Plastid DNA Homogeneity in Celtis australis L. (Cannabaceae) and Nerium oleander L. (Apocynaceae) throughout the Mediterranean Basin

International audiencePremise of research. Riparian plants are highly dependent on water sources; consequently, general climatic conditions are less important to these taxa relative to woodland and shrubland species. This leads to interesting research questions regarding riparian plant taxa. Research on phylogeography of Mediterranean riparian tree and shrub species is scarce. In this article, we investigated the plastidial genetic diversity in Celtis australis L. (hackberry) and Nerium oleander L. (oleander) throughout the Mediterranean Basin. Both species are distributed in gullies, rivers, and stream banks under warm temperate climates. Methodology. Eighteen cpSSR loci and three noncoding cpDNA regions (rps16, rpl32-trnL, and trnQ-5′-rps16) were examined to assess the levels and geographic distribution of sequence variation in 41 hackberry and 56 oleander populations. The rpl32-trnL intergenic region was used for molecular dating analysis. Pivotal results. The respective 2762- and 3134-bp noncoding cpDNA regions sequenced in C. australis and N. oleander, as well as the 22 cpSSR analyzed fragments, exhibited the absence of variability in natural populations throughout the Mediterranean Basin. In N. oleander, two regions (rps16, rpl32-trnL) exhibited variability in three positions, and four cpSSR microsatellite motifs were polymorphic. The polymorphisms were geographically structured, and three haplotypes were characterized, two from Saharan populations and one from Mediterranean populations. Phylogeny and molecular dating analyses resulted in a tree with high consistency values of posteriori probability (PP p 1) and bootstrap support (98%) from the ingroup (N. oleander). The relaxed molecular clock model applied to the calibration of the tree estimates that the diversification of these haplotypes occurred in a range of 7.2–1.2 Ma. Conclusions. The absence of plastid variability in both hackberry and oleander is explained by a low mutation rate and/or recent recolonization of the Mediterranean Basin. Low temperatures during the Last Glacial Maximum produced freezing water along riparian corridors, with the consequent drastic contraction or even disappearance of both species in the Mediterranean Basi