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

Abstract

BackgroundSchmidtea 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.ResultsA 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.ConclusionsS. 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.

Evolutionary history of the Tricladida and the Platyhelminthes: an up-to-date phylogenetic and systematic account

“…It is worth noting that the molecular analyses revealed a low level of diversification of the group as compared to other Dugesiidae genera of similar age, particularly Dugesia. It is possible that these four species have a recent origin, but molecular trees seem to show an old diversification, which could give support to the idea that they are the remnants of a more widespread diversity (Lazaro et al, 2011).…”

“…While there is no specific study analysing the relationships among the Schmidtea species, they have been included in some molecular studies. Such analyses have shown a closer relationship between S. polychroa and S. mediterranea, whereas the relationship between S. lugubris and S. nova is not clear due to the lack of good molecular information for the latter (Álvarez-Presas et al, 2008;Lazaro et al, 2011). It is worth noting that the molecular analyses revealed a low level of diversification of the group as compared to other Dugesiidae genera of similar age, particularly Dugesia.…”

Planarian (Platyhelminthes, Tricladida) Diversity and Molecular Markers: A New View of an Old Group

“…In the remaining distribution area, only sexual diploids are found. The molecular study of these populations, based on three markers (Cox1 and cytochrome b (cob) and the 13th nuclear intron of the netrin receptor-like protein gene (N13)), showed that the species is extremely structured [22]. It is divided into three clades that do not share haplotypes, suggesting that the distribution of the species is a consequence of the geological history of the region and it inhabits; its ancestor may have been present in the microplaque that gave rise to the Balearic Islands, Corsica and Sardinia, which implies a quite old age for the species.…”

“…EF in [31]). The amplified fragment of the ATPase α (also tested in [31]) has a small intron that in the future may be useful for intraspecific analyses, such as the nuclear intron (N13, [22]), which have already proved to be informative for S. mediterranea. Also, in a very recent study, a fragment of the β-actin was used to resolve the phylogeny of a group of planarians from Lake Baikal [43], and it seems to be a good candidate for further molecular studies.…”

Comparative selenoproteome analysis reveals a reduced utilization of selenium in parasitic platyhelminthes

“…mediterranea is a free-living, freshwater planarian that lives in southern Europe and Tunisia (Lazaro et al 2011;Harrath et al2004). It is a model organism for the research of regeneration and development of tissues such as the brain and germline (Reddien & Sanchez Alvarado 2004;Salo & Baguna 2002).…”