The phylogeographic importance of the Strait of Gibraltar as a gene flow barrier in terrestrial arthropods: a case study with the scorpion Buthus occitanus as model organism

Abstract

The phylogenetic relationship between Buthus occitanus populations across the Strait of Gibraltar was investigated using nuclear 18S/ITS-1 DNA sequences and mitochondrial 16S and COI DNA sequences. All analyses showed that the European samples are highly separated from North African samples, and also suggest the existence of three main groups within this species complex, i.e., an European, an Atlas (=Moroccan samples) and a Tell-Atlas group (=Tunisian samples). The European clade was subdivided into three distinct subclades. The application of a previous calibration of the molecular clock of another buthid species suggested that most of the detected mitochondrial DNA lineages including the European lineages are about three times older than the re-opening of the Gibraltar Strait, and consequently, that other and older vicariant events are responsible for the observed phylogeographic structure of this species complex. Concerning the Moroccan samples, a discordance between nuclear and mitochondrial gene markers was observed. The 18S/ITS-1 gene tree could not resolve the phylogenetic relationships among the Moroccan B. occitanus subspecies and the closely related species B. atlantis, whereas mitochondrial genes suggested the co-existence of several old phylogenetic lineages in Morocco. We hypothesized that this difference may be explained by male-biased gene flow and gene conversion at the tandemly repeated 18S/ITS-1 gene regions.

Introduction

The Strait of Gibraltar separates the Iberian fauna from the Mahgreb and was an essential geographic barrier for gene flow during the last 5 Myr, as was shown for terrestrial animals (Busack, 1986; Gómez-Zurita et al., 2000; Palmer and Cambefort, 2000; Palmer et al., 1999), and more recently, for flying animals such as bats (Castella et al., 2000). The Miocene desiccation of the Mediterranean Basin, which was a direct consequence from the closing of the Strait about 5.6 Myr BP (Hsü, 1972; Hsü et al., 1977), created land bridges, which hypothetically allowed the exchanges between the Iberian and Mahgreb fauna. Recent studies that focused on the Western Mediterranean fauna made clear that such dramatic geological changes may have lead to vicariance, and subsequently to genetic differentiation (Beerli et al., 1996; Castella et al., 2000; De Jong, 1998; Prüser and Mossakowski, 1998).

Buthus occitanus (Amoreux, 1789), a scorpion of considerable medical importance (Keegan, 1998), occurs on the Iberian Peninsula and in North Africa, where this species is more diverse and several subspecies were described based on morphological characters (Vachon, 1952). About 5.6 Myr ago the Mediterranean Sea almost completely dried out due to the closing of the Gibraltar-Strait. Then the Strait re-opened 5.2 Myr ago and the Mediterranean Basin was refilled in a rather short period (∼100 years) (Hsü, 1972; Hsü et al., 1977). Consequently, European and African B. occitanus populations should have been genetically isolated since 5.2 Myr, and thus, a high genetic differentiation between these two groups can be expected. However, the separation of European and African populations by the Gibraltar-Strait might not be the only major event that influenced the phylogenetic structure of the B. occitanus species complex. The kinematic reconstruction of the geological history of the Western Mediterranean of the last 23 Myr (De Jong, 1998) indicated that climatic conditions in these regions varied as well as parts of land masses were submerged during some stages. Furthermore, scorpions are considered as sit-and-wait-predators and therefore, are generally known as poor dispersers (Polis et al., 1985; Polis and Sissom, 1990). It is thus possible that African and/or European populations of B. occitanus are also highly genetically structured due to events that occurred before the re-opening of the Gibraltar-Strait.

The primary objective of this study was to evaluate the importance of the re-opening of the Gibraltar-Strait for the current phylogenetic structure of the genus Buthus based on nuclear 18S/ITS-1 DNA sequences and mitochondrial (mt) 16S and COI DNA sequences.