Abstract
BackgroundHybridization between incipient species is expected to become progressively limited as their genetic divergence increases and reproductive isolation proceeds. Amphibian radiations and their secondary contact zones are useful models to infer the timeframes of speciation, but empirical data from natural systems remains extremely scarce. Here we follow this approach in the European radiation of tree frogs (Hyla arborea group). We investigated a natural hybrid zone between two lineages (Hyla arborea and Hyla orientalis) of Mio-Pliocene divergence (~5 My) for comparison with other hybrid systems from this group.ResultsWe found concordant geographic distributions of nuclear and mitochondrial gene pools, and replicated narrow transitions (~30 km) across two independent transects, indicating an advanced state of reproductive isolation and potential local barriers to dispersal. This result parallels the situation between H. arborea and H. intermedia, which share the same amount of divergence with H. orientalis. In contrast, younger lineages show much stronger admixture at secondary contacts.ConclusionsOur findings corroborate the negative relationship between hybridizability and divergence time in European tree frogs, where 5 My are necessary to achieve almost complete reproductive isolation. Speciation seems to progress homogeneously in this radiation, and might thus be driven by gradual genome-wide changes rather than single speciation genes. However, the timescale differs greatly from that of other well-studied amphibians. General assumptions on the time necessary for speciation based on evidence from unrelated taxa may thus be unreliable. In contrast, comparative hybrid zone analyses within single radiations such as our case study are useful to appreciate the advance of speciation in space and time.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0385-2) contains supplementary material, which is available to authorized users.
Highlights
Hybridization between incipient species is expected to become progressively limited as their genetic divergence increases and reproductive isolation proceeds
The geographic distribution of these STRUCTURE groups closely matches mitochondrial data, with intermediate admixture proportions in parapatric populations likely resulting from genetic introgression
Introgression between the two gene pools appears very limited, with patterns of spatial structure testifying to sharp transitions across the contact zone in both transects
Summary
Hybridization between incipient species is expected to become progressively limited as their genetic divergence increases and reproductive isolation proceeds. Amphibian radiations and their secondary contact zones are useful models to infer the timeframes of speciation, but empirical data from natural systems remains extremely scarce. We follow this approach in the European radiation of tree frogs (Hyla arborea group). Depending on how advanced the speciation process is, hybridization at secondary contact may either merge back recently diverged gene pools, or, contrarily, select for pre-zygotic barriers (i.e. preventing interspecific mating and incompatible hybrids) and reinforce reproductive isolation [3]. Hybrid zone analyses showed that complete isolation can Dufresnes et al BMC Evolutionary Biology (2015) 15:155 already be reached by Pliocene-diverged taxa (2.6-5.3 Mya; e.g. Bombina toads [6, 7]; hylid frogs [8, 9]; bufonid toads [10]), and that only recently diverged lineages (
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