Reticulation or divergence: the origin of a rare serpentine endemic assessed with chloroplast, nuclear and RAPD markers

Abstract

Species of Armeria from the southern part of the Iberian Peninsula have been postulated to undergo extensive reticulate evolution based on ITS sequences as well as morphometric, eco-geographical evidence and crossing experiments. The hypothesis that a rare serpentine endemic (A. villosa subsp. carratracensis) originated from a cross between another serpentine tolerant endemic (A. colorata) and a widespread limestone species (A. villosa subsp. longiaristata) is tested with molecular markers. Sequences from two chloroplast regions [trnL (UAA)-trnF (GAA) and trnD (GUC)-trnT (GGU)], from nuclear ribosomal DNA (ITS-1 + 5.8S + ITS-2) as well as RAPD data are used. Variation in chloroplast and nuclear sequence data shows a clear geographical structure irrespective of taxonomic ascription. The putative hybrid taxon shares locally restricted haplotypes and ribotypes both with A. colorata and populations of A. villosa subsp. longiaristata. Species specific markers for the progenitors have not been found and thus a divergent scenario cannot be conclusively excluded. A principal coordinate analysis as well as a UPGMA analysis of the RAPD data based on the Dice coefficient are consistent with the hybrid origin. A joint examination of the various markers together with the previous biological evidence and the geographical structure of the chloroplast and ITS markers raises the likelihood of the reticulate hypothesis. Reasons for inconclusiveness are discussed. Using PCR-RFLPs of artificially synthesized hybrids and backcrosses, evidence for a maternal inheritance of chloroplast DNA in Armeria is provided.