Exploring the history of Serapias politisii (Orchidaceae), a narrow endemic on the opposite coasts of the Otranto channel; insights from molecular investigations

Knowledge of genetic spatial structure may provide insights into the causes of population disjunctions in plants.Serapias politisii is a narrow endemic with only a few populations scattered along the opposite coasts of theOtranto strait (southern Adriatic Sea). It was originally considered to be of hybrid origin between S. vomeracea ssp.laxiflora and S. parviflora, a possibility suggested also by a DNA phylogenetic study that grouped Italian and Greekpopulations in two distinct clades. In this study we have carried out additional plastid DNA sequencing and anAFLP analysis of the three taxa. Whereas the geographical distribution of four plastid DNA haplotypes supportsthe likelihood of a double hybrid origin or of a plastid capture, AFLP data do not support such a hypothesis, becauseS. politisii shows several private alleles, some of which are shared by Italian and Greek populations. In light ofthe floristic specificity of the coasts bordering the Otranto strait, we consider that the present-day disjunction ofS. politisii could have originated either by a long-distance seed dispersal or by a fragmentation of an oldpolymorphic population. The pairwise mismatch distribution excludes a recent expansion of the populationsexamined. In light of recent evidence concerning the Mediterranean Lago-Mare period and the westward haplotypeevolution detected in some Euro-Mediterranean trees, we argue that fragmentation of a previously continuouspopulation could be an intriguing possibility.