A phylogeographic study of Arenaria ciliata and Arenaria norvegica in Ireland and Europe

Abstract

The unusual distribution of Irish ora and fauna has intrigued and eluded biogeographers for centuries and remains the subject of ongo- ing debate. The conventional hypothesis for the postglacial coloniza- tion of Ireland across an Irish-British land-bridge has come under in- creasing scrutiny, with a growing body of evidence suggesting Ireland may have been a refugium during the last glacial cycle. In addition a strong affinity to Iberian populations is evident among much of the islands's native ora and fauna. This study focused on the disjunctly distributed arctic-alpine plant Arenaria ciliata, and its close arctic relative, A. norvegica, in an effort to characterise and date the earli- est links between Ireland and Continental Europe and to investigate the possibility for in situ survival of populations of these species in Ireland during the last glacial maximum. Twenty-nine populations of the target species were sampled through- out their range in Europe. Four separate chloroplast regions were sequenced: psbA-trnH, rpl32 -trnL, trnK -matK and matK, and the in- ternal transcribed spacer region of the nuclear ribosomal DNA (ITS). The chloroplast and ITS sequences were analyzed using maximum likelihood, haplotype networks, and a molecular clock analysis using Bayesian inference. Amplified fragment length polymorphism (AFLP) markers were generated for all populations sampled using three selec- tive primers and the obtained data were analysed using neighbour- joining analysis, networks, AMOVAs and direct analysis of the frag- ments. The results indicate a complex glacial history for these two species, where the consensus of the analysis demonstrates phylogeographic patterns indicative of Pleistocene glacial survival in situ, with evi- dence for cryptic refugia in Ireland, Rum and Svalbard, and multiple colonization events for A. ciliata. In contrast, the phylogeographic patterns obtained for A. norvegica are, for the most part, typical of a recent post-glacial expansion, with very low sequence divergence between individuals and populations. Evidence for in situ survival of the Ben Bulben population of A. ciliata is presented. This population contains multiple unique hap- lotypes, many private AFLP fragments, distinct genetic structuring and the molecular clock analysis indicates an ancient divergence for the Ben Bulben population. This indicates an origin for this resident population much earlier than the last glacial maximum, predating the origin of the A. norvegica haplotypes. AFLP genotypes show distinct grouping for the Ben Bulben and Svalbard populations. Similari- ties in the genetic distinctiveness of populations from Rum, Svalbard and Ben Bulben indicate the possiblity that these locations represent cryptic Pleistocene refugia for A. ciliata. Taxonomic distinctions between A. ciliata and A. norvegica are com- plex, though, a distinct genetic grouping for individuals of A. norvegica is demonstrated, albeit as a monophyletic group within A. ciliata. In- congruences between the chloroplast and nuclear ITS sequences con- found this issue, and highlight the possibility of hybridisation events between these species. AFLP markers show a distinct clustering for A. norvegica genotypes and demonstrate the suitability of AFLP markers in discriminating between closely related taxa. None of the recognized subspecies of the A. ciliata complex could be clearly discriminated based on the molecular markers implemented in this study. This study provides the first evidence for the survival of a terrestrial plant species in situ in Ireland throughout the Pleistocene glacial cycles.