Around 30 millions years ago, the Western Mediterranean basin opened as a result of the tectonic collision of the African and Eurasian plates. This geologic event, described in the scientific literature, was the starting point for the diversification of an endemic group of Western Mediterranean spiders, as has been demonstrated by a new study by Miquel A. Arnedo and Leticia Bidegaray-Batista, from the Department of Animal Biology of the University of Barcelona (UB) and the Biodiversity Research Institute (IRBio).
The paper, published in BMC Evolutionary Biology, focuses on the genus Parachtes, a group of nocturnal ground spiders that spend daylight in silk retreats under stones or dead logs. The spiders of this genus are active hunters that only use silk to build intricate cocoons to protect themselves and their eggsacs. The genus, endemic to the Western Mediterranean, includes close to a dozen species distributed in the Balearic Islands, Corsica, Sardinia and the Iberian and Italic peninsulas. According to Miquel A. Arnedo, leader of the Arthropod, Systematic and Evolution Laboratory of the UB, "this is the first scientific work that proves that such an important tectonic event in the Mediterranean had a relevant impact on the diversification of the local fauna."
Examining hypotheses regarding Mediterranean geology
According to the expert Leticia Bidegaray-Batista, paper's first author, "so far it was assumed that the tectonic collision of the African and Eurasian plates originated the diversification of the fauna endemic to the Western Mediterranean. Nonetheless, no scientific study had confirmed that possibility. With this research, we prove for the first time that the distribution of the genus Parachtes is linked to the tectonic events that occurred in the Mediterranean more than 25 million years ago."
Spiders provide an interesting model for the study of biological and evolutionary processes, which allows for examining hypotheses not yet contrasted, within a temporal context well supported by geological studies. Utilizing molecular techniques to elucidate phylogenetic relationships (three nuclear and five mitochondrial genes), and with biogeographic and fossil points of external calibration, the experts have confirmed that the origin of the spiders diversification matches the geological changes undergone by the Mediterranean basin, an area particularly active during the Oligocene.
Following the shifting of the tectonic plates
"Most likely, the ancestor of the genus Parachtes -- explains Miquel A. Arnedo -- arrived to the Iberian Peninsula from the Middle East via Europe. From this point, the diversification of the different lineages in the Mediterranean can be followed through the movement of the tectonic plates, as a conveyor belt. However the temporal framework for the observed separation between Corsica and Sardinia was unexpected: according to the study, there was a quite early differentiation between the lineages of these islands, in spite of their geographic proximity."
Characterizing the factors that drive biodiversity is key to identify and understand the natural richness of the Mediterranean, a biogeographic region characterized by a high number of endemism. According to Leticia Bidegaray-Batista, "several alternative hypotheses could explain the distribution of the genus Parachtes in the Mediterranea. In this specific case, the tectonic dynamic of the Mediterranean was singled out as the main factor driving species diversification."
Moreover, this study also provides information regarding the rate of substitution in genetic markers commonly used in phylogenetic studies, which allows absolute time inferences using molecular clocks. The conclusions of this research contrast with other phylogenetic studies conducted in Mediterranean islands, where the dynamics of the colonization and diversification of the fauna are associated with the Messinian salinity crisis, a dramatic event that caused the drying of the Mediterranean Sea at the end of the Miocene.
- Bidegaray-Batista, L.; Arnedo, M. A. Gone with the plate: the opening of the Western Mediterranean basin drove the diversification of ground-dweller spiders. BMC Evolutionary Biology, October 2011 DOI: 10.1186/1471-2148-11-317
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