Genetically depauperate in the continent but rich in oceanic islands: Cistus monspeliensis (Cistaceae) in the Canary Islands

Abstract

Background: Population genetic theory holds that oceanic island populations are expected to have lower levels of genetic variation than their mainland counterparts, due to founder effect after island colonization from the continent. Cistus monspeliensis (Cistaceae) is distributed in both the Canary Islands and the Mediterranean region. Numerous phylogenetic results obtained in the last years allow performing further phylogeographic analyses in Cistus.

Methodology/principal findings: We analyzed sequences from multiple plastid DNA regions in 47 populations of Cistus monspeliensis from the Canary Islands (21 populations) and the Mediterranean basin (26 populations). The time-calibrated phylogeny and phylogeographic analyses yielded the following results: (1) a single, ancestral haplotype is distributed across the Mediterranean, whereas 10 haplotypes in the Canary Islands; (2) four haplotype lineages are present in the Canarian Islands; (3) multiple colonization events across the archipelago are inferred; (4) the earliest split of intraspecific lineages occurred in the Early to Middle Pleistocene (<930,000 years BP).

Conclusions/significance: The contrasting pattern of cpDNA variation is best explained by genetic bottlenecks in the Mediterranean during Quaternary glaciations, while the Canarian archipelago acted as a refugium of high levels of genetic diversity. Active colonization across the Canarian islands is supported not only by the distribution of C. monspeliensis in five of the seven islands, but also by our phylogeographic reconstruction in which unrelated haplotypes are present on the same island. Widespread distribution of thermophilous habitats on every island, as those found throughout the Mediterranean, has likely been responsible for the successful colonization of C. monspeliensis, despite the absence of a long-distance dispersal mechanism. This is the first example of a plant species with higher genetic variation among oceanic island populations than among those of the continent.

Figure 1. Phylogeographic analysis of Cistus monspeliensis based on cpDNA sequences.

Sampled populations of Cistus monspeliensis in the Mediterranean (A) and the Canary Islands (B) indicating geographical location of the 11 cpDNA haplotypes (colours) inferred from sequences of the trnS-trnG and psbK-trnS regions. Chart sizes in B are proportional to the number of sequenced individuals. Canarian populations are numbered as in Table&3146;1. Maps: SRTM Shaded Relief, ESRI. (C) Statistical parsimony network of C. monspeliensis haplotypes (indicated by letters) and six closely related species. Lines represent single nucleotide substitutions; dots indicate absent haplotypes (extinct or not found). Colours are as depicted for A and B. (D) Strict consensus tree of the 1322 shortest trees of 89 steps (CI = 0.88; RI = 0.93) from the combined analysis of trnS-trnG and psbK-trnS sequences. Numbers above branches are bootstrap values; numbers below branches are Bayesian posterior probabilities.

Figure 2. Relaxed molecular-clock chronogram and phylogeographic reconstruction of Cistus monspeliensis based on cpDNA sequences.

Maximum clade credibility tree produced by analysis of C. monspeliensis trnS-trnG and psbK-trnS sequences in BEAST, using C. populifolius as the outgroup. Branches are coloured according to the most probable range of their descendent nodes. Pie charts represent posterior probability distributions of ancestral range at well supported (PP>0.95) nodes of interest. 95% highest posterior density intervals for the divergence time estimates of the same nodes are shown. Other nodes with PP>0.95 are indicated with an asterisk (*). Colonization routes supported by a BF>3 are shown on the map. The colour of each route represent its relative support, with darker colours indicating stronger support. The map is based on satellite images available in Google Earth (http://earth.google.com).