02687nas a2200421   4500008004100000020001400041245005700055210005600112260000800168490000600176520162900182100001401811700001601825700001401841700001801855700001501873700001701888700001701905700002001922700002001942700001701962700001401979700001501993700001502008700001802023700001502041700001802056700001602074700001502090700002002105700001402125700001602139700001402155700001402169700001902183700001702202856004602219       2009                        eng d  a1932-620300aTo Be or Not to Be a Flatworm: The Acoel Controversy0 aTo Be or Not to Be a Flatworm The Acoel Controversy  cMay0 v43 aSince first described, acoels were considered members of the flatworms (Platyhelminthes). However, no clear synapomorphies among the three large flatworm taxa - the Catenulida, the Acoelomorpha and the Rhabditophora have been characterized to date. Molecular phylogenies, on the other hand, commonly positioned acoels separate from other flatworms. Accordingly, our own multi-locus phylogenetic analysis using 43 genes and 23 animal species places the acoel flatworm Isodiametra pulchra at the base of all Bilateria, distant from other flatworms. By contrast, novel data on the distribution and proliferation of stem cells and the specific mode of epidermal replacement constitute a strong synapomorphy for the Acoela plus the major group of flatworms, the Rhabditophora. The expression of a piwi-like gene not only in gonadal, but also in adult somatic stem cells is another unique feature among bilaterians. These two independent stem-cell-related characters put the Acoela into the Platyhelminthes-Lophotrochozoa clade and account for the most parsimonious evolutionary explanation of epidermal cell renewal in the Bilateria. Most available multigene analyses produce conflicting results regarding the position of the acoels in the tree of life. Given these phylogenomic conflicts and the contradiction of developmental and morphological data with phylogenomic results, the monophyly of the phylum Platyhelminthes and the position of the Acoela remain unresolved. By these data, both the inclusion of Acoela within Platyhelminthes, and their separation from flatworms as basal bilaterians are well-supported alternatives.1 aEgger, B.1 aSteinke, D.1 aTarui, H.1 aDe Mulder, K.1 aArendt, D.1 aBorgonie, G.1 aFunayama, N.1 aGschwentner, R.1 aHartenstein, V.1 aHobmayer, B.1 aHooge, M.1 aHrouda, M.1 aIshida, S.1 aKobayashi, C.1 aKuales, G.1 aNishimura, O.1 aPfister, D.1 aRieger, R.1 aSalvenmoser, W.1 aSmith, J.1 aTechnau, U.1 aTyler, S.1 aAgata, K.1 aSalzburger, W.1 aLadurner, P.  uhttps://nematodes.myspecies.info/node/12402812nas a2200253   4500008004100000020001400041245008500055210006900140260000800209490000600217520210800223100001802331700001502349700001602364700001602380700001402396700002002410700001502430700001802445700001502463700001702478700001702495856004602512       2009                        eng d  a1471-213X00aCharacterization of the stem cell system of the acoel <i>Isodiametra pulchra</i>0 aCharacterization of the stem cell system of the acoel iIsodiamet  cDec0 v93 aBackground: Tissue plasticity and a substantial regeneration capacity based on stem cells are the hallmark of several invertebrate groups such as sponges, cnidarians and Platyhelminthes. Traditionally, Acoela were seen as an early branching clade within the Platyhelminthes, but became recently positioned at the base of the Bilateria. However, little is known on how the stem cell system in this new phylum is organized. In this study, we wanted to examine if Acoela possess a neoblast-like stem cell system that is responsible for development, growth, homeostasis and regeneration. Results: We established enduring laboratory cultures of the acoel Isodiametra pulchra (Acoela, Acoelomorpha) and implemented in situ hybridization and RNA interference (RNAi) for this species. We used BrdU labelling, morphology, ultrastructure and molecular tools to illuminate the morphology, distribution and plasticity of acoel stem cells under different developmental conditions. We demonstrate that neoblasts are the only proliferating cells which are solely mesodermally located within the organism. By means of in situ hybridisation and protein localisation we could demonstrate that the piwi-like gene ipiwi1 is expressed in testes, ovaries as well as in a subpopulation of somatic stem cells. In addition, we show that germ cell progenitors are present in freshly hatched worms, suggesting an embryonic formation of the germline. We identified a potent stem cell system that is responsible for development, homeostasis, regeneration and regrowth upon starvation. Conclusions: We introduce the acoel Isodiametra pulchra as potential new model organism, suitable to address developmental questions in this understudied phylum. We show that neoblasts in I. pulchra are crucial for tissue homeostasis, development and regeneration. Notably, epidermal cells were found to be renewed exclusively from parenchymally located stem cells, a situation known only from rhabditophoran flatworms so far. For further comparison, it will be important to analyse the stem cell systems of other key-positioned understudied taxa.1 aDe Mulder, K.1 aKuales, G.1 aPfister, D.1 aWillems, M.1 aEgger, B.1 aSalvenmoser, W.1 aThaler, M.1 aGorny, A., K.1 aHrouda, M.1 aBorgonie, G.1 aLadurner, P.  uhttps://nematodes.myspecies.info/node/126