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.