Abstract
Secondary contact between closely related species can lead to the formation of hybrid zones, allowing for interspecific gene flow. Hybrid zone movement can take place if one of the species possesses a competitive advantage over the other, ultimately resulting in species replacement. Such hybrid zone displacement is predicted to leave a genomic footprint across the landscape in the form of asymmetric gene flow (or 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 asymmetric gene flow and a distribution relict (i.e., an enclave) of Triturus marmoratus in the range of T. pygmaeus. We developed a panel of nuclear and mitochondrial SNP markers to test for the presence of a T. marmoratus genomic footprint in the Lisbon peninsula, south of the enclave. We found no additional populations of T. marmoratus. Analysis with the software Structure showed no genetic traces of T. marmoratus in T. pygmaeus. A principal component analysis showed some variation within the local T. pygmaeus, but it is unclear if this represents introgression from T. marmoratus. The results may be explained by (a) species replacement without introgressive hybridization and (b) displacement with hybridization followed by the near‐complete erosion of the footprint by purifying selection. We predict that testing for a genomic footprint north of the reported enclave would confirm that species replacement in these marbled newts occurred with hybridization.
Highlights
Population genetic structure observed today has been shaped by demographic processes that operated in the past (Avise, 2004)
We employed a panel of presumably unlinked neutral markers to test for genetic traces of species replacement with hybridization in two species of marbled newts in Portugal
A northward hybrid zone movement has been proposed for this system, with a documented T. marmoratus enclave signaling the competitive advance of T. pygmaeus
Summary
Population genetic structure observed today has been shaped by demographic processes that operated in the past (Avise, 2004). The genus includes the northern marbled newt Triturus marmoratus (Latreille, 1880), which inhabits central and southern France as well as the northern part of the Iberian Peninsula, and the pygmy marbled newt T. pygmaeus (Wolterstorff, 1905), which occupies the southwestern parts of the Iberian Peninsula These are sister species that engage in a hybrid zone spanning ca. We previously documented an enclave of T. marmoratus in the northwest of the Lisbon Peninsula, near the town Caldas da Rainha, composed of pure and introgressed populations (Figure 1b). Another remarkable spatial signature is the presence of T. pygmaeus in and along the coastal dunes, approximately from Caldas da Rainha to the Aveiro Lagoon (Figure 1). We modeled the mutual species' distributions from climatic conditions of the present day, the Mid-Holocene and the Last Glacial Maximum, to investigate whether climate change would support the scenario of species replacement
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