INTRODUCTION
In
recent years, many insect species have experienced pronounced changes in
their distribution, including several documented cases of range
expansions (González-Tokman et al., 2020; Sánchez-Guillén et al., 2016).
A common signature of range expansion is decreasing genetic diversity at
the range limit, due to frequent founder effects and genetic drift
(Slatkin & Excoffier, 2012). However, such a loss of diversity can be
counteracted by hybridization and introgression in situations where the
expanding species comes in contact with a formerly allopatric sister
taxa (Rieseberg et al., 2007). This process is of increasing interest as
many regions are facing changes due to anthropogenic pressures and
ongoing climate change, which can lead to novel interactions and
hybridization among expanding species (Sánchez-Guillén et al., 2016).
Evidence is accumulating that both the extent and pattern of
hybridization have important conservation implications due to their
direct impact on biodiversity as a whole, and because this process can
sometimes lead to the replacement of one of the hybridizing taxa (Abbott
et al., 2013; Seehausen, 2004; Todesco et al., 2016). Thus,
understanding evolutionary outcomes of hybridization is of increasing
interest for gaining general insights into species dynamics and
conservation in response to environmental change (Abbott et al., 2013).
Hybridization outcomes are manifold and include transfer of genetic
material between species (potentially facilitating their adaptive
evolution), fusion of species, genetic swamping of one species by
another, elicit reinforcement of reproductive isolation between
incompletely isolated species, and the origin of new species (Seehausen,
2004). The likelihood of these outcomes is highly dependent on intrinsic
factors, such as the extent of reproductive isolation, as well as
extrinsic factors, because hybrid fitness may be dependent on the
environment (Coyne, 2004). The study of multiple contact regions of the
same species pair allows to test the consistency of hybridization
outcomes (Harrison & Larson, 2016). Thus, when the range of two closely
related species is large and overlapping, and if this overlap results in
hybridization, then hybridization at several spatial scales can provide
insights into parallel patterns of hybridization in different
demographic and ecological contexts (Abbott et al., 2013). Several
empirical studies, mainly from plants (Buerkle & Rieseberg, 2001;
Haselhorst & Buerkle, 2013; Sweigart, Mason, & Willis, 2007) and
fishes (Aboim, Mavárez, Bernatchez, & Coelho, 2010; Mandeville et al.,
2017; Nolte, Gompert, & Buerkle, 2009), but also from mammals (Good,
Handel, & Nachman, 2008), amphibians (Vines et al., 2003) and insects
(Gompert et al., 2014), have shown that the outcomes of hybridization
varies among hybrid regions of the same species pairs while only few
studies have shown consistency in the pattern, e.g., in plants (Buerkle
& Rieseberg, 2001) and insects (Larson, White, Ross, & Harrison,
2014). Detecting parallelism in the prevailing genomic signatures allows
gaining insights into the process of hybridization itself, for example,
if species have genomic regions that introgress more readily than
others.
Odonata are a group that is heavily affected by increasing temperatures
and many species are changing their distributions (Hassall & Thompson,
2008; Hassall, Thompson, & French, 2007; Hickling, Hill, & Thomas,
2005; Lancaster et al., 2016; Ott, 2010). Within odonates, the damselfly
genus Ischnura is extremely species rich (around 70 species)
(Dijkstra & Kalkman, 2012; Sánchez-Guillén et al., 2020) and includes
many closely related and recently diverged species that co-occur in
partial sympatry over parts of their range (Sánchez-Guillén, Muñoz,
Rodríguez-Tapia, Arroyo, & Córdoba-Aguilar, 2013; Wellenreuther &
Sánchez-Guillén, 2015). This genus has also other interesting properties
such as high local abundances and wide-ranging distribution patterns,
and it is highly enabled for field identification and sampling, making
this a suitable group to explore adaptive introgression caused by
environmental change. Indeed, there are several examples of extensive
hybridization between species within the Ischnura genus, for
example, between I. denticollis and the endangered I.
gemina in the San Francisco Bay (Leong & Hafernik, 1992;
Sánchez-Guillén et al., 2014), and in Europe between I. graellsiiand I. saharensis, and between I. genei and I.
elegans (Sánchez-Guillén, Córdoba-Aguilar, Cordero-Rivera, &
Wellenreuther, 2014).
However, the best documented and explored case of hybridization inIschnura is between I. elegans and I. graellsii in
Spain (Monetti, Sánchez-Guillén, & Cordero Rivera, 2002;
Sánchez-Guillén, van Gossum, & Cordero-Rivera, 2005; Sánchez-Guillén,
Wellenreuther, & Cordero-Rivera, 2012; Wellenreuther et al., 2018).Ischnura elegans and I. graellsii are among the most
frequently studied damselfly species within the order Odonata, and as
such have become eco-evolutionary model species that have been studied
extensively for hybridization and reproductive isolation (Monettiet al. , 2002; Sánchez-Guillén et al. , 2005, 2011, 2012,
2014b; Wellenreuther & Sánchez-Guillén, 2016; Wellenreuther et
al. , 2018). Ischnura elegans and I. graellsii are closely
related and share many morphological, genetic and phenotypic traits
(Sánchez-Guillén et al., 2011), including preference traits for
habitats, and they occupy similar ecological niche space in Spain
(Wellenreuther et al., 2018). Ischnura elegans has dramatically
expanded its distribution in Spain during last 40 years (Sánchez-Guillén
et al., 2011), where its distributional range nowadays overlaps almost
completely with I. graellsii (Fig. 1A-B), which is endemic
to Spain and Maghreb in north-western Africa. The Mediterranean coast
represents the oldest hybrid region with the firsts records of I.
elegans in the late 19th century (Ocharan, 1987),
with increasing records over the country from the early 1980s and
onwards (Ocharan, 1987), and a fast expansion from 2000 to the present
(Fig. 1B). A previous molecular study with microsatellites
estimated that in the Spanish hybrid populations 55-60% of the I.
elegans individuals were introgressed or backcrossed, and that from
10-30% were F1 or F2 hybrids
(Sánchez-Guillén et al., 2011). In the Spanish hybrid region, I.
elegans has replaced I. graellsii from the coast and modified
its original environmental niche to become more similar to the I.
graellsii niche (Wellenreuther et al., 2018).
In the present study, we focus on two hybrid regions in Spain, one in
north-west Spain (Galicia) and one in north-central Spain (La Rioja,
Navarra and Avila). We used
Restriction site–Associated DNA (RAD) sequencing and a recent I.
elegans reference genome assembly (Chauhan et al., 2021) to identify
genome-wide SNPs in individuals from the two hybrid regions as well as
from eight allopatric populations from both species in Spain and
adjacent countries. First, we used species-specific SNPs to analyze the
fine-scale individual ancestry and proportion of individuals in
different hybrid classes (ranging from pure individuals to
F1 and F2 hybrids). With this analysis
we were particularly interested in confirming the occurrence of hybrids
and backcrosses, and in evaluating the local and regional scales of
hybridization dynamics. We used data on the strength of reproductive
barriers and colonization history in an aim to explain the present-day
hybridization patterns. Second, we used the full set of SNPs to compare
the degree genetic variation and genetic differentiation in the Spanish
hybrid regions and adjacent allopatric regions. With these analyses we
were interested in evaluating whether hybridization and introgression
have increased the level of genetic variation of individuals in the
different hybrid classes. We discuss our results in light of parallelism
in hybridization and introgression within and between regions, or the
lack thereof, and in relation to previous data on biased mating success
among I. elegans and I. graellsii . This work is a step
towards understanding the potential role of hybridization and
introgression in facilitating range expansions.