Combining the use of molecular techniques and archival documentary evidence to trace the origin of Populus alba in a Central Mediterranean archipelago

The nativity of Populus alba in the Mediterranean has only been confirmed in the last decade, following the discovery of 8,000-year-old leaf imprints in Southern France. Recent evidence has even emerged from molecular studies suggesting that the species is native to some of the islands, and these populations may be relicts of a native flora that arrived there much earlier than previously thought. In view of this, samples obtained from the Central Mediterranean archipelago of Malta and other neighbouring regions were analysed to determine the native status of the Maltese populations and possibly trace their origins. All 38 samples were investigated in order to assess the genetic diversity and origin of Maltese trees. Nuclear microsatellite analysis revealed that all 28 trees sampled from the two islands of Malta belonged to one clone. Chloroplast data suggested relatedness of the Maltese clone to Italian P. alba samples. However, nuclear data suggested additional admixture through pollen from North Africa. Existing archival and palaeontological records were also examined for any supporting evidence. On considering the latter records in combination with molecular evidence, we arrived to the conclusion that arrival of this clone in Malta through human introduction in the sixteenth century is the most likely explanation, since alternative scenarios like autovegetative propagation or arrival by seed seem highly unlikely.