Proximity to the introduction site and local dynamics
Our admixture analysis with the set of species-specific loci showed
bidirectional introgressive hybridization between I. elegans andI. graellsii . Individuals from both hybrid regions, north-west
and north-central Spain, mainly belonged to the introgressed I.
elegans and introgressed I. graellsii classes, and less
frequently to the backcross to I. elegans , backcross to I.
graellsii , F1 and F2 hybrid classes.
Almost no pure I. elegans or pure I. graellsii were found
in the hybrid regions.
When comparing the distribution of
parental and hybrid classes between north-west and north-central hybrid
regions, all classes were at similar frequency proportions except for
the introgressed I. elegans class, which was significantly higher
in the north-west hybrid region.
The proximity to the introduction
or source locality could affect hybridization outcomes (Fitzpatrick et
al., 2010; Lepais et al., 2009), as the rate and directionality of
introgression can be influenced by the species’ relative abundance and
therefore by gene flow from the native to the non-native species. A
non-native colonising species is usually rare and matings with the
native species are likely, as was detected in other studied organisms
(Fitzpatrick et al., 2010; Lepais et al., 2009; Quilodrán, Nussberger,
Montoya-Burgos, & Currat, 2019). Introgressed genes can reach high
frequency in the non-native species by a rapid demographic growth,
resulting in asymmetric introgression of neutral genes (Currat, Ruedi,
Petit, & Excoffier, 2008). The north-central hybrid region is closer to
the allopatric distribution of I. elegans and disconnected to theI. elegans populations from the north-west hybrid region. In
fact, the number of populations dominated by I. elegans , and the
abundance of I. elegans in populations with both species in the
north-western hybrid region is lower than in the
north-central hybrid region which
is closer to the allopatric distribution of I. elegans (Boudot et
al., 2009; Sanchez-Guillen, Van Gossum, & Cordero Rivera, 2005;
Sánchez-Guillén et al., 2011)
In both hybrid regions local populations showed pronounced variation in
species composition and in the degree of hybridization and
introgression, which suggests that the local dynamics is even more
distinct than the regional dynamics in this system (Fig. 2A-B). In four
out of the six study populations where both species co-occur at
different numbers (Cañas, Louro, Perdiguero and Villar), nine
individuals morphologically classified I. graellsii were
genetically assigned to F1 or F2 hybrid
classes. This is consistent with observations in the laboratory that
F1 hybrids are morphologically similar to I.
graellsii (Sanchez-Guillen et al., 2005), and independent of the
reciprocal cross direction (I. elegans male mated with I.
graellsii or hybrid female, or I. graellsii male mated withI. elegans or hybrid female) that has produced the
F1 or the F2 hybrid (RAS-G personal
observation). Local dynamics was dominated by periodic dry-ups followed
by recolonizations by one or both species. For instance, Cañas and
Villar (north-central populations) were emptied and recolonized a couple
of years before we sampled them. Similarly, the north-western population
Louro became extinct due to salobrisation by seawater in 2009, and was
recolonized by both species, forming a hybrid population with high
levels of admixture. In Arreo and Valbornedo we did not detect
F1 or F2 hybrids, even though both
species are present. Even the six studied populations with only one of
the genotypical-parental species [Doniños, Laxe and Menorca (onlyI. elegans ), and Mateo, Valpierre and Xuño (only I.
graellsii )] showed different levels of introgression or back crosses,
indicating that although only one species was detected in the sampling
years both species were possibly present in the population in the years
before. This can be the case of Xuño that had both species during
2003-2006 and was naturally dried in 2007 and after that recolonized byI. graellsii , which explains the observed levels of admixture.