Molecular markers reveal a strong genetic differentiation between two European relic tree species: Zelkova abelicea (Lam.) Boissier and Z-sicula Di Pasquale, Garfi & Quezel (Ulmaceae)

Chloroplast (trnL) and ribosomal (ITS2) sequences and chloroplast DNA (PCR-RFLP and SSR) markers were analysed in two relic Ulmaceae tree species: Zelkova abelicea, from Crete, and Z sicula, from Sicily. The analysis of the plastidial trnL intron and of ITS2 ribosomal sequences revealed their divergence from the related species Z carpinifolia, widespread in the Caucasian region; one base substitution in the trnL intron was detected between the two Mediterranean species, thus suggesting their recent separation. Molecular markers (plastidial PCR-RFLP and SSR) showed an evident genetic differentiation between Z sicula and Z abelicea, the two species being characterised by different haplotypes. No within population variation was detected using different chloroplast markers in Z abelicea and Z sicula. Paleobotanical data proved that the genus Zelkova was abundant and widespread in central Italy until it became extinct in the continental part of Europe during last glaciation events and survived only in two Mediterranean islands. The segregation of the two Mediterranean relic species might have occurred as a consequence of the strong reduction of their distribution and the following geographic isolation. Genetic drift may have determined the drastic reduction of within stand diversity as observed in other small, peripheral and geographically isolated plant populations. The priorities for conservation programs are discussed in the light of the different genetic resources represented by the two taxa.