Schmidtea mediterraneaphylogeography: an old species surviving on a few Mediterranean islands?

Background:Schmidtea mediterranea(Platyhelminthes, Tricladida, Continenticola) is found in scattered localities on a few islands and in coastal areas of the western Mediterranean. AlthoughS. mediterraneais the object of many regeneration studies, little is known about its evolutionary history. Its present distribution has been proposed to stem from the fragmentation and migration of the Corsica-Sardinia microplate during the formation of the western Mediterranean basin, which implies an ancient origin for the species. To test this hypothesis, we obtained a large number of samples from across its distribution area. Using known and new molecular markers and, for the first time in planarians, a molecular clock, we analysed the genetic variability and demographic parameters within the species and between its sexual and asexual populations to estimate when they diverged.Results: A total of 2 kb from three markers (COI,CYBand a nuclear intronN13) was amplified from ~200 specimens. Molecular data clustered the studied populations into three groups that correspond to the west, central and southeastern geographical locations of the current distribution ofS. mediterranea. Mitochondrial genes show
low haplotype and nucleotide diversity within populations but demonstrate higher values when all individuals are
considered. The nuclear marker shows higher values of genetic diversity than the mitochondrial genes at the
population level, but asexual populations present lower variability than the sexual ones. Neutrality tests are
significant for some populations. Phylogenetic and dating analyses show the three groups to be monophyletic,
with the west group being the basal group. The time when the diversification of the species occurred is between
~20 and ~4 mya, although the asexual nature of the western populations could have affected the dating analyses.Conclusions:S. mediterraneais an old species that is sparsely distributed in a harsh habitat, which is probably the consequence of the migration of the Corsica-Sardinia block. This species probably adapted to temperate climates in the middle of a changing Mediterranean climate that eventually became dry and hot. These data also suggest that in the mainland localities of Europe and Africa, sexual individuals ofS. mediterraneaare being replaced by asexual individuals that are either conspecific or are from other species that are better adapted to the Mediterranean climate.