Strength of the prezygotic reproductive barriers
Differential outcomes of hybridization may partly be explained by the
strength of the prezygotic
reproductive barriers (Lepais et al., 2009; Mandeville et al., 2017;
Vines et al., 2003) which varies across species (Good et al., 2008;
Sánchez-Guillén et al., 2014) and in hybridizing species, with the time
since sympatry (Kronforst, Young, & Gilbert, 2007; Lemmon & Juenger,
2017; Liao et al., 2019). The frequency distribution of hybrid classes
in a hybrid zone may reflect the strength and direction of reproductive
barriers. A bimodal distribution of the genotypical-like classes
(parental or introgressed individuals) are thought to be the consequence
of strong but incomplete reproductive isolation (e.g., Harrison &
Bogdanowicz, 1997), while lower levels of prezygotic isolation resulting
in a unimodal distribution on the genotypic classes (the distribution
spans a range of admixture and backcrosses toward both parents) (e.g.,
Szymura & Barton, 1991) (Gompert, Mandeville, & Buerkle, 2017; Jiggins
& Mallet, 2000).
The high prevalence of admixed individuals in north-central Spain can be
explained by its level of reproductive isolation as was, for instance,
detected in Catostomus fish species (Mandeville et al., 2017) and
intertidal snails (Littorinids) (Stankowski et al., 2020). In
north-central Spain, 70% of the heterospecific matings between
phenotypically I. elegans and I. graellsii were prevented,
while the backcrosses to I. elegans and the backcrosses toI. graellsii were less and similarly prevented (54% and 50%,
respectively). In fact, from the north-central hybrid region, five
populations (Arreo, Cañas, Perdiguero, Valbornedo and Villar) show a
bimodal distribution (introgressed individuals with I. elegansand I. graellsii ) and Mateo and Valpierre presented an
introgressed distribution (introgressed individuals with I.
graellsii ). However in the north-west hybrid region, heterospecific
matings and backcrosses with I. elegans were similar and strongly
prevented (80% and 79% respectively), while only 46% of the
backcrosses with I. graellsii were prevented. Arce-Valdéset al. (under review) found evidence of an incipient
strengthening of the prezygotic isolation in this hybrid region. From
the north-west hybrid region, three populations (Doniños, Laxe, Xuño)
presented an introgressed distribution towards I. elegans(Doniños, Laxe) and towards I. graellsii (Xuño), and one
population (Louro) presented a unimodal pattern.
Differences in the admixture-class
distributions between the Spanish hybrid regions (I. elegansclass was significantly higher in the north-west hybrid region) can be
explained by the reinforcement of the strength of the reproductive
isolation between I. elegans and I. graellsii in the
north-west hybrid region.