Abstract
BackgroundMediterranean islands host a disproportionately high level of biodiversity and endemisms. Growing phylogeographic evidence on island endemics has unveiled unexpectedly complex patterns of intra-island diversification, which originated at diverse spatial and temporal scales. We investigated multilocus genetic variation of the Corsican-Sardinian endemic lizard Podarcis tiliguerta with the aim of shedding more light on the evolutionary processes underlying the origin of Mediterranean island biodiversity.ResultsWe analysed DNA sequences of mitochondrial (12S and nd4) and nuclear (acm4 and mc1r) gene fragments in 174 individuals of P. tiliguerta from 81 localities across the full range of the species in a geographic and genealogical framework. We found surprisingly high genetic diversity both at mitochondrial and nuclear loci. Seventeen reciprocally monophyletic allopatric mitochondrial haplogroups were sharply divided into four main mitochondrial lineages (two in Corsica and two in Sardinia) of Miocene origin. In contrast, shallow divergence and shared diversity within and between islands was observed at the nuclear loci. We evaluated alternative biogeographic and evolutionary scenarios to explain such profound discordance in spatial and phylogenetic patterning between mitochondrial and nuclear genomes. While neutral models provided unparsimonious explanations for the observed pattern, the hypothesis of environmental selection driving mitochondrial divergence in the presence of nuclear gene flow is favoured.ConclusionsOur study on the genetic variation of P. tiliguerta reveals surprising levels of diversity underlining a complex phylogeographic pattern with a striking example of mito-nuclear discordance. These findings have profound implications, not only for the taxonomy and conservation of P. tiliguerta. Growing evidence on deep mitochondrial breaks in absence of geographic barriers and of climatic factors associated to genetic variation of Corsican-Sardinian endemics warrants additional investigation on the potential role of environmental selection driving the evolution of diversity hotspots within Mediterranean islands.
Highlights
Mediterranean islands host a disproportionately high level of biodiversity and endemisms
These islands offer an ideal setting for investigating the evolutionary processes behind the origin and the structure of current patterns of insular biodiversity hotspots because: (i) they have a complex topography with a diversity of landscapes and microclimatic regions, spanning from Mediterranean to alpine climates [5, 6], which combined with the imprints of Plio-Pleistocene climatic oscillations [7,8,9,10] contribute to the diversification and persistence of old lineages [11,12,13]; (ii) the palaeogeographical evolution of the Corsican-Sardinian system within the Western Mediterranean is well established [14,15,16,17,18], providing a useful framework for biogeographic and molecular inferences [19]; and (iii) emerging phylogeographic and phylogenetic data on many endemics allow conclusions to be drawn within a comparative framework [see e.g. [19, 20]
Corsican-Sardinian species have been the subject of intensive phylogeographic surveys, especially regarding amphibians and reptiles [12, 13, 20,21,22,23,24,25,26,27,28,29,30,31,32,33,34], which have revealed how an essential component of the Tyrrhenian biodiversity hotspot is represented by the genetic variation held within and among populations of these endemic species
Summary
Mediterranean islands host a disproportionately high level of biodiversity and endemisms. Essentially each phylogeographical reconstruction carried out so far on Corsican-Sardinian species suggested an idiosyncratic scenario for the evolution of the current geographical patterns of intraspecific genetic diversity [12, 19, 20, 30, 31] This suggests that we are still far from either an exhaustive inventory or a deep understanding of the evolutionary processes underlying the origins and diversity of this biodiversity hotspot
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