Diversification and biogeographic history of the Western Palearctic freshwater flatworm genusSchmidtea(Tricladida: Dugesiidae), with a redescription ofSchmidtea nova

Abstract

The freshwater flatworm genus Schmidtea is endemic in the Western Palearctic region, where it is represented by only four species, thus contrasting with the high species diversity of the closely related genus Dugesia within Europe. Although containing an important model species in developmental and regeneration research, viz. Schmidtea mediterranea, no evolutionary studies on the genus Schmidtea have been undertaken. For the first time, we present a well‐resolved molecular phylogenetic tree of the four species of the genus, inferred on the basis of two molecular markers, and provide also the first detailed morphological account of Schmidtea nova. The phylogenetic tree generated corroborates an earlier speciation hypothesis based on karyological data and points to chromosomal rearrangements as the main drivers of speciation in this genus. The high genetic divergence between the four species, in combination with previous dating studies and their current geographic distribution, suggests that Schmidtea could have originated in Laurasia but lost most of its diversity during the Oligocene. Thus, its present distribution pattern may be the result of the expansion of three of its four relictual species over Europe, probably after the Pleistocene glaciations. Our detailed morphological study of S. nova revealed that it shows a number of remarkable features: interconnected testis follicles, parovaria, an ejaculatory duct exiting into the primary as well as the secondary seminal vesicle by means of a nipple, and the wall of the distal section of the ejaculatory duct being sclerotic or chitinized.

Outstanding intraindividual genetic diversity in fissiparous planarians (Dugesia, Platyhelminthes) with facultative sex

“…Moreover, the presence of this high intraindividual genetic diversity due to fissiparous reproduction may allow them also to overcome evolutionary problems such as those generated by population bottlenecks (a situation that, for instance, species of Dugesia endure each year in the Mediterranean region), since they will not result in a loss of genetic diversity. All these reasons could help explain the evolutionary success of the genus Dugesia , with more than 85 species distributed in Africa, Asia and Europe [83], compared to its sister genera Schmidtea [45] and Recurva [84], which principally reproduce sexually or by parthenogenesis and at present include only 4 and 2 species, respectively, distributed only in Northern Africa and Europe.…”

“…The TMED9 gene was selected because it is a single copy gene in the species Schmidtea mediterranea containing a long intronic region (total amplified exonic region: 197 bp; total amplified intronic region: 751 bp) [43, 44]. The conditions for the PCR reactions for Cox1 were as previously published [45]. The amplification conditions for TMED9 were the following: 1) 2 min at 94 °C, 2) 45 s at 94 °C, 3) 50 s at 58 °C, 4) 40 s at 72 °C and 5) 3 min at 72 °C.…”

Schmidtea sp., from the S-W Romania (Platyhelminthes, Tricladida, Dugesiidae)

“…Schmidtea is a genus of freshwater flatworms belonging to Order Tricladida, Suborder Continenticola, Family Dugesiidae (Sluys et al 2009). It consists of only four species with west-palearctic distribution: S. lugubris (Schmidt, 1861), S. polychroa (Schmidt, 1861), S. mediterranea (Benazzi, Baguñà, Ballester, Puccinelli, del Papa, 1975) and S. nova (Benazzi, 1982) (Leria et al 2018).…”

“…In Romania, the genus Schmidtea is represented only by the species lugubris and nova. S. nova was found in the north-western part of Romania at Huedin (Leria et al2018). The literature registers the presence of S. lugubris in the south and east regions - South Plain (Romanian Plain), Dobrogea, Danube Delta, Siret Plain (Năstăsescu 1973) and in the north-western part at Aiud (Leria et al 2018).…”

“…S. nova was found in the north-western part of Romania at Huedin (Leria et al2018). The literature registers the presence of S. lugubris in the south and east regions - South Plain (Romanian Plain), Dobrogea, Danube Delta, Siret Plain (Năstăsescu 1973) and in the north-western part at Aiud (Leria et al 2018). On S. lugubris for Wallachian Plain (SW Plain or Oltenia Plain), data on the geographical distribution, habitat and biocenoses are brought by Rogoz (1979), yet no anatomical character was published.…”