stoppers to those made of synthetic materials or metal (WWF 2006).
The evolutionary importance of introgres-sive hybridization involving Q. suber with other Quercus lineages has been detected by numerous studies. For example, Toumi and Lumaret (1998) used allozyme markers to test for patterns of genetic variation in cork oak. In addition to phy-logeographic discontinuities, these authors found apparent introgression of allozyme alleles from holm oak (Quercus ilex) into cork oak individuals. Similarly, Coelho et al. (2006) described a pattern of shared nuclear alleles (i.e., amplified fragment length polymorphisms [AFLP]) between these two species that was consistent with introgres-sion from Q. ilex into Q. suber. Several analyses of cpDNA variation have also led to the inference of introgressive hybridization between cork oak and holm oak populations. In particular, Jiménez et al.
(2005) reported cpDNA variation in these two species resulting in the following conclusions: (1) cpDNA introgression had occurred between Q. suber and Q. ilex in numerous regions of overlap; (2) there was asymmetry in the introgression event with cpDNA genotypes from Q. ilex having intro-gressed into Q. suber at a much higher frequency than from Q. suber into Q. ilex; (3) following intro-gression of Q. ilex cpDNA into the cork oak lineage, mutations have occurred leading to multiple cpDNA types in cork oak (i.e., derived from both the introgressed and native cpDNA); and (4) the presence of Q. ilex cpDNA in individuals that were Q. suber on the basis of nuclear loci may reflect the "migration" of cork oak through pollen-mediated introgression. The inference of asymmetric introgression into cork oak by holm oak has also been supported by a recent and extensive phylo-geographic analysis of cpDNA variation by Magri et al. (2007). Indeed, these latter authors reported data supporting the hypothesis of introgression between Q. suber and Q. ilex, but also between Q. suber and the deciduous oak species, Quercus cerris (Turkey oak; see later).
Another class of observation also indicates the importance of reticulate evolutionary processes involving Q. suber. However, in this case, rather than being the recipient of genetic material through introgression, the cork oak lineage has acted as one of the progenitors for hybrid species. Specifically, both Quercus afares and Quercus crenata have been recognized as hybrid species. Q. afares is a North African endemic that is sympatric with both Q. suber and its other putative parent, Quercus canariensis (Mir et al. 2006). Surprisingly (because cork oak is evergreen, while Q. canarien-sis is a semideciduous species), Q. afares reflects an admixture of morphological, physiological, and ecological traits characteristic of the two hypothesized progenitors (see discussion in Mir et al. 2006). Consistent with a hybrid origin for Q. afares was the finding of combinations of nuclear and cpDNA loci from Q. suber and Q. canariensis in Q. afares individuals/populations (Mir et al. 2006). Likewise, a hybrid derivation of Q. crenata from crosses involving Q. suber, but in this case with Q. cerris, has also been inferred (Conte et al. 2007). Nuclear genotypes in Q. crenata populations indicated its intermediate position between cork and Turkey oak genotypes (Conte et al. 2007). The intermediacy of the genotypic scores for Q. crenata is consistent with an admixture of alleles and thus a hybrid derivation from Q. suber and Q. cerris.
Both the introgression of genomic material between cork oak and other species of Mediterranean oaks, as well as Q. suber's role in the origin of hybrid species, again reflects the evolutionary significance of reticulate evolution in general. Furthermore, reticulate evolution has impacted a species that has a keystone position in terms of its ecological, cultural, and economic value.
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