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Macrostomum lignano is a free-living flatworm that is emerging as an attractive experimental animal for research on a broad range of biological questions. One feature setting it apart from other flatworms is the successful establishment of transgenesis methods, facilitated by a steady supply of eggs in the form of single-cell zygotes that can be readily manipulated. This, in combination with the transparency of the animal and its small size, creates practical advantages for imaging and fluorescence-activated cell sorting in studies related to stem cell biology and regeneration. M. lignano can regenerate most of its body parts, including the germline, thanks to the neoblasts, which represent the flatworm stem cell system. Interestingly, neoblasts seem to have a high capacity of cellular maintenance, as M. lignano can survive up to 210 Gy of γ-irradiation, and partially offset the negative consequence of ageing. As a non-self-fertilizing simultaneous hermaphrodite that reproduces in a sexual manner, M. lignano is also used to study sexual selection and other evolutionary aspects of sexual reproduction. Work over the past several years has led to the development of molecular resources and tools, including high-quality genome and transcriptome assemblies, transcriptional profiling of the germline and somatic neoblasts, gene knockdown, and in situ hybridization. The increasingly detailed characterization of this animal has also resulted in novel research questions, such as bio-adhesion based on its adhesion-release glands and genome evolution due to its recent whole-genome duplication.

Background Proseriates (Proseriata, Platyhelminthes) are free-living, mostly marine, flatworms measuring at most a few millimetres. In common with many flatworms, they are known to be capable of regeneration; however, few studies have been done on the details of regeneration in proseriates, and none cover cellular dynamics. We have tested the regeneration capacity of the proseriate Monocelis sp. by pre-pharyngeal amputation and provide the first comprehensive picture of the F-actin musculature, serotonergic nervous system and proliferating cells (S-phase in pulse and pulse-chase experiments and mitoses) in control animals and in regenerates. Results F-actin staining revealed a strong body wall, pharynx and dorsoventral musculature, while labelling of the serotonergic nervous system showed an orthogonal pattern and a well developed subepidermal plexus. Proliferating cells were distributed in two broad lateral bands along the anteroposterior axis and their anterior extension was delimited by the brain. No proliferating cells were detected in the pharynx or epidermis. Monocelis sp. was able to regenerate the pharynx and adhesive organs at the tip of the tail plate within 2 or 3 days of amputation, and genital organs within 8 to 10 days. Posterior pieces were not able to regenerate a head. The posterior regeneration blastema was found to be a centre of cell proliferation, whereas within the pharynx primordium, little or no proliferation was detected. The pharynx regenerated outside of the blastema and was largely, but not solely formed by cells that were proliferating at the time of amputation. Conclusions Our findings suggest that proliferating cells or their offspring migrated to the place of organ differentiation and then stopped proliferating at that site. This mode of rebuilding organs resembles the mode of regeneration of the genital organs in another flatworm, Macrostomum lignano. Pharynx regeneration resembles embryonic development in Monocelis fusca and hints at the vertically directed pharynx being plesiomorphic in proseriates. Proliferation within the regeneration blastema has been detected in anterior and posterior blastemas of other flatworms, but is notably missing in triclads. The phylogenetic relationships of the flatworms studied indicate that proliferation within the blastema is the plesiomorphic condition in Platyhelminthes. Electronic supplementary material The online version of this article (doi:10.1186/2041-9139-5-37) contains supplementary material, which is available to authorized users.

Polyclad flatworms are an early branching clade within the rhabditophoran Platyhelminthes. They provide an interesting system with which to explore the evolution of development within Platyhelminthes and amongst Spiralia (Lophotrochozoa). Unlike most other flatworms, polyclads undergo spiral cleavage (similar to that seen in some other spiralian taxa), they are the only free-living flatworms where development via a larval stage occurs, and they are the only flatworms in which embryos can be reared outside of their protective egg case, enabling embryonic manipulations. Past work has focused on comparing early cleavage patterns and larval anatomy between polyclads and other spiralians. We have selected Maritigrella crozieri, the tiger flatworm, as a suitable polyclad species for developmental studies, because it is abundant and large in size compared to other species. These characteristics have facilitated the generation of a transcriptome from embryonic and larval material and are enabling us to develop methods for gene expression analysis and immunofluorescence techniques. Here we give an overview of M. crozieri and its development, we highlight the advantages and current limitations of this animal as a potential evo-devo model and discuss current lines of research.

Flatworm embryology has attracted attention since the early beginnings of comparative evolutionary biology. Considered for a long time the most basal bilaterians, the Platyhelminthes (excluding Acoelomorpha) are now robustly placed within the Spiralia. Despite having lost their relevance to explain the transition from radially to bilaterally symmetrical animals, the study of flatworm embryology is still of great importance to understand the diversification of bilaterians and of developmental mechanisms. Flatworms are acoelomate organisms generally with a simple centralized nervous system, a blind gut, and lacking a circulatory organ, a skeleton and a respiratory system other than the epidermis. Regeneration and asexual reproduction, based on a totipotent neoblast stem cell system, are broadly present among different groups of flatworms. While some more basally branching groups - such as polyclad flatworms - retain the ancestral quartet spiral cleavage pattern, most flatworms have significantly diverged from this pattern and exhibit unique strategies to specify the common adult body plan. Most free-living flatworms (i.e. Platyhelminthes excluding the parasitic Neodermata) are directly developing, whereas in polyclads, also indirect developers with an intermediate free-living larval stage and subsequent metamorphosis are found. A comparative study of developmental diversity may help understanding major questions in evolutionary biology, such as the evolution of cleavage patterns, gastrulation and axial specification, the evolution of larval types, and the diversification and specialization of organ systems. In this review, we present a thorough overview of the embryonic development of the different groups of free-living (turbellarian) platyhelminths, including the Catenulida, Macrostomorpha, Polycladida, Lecithoepitheliata, Proseriata, Bothrioplanida, Rhabdocoela, Fecampiida, Prolecithophora and Tricladida, and discuss their main features under a consensus phylogeny of the phylum. publishedVersion