against the average ROH length. A total of 15 Bisaro (BIO1) pigs from Portugal, 31 Iberian (IB) pigs from Spain, and 18 wild boars (WB) from
Portugal and Spain have been typed with the Porcine 60K SNP BeadChip (Illumina). Each dot represents an individual. It can be seen that Bisaro pigs have a lower
percentage of their genomes covered by ROH (0–200 Mb) when compared to Iberian pigs (200–800 Mb) and wild boars (200–600 Mb), suggesting that this breed
has been less affected by consanguinity and demographic recession. The genomes of three Bisaro and three Iberian pigs were covered by ROH to a significant
extent (> 600 Mb), suggesting the occurrence of recent inbreeding. This genomic information is important for conservation purposes because ROH are enriched in
deleterious mutations. Figure extracted from Herrero-Medrano et al. (2013) with permission of the editors and authors.
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