Genetic variability of Chamaerops humilis (Arecaceae) throughout its native range highlights two species movement pathways from its area of origin

The European fan palm (Chamaerops humilis, Arecaceae) is the only native palm in continental Europe, providing ecosystem services that are hard to obtain from other species. However, its populations are declining in some areas due to anthropogenic effects including climate change. Knowledge of genetic variability among natural populations is needed to establish conservation plans, to prevent genetic contamination of native stands by cultivated germplasm and to exploit it as an ornamental species. However, information on the genetic similarities among C. humilis populations is scarce. The aims of this work were to study genetic structure in C. humilis using a set of specifically designed genetic markers and to highlight genetic similarities and their relationships with geographical proximity. We sampled 301 specimens from 42 natural populations throughout the distribution area and analysed these with ten di-, tri- and tetra-nucleotide simple sequence repeats. Relationships between genetic similarities and geographical distances were analysed and populations grouped according to a genetic, geographical or national clustering. We found lower variability in populations from the eastern half of the distribution, and this lower variability was accompanied by a stronger relationship between genetic differences and spatial proximity. In addition, we found that C. humilis probably showed two patterns of spread and further differentiation: one from Morocco to southern continental Spain and then to Portugal and the Balearic Islands, and one from Morocco to Algeria, Tunisia, Sicily and continental Italy. Populations from Sardinia and France showed similarities to those from Spain and Tunisia, respectively, and may have arisen from multiple colonization events. Our results support the hypothesis that isolation on large islands may have increased diversification of the species even if all populations shared the same founder. These results have important implications for both the ecological management and the conservation of the species.