5.2 Intraspecific hybridization
As discussed in Part 3.2, a longstanding challenge in invasion biology is to explain how invasive species overcome or avoid the deleterious consequences of a demographic bottleneck. One solution to this challenge is seen where invasions involve admixture among multiple genetically differentiated source populations (Cristescu, 2015; Smith et al., 2020). For example, population genomic studies of the invasive fungusCryphonectria parasitica (causing chestnut blight) and the fall armyworm (Spodoptera frugiperda ) show that gene flow among invasive lineages maintains genetic diversity (Demené et al., 2019; Tay et al., 2020; Yainna et al., 2020). Admixture not only alleviates the effects of inbreeding depression but can lead to the sorting of adaptive alleles into beneficial combinations. This may often explain the ‘bridgehead effect’, where an initially successful invasion acts as a source of colonists for subsequent invasions (Lombaert et al., 2010). Rispe et al. (2020) provided a recent example, showing that multiple native North American populations of the viticultural pestDaktulosphaira vitifoliae were introduced to France. The invasive hybrid French population then acted as a bridgehead for the subsequent invasion of vineyards throughout the rest of Europe, and possibly throughout the Southern Hemisphere.
Strong genomic evidence for the sorting of adaptive alleles following the hybridization of genetically differentiated populations has also come from replicate studies of geographic clines. In introduced Australian and North American populations of Drosophila melanogaster , an FST outlier scan was used to identify polymorphisms responsible for parallel latitudinal clines in both continents (Bergland, Tobler, Gonzalez, Schmidt, & Petrov, 2016). In both cases, invasive populations were the result of hybridization between African and European populations. High-latitude populations in both North America and Australia share more ancestry with native European populations, in contrast to low-latitude populations that share more African ancestry.
[ FIGURE 5 ]
Similarly, African honey bees (Apis mellifera scutellata ) were introduced to the Americas in 1957 where they hybridize with, and often outcompete, conspecifics with European ancestry (Figure 5A) (Calfee, Agra, Palacio, Ramírez, et al., 2020). Genome-wide geographic clines in African ancestry in both California and Argentina (Figure 5B) are concordant with phenotypic clines in wing length, consistent with a model of highly polygenic divergent selection in response to differences in climate. Using this framework of migration-selection balance, loci that introgressed further along the transect than expected (i.e. , those loci important for spread, subjected to different spatially
varying selection pressure) could be identified (Figure 5C). In both of these examples, spatially varying selection in hybrid populations has sorted standing genetic variation along environmental gradients, facilitating the rapid adaptation of these invasive species.