Acoel single-cell atlas reveals expression dynamics and heterogeneity of adult pluripotent stem cells.

Adult pluripotent stem cell (aPSC) populations underlie whole-body regeneration in many distantly-related animal lineages, but how the underlying cellular and molecular mechanisms compare across species is unknown. Here, we apply single-cell RNA sequencing to profile transcriptional cell states of the acoel worm Hofstenia miamia during postembryonic development and regeneration. We identify cell types shared across stages and their associated gene expression dynamics during regeneration. Functional studies confirm that the aPSCs, also known as neoblasts, are the source of differentiated cells and reveal transcription factors needed for differentiation. Subclustering of neoblasts recovers transcriptionally distinct subpopulations, the majority of which are likely specialized to differentiated lineages. One neoblast subset, showing enriched expression of the histone variant H3.3, appears to lack specialization. Altogether, the cell states identified in this study facilitate comparisons to other species and enable future studies of stem cell fate potentials.

Embryonic origins of adult pluripotent stem cells.

Although adult pluripotent stem cells (aPSCs) are found in many animal lineages, mechanisms for their formation during embryogenesis are unknown. Here, we leveraged Hofstenia miamia, a regenerative worm that possesses collectively pluripotent aPSCs called neoblasts and produces manipulable embryos. Lineage tracing and functional experiments revealed that one pair of blastomeres gives rise to cells that resemble neoblasts in distribution, behavior, and gene expression. In Hofstenia, aPSCs include transcriptionally distinct subpopulations that express markers associated with differentiated tissues; our data suggest that despite their heterogeneity, aPSCs are derived from one lineage, not from multiple tissue-specific lineages during development. Next, we combined single-cell transcriptome profiling across development with neoblast cell-lineage tracing and identified a molecular trajectory for neoblast formation that includes transcription factors Hes, FoxO, and Tbx. This identification of a cellular mechanism and molecular trajectory for aPSC formation opens the door for in vivo studies of aPSC regulation and evolution.

New records of cotylean flatworms (Platyhelminthes: Polycladida: Rhabditophora) from coastal habitats of Israel.

Seven new records of cotylean flatworms belonging to two families and four genera were identified from Israel collected in the eastern Mediterranean Sea and the northern Red Sea. The family Pseudocerotidae was represented by three genera and six species (Pseudoceros duplicinctus, Pseudobiceros apricus, Pseudobiceros damawan, Pseudobiceros murinus, Pseudobiceros stellae, and Thysanozoon brocchii), followed by the Euryleptidae with a single species (Maritigrella fuscopunctata). Pseudoceros duplicinctus is recognized as a senior synonym of Pseudoceros prudhoei and color and pattern variations are reported for Maritigrella fuscopunctata. In addition, the presence of Pericelis byerleyana in the northern Gulf of Aqaba is confirmed, illustrated, and properly described with morphological and molecular data. Pseudoceros duplicinctus and Pseudobiceros stellae represent new geographic records for the Mediterranean Sea and Pseudobiceros apricus and Pseudobiceros damawan are new for the Red Sea. Detailed morphological analysis with emphasis on the color and pattern was applied for species identifications including high quality photographs of live and fixed animals. Partial sequences of the 28S ribosomal DNA (rDNA) of four of the species were obtained and deposited in GenBank with accession numbers provided with the corresponding species description. Our results represent a significant taxonomic contribution for the diversity and distribution of the Polycladida in Israel pointing out the importance for more integrative and comprehensive surveys in these two regions to advance the knowledge of marine biodiversity worldwide and along both Israeli coasts.

Embryonic muscle development in direct and indirect developing marine flatworms (Platyhelminthes, Polycladida).

We compared embryonic myogenesis of the direct-developing acotylean polyclad Melloplana ferruginea with that of Maritigrella crozieri, a cotylean that develops via a larval stage. Fluorescently labeled F-actin was visualized with laser confocal microscopy. Developmental times are reported as percentages of the time from oviposition to hatching: 7 days for M. crozieri and 22 days for M. ferruginea. The epithelium began to form at 30% development in M. crozieri and at 15% development in M. ferruginea. Random myoblasts appeared in peripheral areas of the embryo at 36% and 22-30% development in M. crozeri and M. ferruginea, respectively. Circular and longitudinal muscle bands formed synchronously at 37-44% development in M. crozieri; yolk obscured observations of early myogenesis in M. ferruginea. An orthogonal muscle grid was established by 45-50% development in both species. Diagonal muscles developed in M. ferruginea at 60-71% development. Hence, juveniles of this species hatch with the same basic body-wall musculature as adults. Larvae of M. crozieri did not hatch with diagonal muscles; these muscles are acquired postmetamorphosis. Additionally, a specialized musculature developed in the larval lobes of M. crozieri. Oral musculature was complex and established by 72% development in both species. Our results are comparable to the muscle differentiation reported for other indirect-developing polyclads and for direct-developing species of macrostomid flatworms. Furthermore, they provide additional support that the orthogonal muscle pattern of circular and longitudinal muscles is a symplesiomorphy of Spiralia.