Phylogeography of the common ivy (Hedera sp.) in Europe: genetic differentiation through space and time
We studied the phylogeography of ivy (Hedera sp.), a liana widespread in Europe, throughout its natural range. The populations sampled belong to four closely related species differing by ploidy levels and morphological characters. Chloroplast (cp) markers were used and 13 haplotypes were detected, usually shared across species, contrary to ribosomal internal transcribed spacer (ITS) variants. We demonstrated the existence of a strong overall cpDNA phylogeographical structure. Several methods of data analysis were conducted to describe how this structure and the genetic diversity change through space and time. Southern populations, especially those from Spain, are the most divergent. Pairwise estimates of differentiation point to isolation by distance, and the existence of a latitudinal gradient of divergence was demonstrated using a regression procedure. Similarly, latitudinal differences in haplotype richness and diversity exist, as shown by population permutations ('differentiation through space'). Finally, we measured differentiation by taking into account successive levels of divergence between haplotypes ('differentiation through time'). Genetic differentiation turns out to be much greater when differences between closely related haplotypes are not considered. Further, these results indicate that the phylogeographical structure is essentially due to the relative distribution of the most similar haplotypes. Diversity decreases from south to north, whereas haplotype frequencies change longitudinally. It appears that Hedera survived in Spanish and Balkan refugia during the last ice age. A third refugium must have been present in the Alps or in Italy. During the northward expansion, the decrease in overall diversity was attenuated by some mixing of lineages at intermediate latitudes, resulting in comparatively higher levels of differentiation in the south.