Chloroplast DNA variation, postglacial recolonisation and hybridisation in hazel, Corylus avellana

To unravel the post-glacial migration history of hazel, Corylus avellana, the genetic variation at two types of chloroplast DNA markers, PCR-RFLP and microsatellites, was assessed in 26 natural hazel populations distributed across the range of Corylus avellana. In addition a sequence of 2468 base pairs, which contains the matK gene, was analysed in seven individuals. Very little variation was detected overall [hT:PCR-RFLP = 0.091, hT:microsatellite = 0.385, p (nucleotide diversity) = 0.00093] but the microsatellite markers, which have the highest levels of variation, show a clear geographic structure that divides Europe into two areas: (i) Italy and the Balkans, on the one hand and (ii) the rest of Europe, on the other hand. These data excludes Italy and the Balkans as possible origins of the post-glacial recolonisation but can not unambiguously show which other area is the origin, since the genetic data does not indicate the direction of spread. If we take the pollen record into account, the most likely scenario would be an expansion from the Biscay area into most of Europe except Italy and the Balkans, and then a local expansion in the latter area. The two main haplotypes identified with both PCR-RFLP and sequencing, A and B, were not only found in Corylus avellana but also in other European Corylus species and cultivars. Haplotype A, which is dominating all investigated natural populations of C. avellana, is also found in the European tree hazel (C. colurna) and haplotype B, which is rare in C. avellana, have been identified in the filbert (C. maxima) and C. avellana cultivars. This pattern seems to indicate a history of past hybridisation among the European Corylus species and cultivars.