5.3 Interspecific hybridization
Interspecific hybridization is now recognised as being reasonably
frequent, occurring in some 10% of animal and 25-30% of plant species
(Mallet, 2005; Rieseberg, Wood, & Baack, 2006). Increasingly, through
unintended anthropogenic dispersal, pairs of species are interacting for
the first time since they last shared a common ancestor (Grabenstein &
Taylor, 2018; McFarlane & Pemberton, 2019; Muirhead et al., 2015;
Seebens et al., 2015). This opens the possibility of adaptive gene
exchange that could contribute to invasion success in much the same way
it does within species (Hovick & Whitney, 2014). However, interspecific
hybridization will more commonly produce unfit offspring compared to
intraspecific admixture, as reproductive incompatibilities are more
likely to have accumulated. The trade-off between the cost of
hybridization and the benefit of adaptive introgression creates ideal
conditions for the study of speciation.
One example of interspecific introgression during invasion is the
Iberian hare, Lepus granatensis , which replaced the now-extinct
Arctic species, L. timidus , in its northern range. IL12B ,
a gene implicated in the inflammatory process and immune response to
viruses in rabbits, underwent adaptive introgression from L.
timidus to L. granatensis , potentially contributing to its
northern range expansion following the last glacial maximum (Seixas,
Boursot, & Melo-Ferreira, 2018). Similarly, some introduced populations
of the three-spined stickleback (Gasterosteus aculeatus ) have
higher genetic diversity as a result of introgression from G.
nipponicus (Yoshida et al., 2016).
In some cases, hybridization might contribute to increased fitness of
native species. For example, the crop pests Helicoverpa armigeraand H. zea developed strong prezygotic barriers to hybridization
following more than one million years of divergence in allopatry (Laster
& Sheng, 1995). H. armigera was introduced to Brazil within the
past decade, where it encountered the native H. zea . Whole genome
resequencing has shown that the pesticide resistance alleleCYP337B3 was subsequently introduced from H. armigera toH. zea , increasing its ability to evade previously effective
control measures (Valencia-Montoya et al., 2020).
Another related phenomenon is hybridization among multiple co-invading
species. Examples include fishes in the genera Hypophthalmichthys(carp) and Cottus (freshwater sculpins), and invasive fungi in
the genus Ophiostoma (the cause of Dutch elm disease)
(Dennenmoser et al., 2017, 2019; Hessenauer et al., 2020; Wang et al.,
2020). Notably, interspecific introgression among Ophiostomaspecies increased genetic diversity and was associated with individual
growth rate (Hessenauer et al., 2020). Moreover, introgression on
chromosome 1 was positively associated with virulence, apparently as a
consequence of adaptive introgression (Hessenauer et al., 2020). These
observations suggest that interspecific hybridization can create novel
combinations of adaptive variants that enhance spread, in addition to
mitigating the negative impacts of population bottlenecks by maintaining
genetic diversity.
Regardless of whether it increases the spread of invasive species,
interspecific hybridization can threaten local biodiversity as a result
of genetic swamping (where local genotypes are replaced by hybrids) or
demographic swamping (where local population growth rates are reduced
via outbreeding depression) (Todesco et al., 2016). Genetic swamping is
both a potentially cryptic mode of extinction (Todesco et al., 2016) and
a mechanism by which genetic material from introduced domesticated
species can dominate populations of wild relatives (Haygood, Ives, &
Andow, 2003). The high resolution of WGR means that it can be a powerful
tool for monitoring and quantifying genetic swamping.