Abstract
Despite increasing appreciation of the “speciation continuum”, delimiting and describing new species is a major yet necessary challenge of modern phylogeography to help optimize conservation efforts. In amphibians, the lack of phenotypic differences between closely-related taxa, their complex, sometimes unresolved phylogenetic relationships, and their potential to hybridize all act to blur taxonomic boundaries. Here we implement a multi-disciplinary approach to evaluate the nature of two deeply-diverged mitochondrial lineages previously documented in Italian tree frogs (Hyla intermedia s. l.), distributed north and south of the Northern Apennine Mountains. Based on evidence from mitochondrial phylogenetics, nuclear phylogenomics, hybrid zone population genomics, niche modelling analyses, and biometric assessments, we propose that these lineages be considered distinct, cryptic species. Both mitochondrial and nuclear data affirm that they belong to two monophyletic clades of Pliocene divergence (~3.5 My), only admixing over a relatively narrow contact zone restricted to the southeast of the Po Plain (50-100km). These characteristics are comparable to similarly-studied parapatric amphibians bearing a specific status. Inferred from their current geographic distribution, the two Italian tree frogs feature distinct ecological niches (<15% of niche overlap), raising questions regarding potential adaptive components contributing to their incipient speciation. However, we found no diagnostic morphological and bioacoustic differences between them. This system illustrates the speciation continuum of Western-Palearctic tree frogs and identifies additional cryptic lineages of similar divergence to be treated as separate species (H. cf. meridionalis). We recommend combined approaches using genomic data as applied here for the future taxonomic assessment of cryptic diversity in alloparapatric radiations of terrestrial vertebrates, especially in controversial taxa. Finally, we formally described the northern Italian tree frogs as a new species, Hyla perrini, sp. nov.
Highlights
Three decades of molecular biogeography have shed light on unsuspected amounts of genetic diversity among terrestrial vertebrate species that were once considered monotypic
We explored the relationships between WesternPalearctic hylids with a Principal Component Analysis (PCA) on the nuclear genomic dataset, discarding the early-diverged H. japonica and H. meridionalis
The nuclear tree is fully resolved (Figure 2), suggesting no signs of past admixture or incomplete lineage sorting, while the mtDNA tree differs slightly at a few unresolved nodes and larger uncertainty in the molecular dating (Figure 1). Both trees fully support the split of H. intermedia s. s. and Hyla perrini sp. nov. as two monophyletic clades, with divergence pointing to the upper Pliocene
Summary
Three decades of molecular biogeography have shed light on unsuspected amounts of genetic diversity among terrestrial vertebrate species that were once considered monotypic. The resulting lineages often remained remarkably cryptic, sharing similar ecologies and featuring no obvious phenotypic differentiation, be it morphology, coloration or behavior, despite millions of years of independent evolution (Bickford et al, 2007, e.g., Wielstra et al, 2013; Diaz-Rodriguez et al, 2017). Notably when species have non-overlapping distribution ranges and do not naturally meet, the amount of genetic divergence (e.g., divergence time) is rather used as a proxy for their level of reproductive isolation. A standardized approach accounting for these limitations is still missing to harmonize species limits in cryptic polytypic radiations
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