beta-catenin-1 localization and transcriptomic profiling provide insights into the early development of planarians

Background: The development of planarians is unique among Spiralians. Instead of the stereotypical spiral cleavage, planarians exhibit a disperse cleavage. There is no apparent gastrulation, and the morphogenesis of the yolk-feeding embryo remains a mystery. In this study, we examine the subcellular localization of βcatenin-1 and the transcriptomic profile during the early embryonic development of Schmidtea polychroa to shed light on these early events. Results: We find that the first nuclearization of βcatenin-1 occurs in yolk cells surrounding the embryonic syncytium. By 24 hours post deposition, βcatenin-1 starts to be nuclearized in blastomeres, coinciding with the activation of signaling and cell motility genes. During morphogenesis of the yolk-feeding embryo, βcatenin-1 is first nuclearized at one pole (gut and pharynx progenitors) and in epidermal progenitors, and afterwards in the embryonic pharynx. At this stage, genes involved in a first morphogenetic event are turned on. Following the yolk ingestion by the embryo, a dramatic transcriptomic shift occurs that coincides with the activation of genes related to cell proliferation. Finally, between 5-7 days post deposition βcatenin-1 is massively nuclearized, and genes involved in the morphogenesis and patterning of the adult tissues get activated. Conclusions: Our findings provide new insights into the early developmental events of Schmidtea polychroa, including cleavage, the involvement of βcatenin-1 in the formation of the embryonic tissues and the morphogenesis of two distinct body plans. These findings are significant to understand the evolution of the peculiar mode of planarian development. RNA-seq time course differential gene expression analysis of Schmidtea polychroa embryonic development from samples at 4, 24, 32, 48, 72, 96, 120, and 168 hours post cocoon deposition, taking 4h as reference time (t_x vs t_4h) or comparisons with previous time points (t_x vs t_(x-1)). The Transcriptome Shotgun Assembly project has been deposited at DDBJ/EMBL/GenBank under the accession GLGS00000000. The version described in this paper is the first version, GLGS01000000.