Introduction
Past population demographics shape present genetic structure (Avise,
2004). Population genetic methods unravelling current allelic patterns
may thus provide insight into past and extant species distribution
dynamics (Avise et al., 1987; Crisci, Katinas, & Posadas, 2003).
Hybrid zones, where genetically distinct populations meet and admix, are
characterised by gene flow through time and space (Barton & Hewitt,
1985; Hewitt, 2001). These ‘natural laboratories’ are often established
when recently diverged species meet, for example following secondary
contact during range expansion of species from glacial refugia
(Excoffier, Foll & Petit, 2009; Hewitt, 1988; Taberlet, Fumagalli,
Wust-Saucy, & Cosson, 1998).
Spatio-temporal dynamics among hybridising taxa can result in the
formation of enclaves (Buggs, 2007; Wielstra, Burke, Butlin, & Arntzen,
2017a). Enclaves form when the population of one species is surrounded
by populations of a competing related species, becoming genetically
isolated from the remainder of the receding species’ range (Arntzen,
1978). Enclaves can therefore illustrate historical species replacement,
particularly in ground-dwelling organisms with low dispersal
capabilities. Moving hybrid zones may leave a specific spatial
signature, in the form of a molecular genomic footprint (Scribner &
Avise, 1993; Wielstra, 2019). As an advancing species spreads into a
contact zone, neutral alleles may flow from the retreating to the
invading taxon and introgression will be more pronounced in the
advancing than in the receding species (Moran, 1981; van Riemsdijk,
Butlin, Wielstra, & Arntzen, 2019). Asymmetric introgression may
reflect neutral alleles left in the wake of the moving hybrid zone and
eventually become geographically stable over time, as the genomic
footprint is solely dependent on drift (Barton & Hewitt, 1985; Currat,
Ruedi, Petit, & Excoffier, 2008). Hence, introgression patterns of
selectively neutral traits can be used to reconstruct the history of
hybrid zones (Wielstra et al. , 2017b).
We previously documented an enclave and limited interspecific gene flow
in the marbled newt Triturus marmoratus (Latreille, 1880) around
Caldas da Rainha in the northwest of the Lisbon Peninsula, and suggested
that the observation was best explained by the competitive advance of
its sister-species, the pygmy marbled newt T. pygmaeus(Wolterstorff, 1905) (Espregueira Themudo & Arntzen, 2007; Wielstra,
Sillero, Vörös, & Arntzen, 2014). Long-distance colonisation or human
introduction seem unlikely to have originated the enclave, since the
minimum distance to the contact zone exceeds the dispersal capacities ofTriturus newts and there is no tradition of newt husbandry in
Portugal (Espregueira Themudo & Arntzen, 2007). The potential for
interspecific gene flow in these species allows exploring genomic
signals and reconstructing hybrid zone movement (Arntzen, Wielstra, &
Wallis, 2014). We here test for the presence of a genomic footprint ofT. marmoratus in T. pygmaeus south of the enclave
employing newly developed single-nucleotide polymorphism (SNP) markers
with species diagnostic allele variants (Garvin, Saitoh, & Gharrett,
2010; Meilink, Arntzen, van Delft, & Wielstra, 2015).