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

Journal Article
Riutort, Eva M Lázaro, Abdul Halim Harrath, Giacinta Stocchino, Maria Pala, Jaume Baguñà, Marta . 2011
Magazine \ Newspaper: 
BMC Evol Biol
Volume Number: 
Publication Abstract: 

Background: Schmidtea mediterranea (Platyhelminthes, Tricladida, Continenticola) is found in scattered localities on
a few islands and in coastal areas of the western Mediterranean. Although S. mediterranea is 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, CYB and a nuclear intron N13) 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 of S. 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. mediterranea is 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 of S. mediterranea are being replaced by
asexual individuals that are either conspecific or are from other species that are better adapted to the
Mediterranean climate.