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.