Aegean reptiles reveal secrets to speciation
Phylogeography uses DNA to study the geographic distribution of biodiversity to determine the evolutionary and ecological processes that have left a genetic imprint on living organisms. This provides a historical perspective, which is vital to the study of speciation, biogeography, taxonomy and conservation biology. Lying close to Africa and Asia, the Aegean is one of the most species-rich regions of Europe and a natural laboratory for phylogeographic studies. The region’s geological history includes many periods when sea barriers appeared and disappeared, isolating populations or allowing them to come into contact with one another. Barriers are essential for the process of speciation, in which new species are created. The CO-PHY-MED project used squamate reptiles (lizards and snakes) as model organisms to conduct a large-scale multi-species comparative phylogeographic analysis in the Aegean region. “Reptiles are outstanding models in ecology and evolution because their temperature dependence makes them particularly sensitive to climatic change and the diversity they exhibit in their ability (or lack of) to overcome barriers,” says lead researcher Panagiotis Kornilios.
A better understanding of speciation
Phylogeography is traditionally based on analyses of mitochondrial DNA, the part of the genome that is usually only inherited from the mother. However, modern studies rely on multiple genetic markers, scattered throughout the entire genome, to give more robust, accurate and reliable results. Researchers studied more than 700 museum samples of over 40 species of lizards and snakes. By using next generation sequencing (NGS) to analyse thousands of independent genomic markers they identified patterns that are quite different from what is currently known about the history of the species studied. “Our understanding of underlying mechanisms is changing as only using mitochondrial DNA in the study of speciation and biogeography may lead to oversimplified results,” explains Kornilios. Combining mitochondrial DNA and NGS analysis gives a more complete picture of the diversification history of the organisms studied, by providing a more detailed record of biodiversity and identifying cryptic biodiversity elements, even new species, and geographic regions and important barriers that have acted as drivers of speciation.
Island castaways
Some results reveal unexpected biogeographical patterns. For example, the green lizards of the genus Lacerta seem to have colonised the Balkan Peninsula from Asia Minor by crossing the ancient Aegean Sea, using the islands of that era as stepping stones. This has left lizard-castaways on the central Aegean islands that, millions of years later, we can identify as a separate ‘cryptic’ species. Meanwhile, results for the four-lined snake showed evidence of islands where populations are genetically admixed between ancient inhabitants and more recent settlers, as a result of multiple colonisations. CO-PHY-MED represents an innovative large-scale approach for comparative phylogeographical reconstructions and was undertaken with the support of the Marie Skłodowska-Curie programme. “The project provides not only detailed evidence of biodiversity but also its drivers. It can, therefore, assist with the effective management and conservation of biodiversity, a major challenge, especially in the face of environmental change,” Kornilios concludes.
Keywords
CO-PHY-MED, biodiversity, Aegean, next generation sequencing (NGS), phylogeography, evolution, mitochondrial DNA, islands, sea barriers