Abstract
Secondary contact between closely related species can lead to the
formation of hybrid zones, allowing for interspecific gene flow among
taxa. Species replacement can take place if one of the species possesses
a competitive advantage over the other, resulting in hybrid zone
movement. This displacement may leave a genomic footprint across the
landscape in the form of asymmetric introgression of selectively neutral
alleles from the displaced to the advancing species. Hybrid zone
movement has been suggested for marbled newts in the Iberian Peninsula,
supported by the presence of a Triturus marmoratus stronghold surrounded
by populations of the supposedly advancing T. pygmaeus in the northwest
of the Lisbon Peninsula, i.e., an enclave. Moreover, a newly constructed
two-species distribution model suggests that climate conditions
following the Last Glacial Maximum may have favoured T. pygmaeus over T.
marmoratus along the Atlantic coast. To test for the presence of a T.
marmoratus genomic footprint in the area that may have witnessed species
displacement, we developed and employed 54 nuclear SNPs and one
mitochondrial DNA marker. We found no additional enclaves nor genetic
traces of T. marmoratus in T. pygmaeus populations. Therefore, two main
hypothesis arise in the absence of a genomic footprint: i) species
replacement without hybridisation, either in allopatry or in sympatry
under strong reproductive isolation; or ii) displacement with
hybridisation where the footprint was eroded due to strong purifying
selection. We predict testing for a genomic footprint north of the
reported enclave could confirm that species replacement in the marbled
newts occurred with hybridisation.