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The influence of geological and evolutionary processes on the distribution of organisms is becoming increasingly well understood thanks to the development of phylogeographical studies focusing on the genetic signal left by past events (e.g. Wiens & Donoghue, 2004). Accordingly, the impact of the last glacial maximum (i.e. 15 000 yr ago) on intraspecific genetic variation, as well as on the location of glacial refugia and postglacial recolonization pathways, has been reported for various species across diverse areas (e.g. Taberlet et al., 1998; Hewitt, 2004; Hampe & Petit, 2005; Médail & Diadema, 2009). By contrast, little is known about the influence of humans on past events, such as the survival, dispersal or range expansion–contraction of wild species.
Agricultural practices emerged in the eastern Mediterranean area (i.e. the Fertile Crescent) c. 10 000 yr ago, and subsequently spread rapidly around the Mediterranean Sea and across Europe (Zeder, 2008). Neolithic farmers might have influenced the current distribution of plant or animal populations in multiple ways, acting as dispersal agents through exchanges or interfering with natural recolonization by modifying the landscape (e.g. deforestation). Given that the Mediterranean region has a long history of farmer occupation, ancient as well as recent human activities have probably had a significant impact on the genetic diversity of plants associated with agroecosystems, such as Abies sp. (Parducci et al., 2001), Castanea sativa (Fineschi et al., 2000), Festuca pratensis (Fjellheim et al., 2006), Lolium sp. (Balfourier et al., 2000), Nigella arvensis (Bittkau & Comes, 2005) and Olea europaea (Besnard et al., 2007).
Aegilops geniculata Roth is a wild relative of wheat and is currently abundant in agricultural landscapes (e.g. in field edges, along roadsides, in pastures or in disturbed areas) of the Mediterranean Basin (Van Slageren, 1994). The species is annual and largely autogamous (Hammer, 1980). The morphological features of spikes (e.g. awns and hairs) ensure an efficient zoochorous dispersal of seeds, which remain intact in the soil for years, supplying a consistent seed bank. Accordingly, human activities, such as pastoralism of goats and ovine livestock or cereal cultivation, might have contributed to long-distance seed dispersal in Ae. geniculata (Zaharieva et al., 2001a). The species thus offers a good model to explore how humans might have interfered with natural expansion. Schematically, two contrasting hypotheses can be postulated to explain the present distribution of this species in the Mediterranean area: Ae. geniculata spread across the region before the last glacial maximum and recolonized its current distribution range from glacial refugia during the postglacial period (i.e. the last 15 000 yr), or Ae. geniculata accompanied Neolithic farming and dramatically expanded its range during the spread of agriculture. Unfortunately, no evidence yet available clearly favours or dismisses one of these hypotheses (Zaharieva et al., 2001b).
Aegilops geniculata is an allotetraploid species (2n = 4x = 28 chromosomes; genome formula MMUU), whose ancestors are Ae. comosa Sm. in Sibth. & Sm. var. comosa (genome MM) and Ae. umbellulata Zhuk. (genome UU; Van Slageren, 1994; Friebe et al., 1999). Based on the current distribution of the diploid progenitor species, most authors have suggested an eastern Mediterranean origin of Ae. geniculata (Chennaveeraiah, 1960; Waines & Barnhart, 1992; Van Slageren, 1994; Resta et al., 1996). Previous molecular studies have also corroborated this hypothesis (Zaharieva et al., 2001a). Nevertheless, its precise origin remains elusive and might have involved multiple polyploidy events (Meimberg et al., 2009). As Ae. geniculata constitutes an important reservoir of genes for wheat improvement (Zaharieva et al., 2001a, 2003), a better understanding of the origin of the species and the evolutionary processes shaping genetic variation across its native distribution range is critical for genetic resource conservation. Accordingly, the present study investigates the spatiotemporal dynamics of native populations of Ae. geniculata over the whole Mediterranean area. In particular, using both chloroplast DNA (cpDNA) sequencing and amplified fragment length polymorphism (AFLP), we aim to describe the distribution of genetic variation across the Mediterranean Basin to characterize the origin of Ae. geniculata, unravel the species’ expansion patterns and the putative human imprints, and highlight the evolutionary processes acting on genetic diversity. We report a strong phylogeographical pattern, suggesting that the polyploid Ae. geniculata evolved multiple times in the area of the Bosphorus Strait, and that independent lineages expanded naturally towards the north or south of the Mediterranean Sea. Accordingly, human-mediated migration was probably limited, but immigrated individuals seem to have influenced genetic variation locally in resident populations through considerable introgression.