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2. Origin and Genetic Variability
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versely, nineteen chlorotypes belong to a very different lineage called ilex– coccifera. A large majority of holm oaks and kermes oaks from Morocco, Iberia, the Balearic Islands, and southern France are also characterized by haplotypes of this lineage, hence its name. Cork oak populations from the current main range (western Iberia and Morocco) show a chlorotype from lineage suber, massively distributed in that area (Belahbib et al. 2001; Jiménez et al. 2004; Lumaret et al. 2005; López de Heredia et al. 2007a). The eastern range of the species is occupied by groups of chlorotypes from lineage suber but distinct from those of the western range (Jiménez et al. 2004; López de Heredia et al. 2005; Lumaret et al. 2005). The disjunct eastern area that comprises Algeria, Tunisia, Sicily, continental Italy, Provence, Sardinia, and Corsica presents higher cpDNA diversity than the western area (Morocco, Iberia, and southwestern France). The occurrence of ilex–coccifera lineage in cork oak has been explained as a result of cytoplasmic introgression from holm oak. Introgression is defined as gene exchange between species mediated by hybridization and backcrossing to a parental species (Rieseberg and Carney 1998). Interspecific barriers to gene flow are diffuse in oaks, promoting natural hybridization events. Actually, hybrid and morphologically intermediate individuals between evergreen oak species occur sympatrically in several areas. Therefore, cork oak is able to hybridize with holm oak (Camus 1938) but also with Turkey oak (Bellarosa et al. 1996) and, in North Africa, with Algerian oak (Quercus canariensis) (Mir et al. 2006). The prevalent direction of hybridization with holm oak is for cork oak to act preferentially as a pollen donor (Boavida et al. 2001). In angiosperms, chloroplasts are mostly maternally inherited; accordingly, cork oak–holm oak hybrids almost always carry an ilex (holm oak) chloroplast. Through subsequent backcrosses of the fertile hybrids with cork oak, the ilex cpDNA would become fixed in the population within a few generations. However, hybridization events seem to be infrequent because phenological overlapping does not occur or is reduced between the species (Elena-Roselló et al. 1992). An ancient hybridization of cork oak in the central area may explain the complete displacement of the suber lineage (López de Heredia et al. 2005; Lumaret et al. 2005). Moreover, even if cork oak shares the same lineage with holm oak in sympatric populations, the chlorotype is not necessarily the same in both species, suggesting the occurrence of distinct mutation events in the cpDNA of each species or of independent posthybridization migration processes. The question arises whether this cytoplasmic introgression is reflected somehow in the nuclear DNA. If the hypothesis of ancient hybridizations is correct, then nuclear contribution of holm oak into cork oak may be diluted