Gone with the plate: the opening of the Western Mediterranean basin drove the diversification of ground-dweller spiders.

Leticia Bidegaray-Batista,Miquel A Arnedo

doi:10.1186/1471-2148-11-317

Abstract

BackgroundThe major islands of the Western Mediterranean--Corsica, Sardinia, and the Balearic Islands--are continental terrenes that drifted towards their present day location following a retreat from their original position on the eastern Iberian Peninsula about 30 million years ago. Several studies have taken advantage of this well-dated geological scenario to calibrate molecular rates in species for which distributions seemed to match this tectonic event. Nevertheless, the use of external calibration points has revealed that most of the present-day fauna on these islands post-dated the opening of the western Mediterranean basin. In this study, we use sequence information of the cox1, nad1, 16S, L1, and 12S mitochondrial genes and the 18S, 28S, and h3 nuclear genes, along with relaxed clock models and a combination of biogeographic and fossil external calibration points, to test alternative historical scenarios of the evolutionary history of the ground-dweller spider genus Parachtes (Dysderidae), which is endemic to the region.ResultsWe analyse 49 specimens representing populations of most Parachtes species and close relatives. Our results reveal that both the sequence of species formation in Parachtes and the estimated divergence times match the geochronological sequence of separation of the main islands, suggesting that the diversification of the group was driven by Tertiary plate tectonics. In addition, the confirmation that Parachtes diversification matches well-dated geological events provides a model framework to infer substitution rates of molecular markers. Divergence rates estimates ranged from 3.5% My-1 (nad1) to 0.12% My-1 (28S), and the average divergence rate for the mitochondrial genes was 2.25% My-1, very close to the "standard" arthropod mitochondrial rate (2.3% My-1).ConclusionsOur study provides the first unequivocal evidence of terrestrial endemic fauna of the major western Mediterranean islands, whose origin can be traced back to the Oligocene separation of these islands from the continent. Moreover, our study provides useful information on the divergence rate estimates of the most commonly used genes for phylogenetic inference in non-model arthropods.

Highlights

The major islands of the Western Mediterranean–Corsica, Sardinia, and the Balearic Islands–are continental terrenes that drifted towards their present day location following a retreat from their original position on the eastern Iberian Peninsula about 30 million years ago
The alignment of the non-protein coding genes, including informative gap characters coded as absence/presence characters, were merged with the protein-coding genes, resulting in a combined matrix of 4454 characters for the “default alignment”, 4462 for the “gappy alignment” and 4447 for the “compressed alignment”
Gap characters were included in the parsimony and Bayesian inference analyses but excluded in maximum likelihood and divergence time analyses

Summary

Introduction

The major islands of the Western Mediterranean–Corsica, Sardinia, and the Balearic Islands–are continental terrenes that drifted towards their present day location following a retreat from their original position on the eastern Iberian Peninsula about 30 million years ago. We use sequence information of the cox, nad1, 16S, L1, and 12S mitochondrial genes and the 18S, 28S, and h3 nuclear genes, along with relaxed clock models and a combination of biogeographic and fossil external calibration points, to test alternative historical scenarios of the evolutionary history of the ground-dweller spider genus Parachtes (Dysderidae), which is endemic to the region. The use of molecular data to estimate divergence times goes back to the mid 60’s of the past century [1], the number of studies that include time estimation has increased rapidly over the last decade due to the ever increasing amount of DNA sequence data and the development of new algorithms that relax the limiting assumptions of Genetic distances are transformed into absolute divergence times by incorporating calibration points. The appropriate use of biogeographic events to calibrate phylogenies requires a well-documented geochronology and the demonstration that the assumed barriers constitute a true obstacle to dispersal for the focal group

Methods

Results

Conclusion