Tracing species replacement in marbled newts

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.