beta-catenin-1 localization and transcriptomic profiling provide insights into the early development of planarians
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 beta-catenin-1 and the transcriptomic profile during the early embryonic development of Schmidtea polychroa to shed light on these early events.We find that the first nuclearization of beta-catenin-1 occurs in yolk cells surrounding the embryonic syncytium. By 24 hours post deposition, beta-catenin-1 starts to be nuclearized in blastomeres, coinciding with the activation of signaling and cell motility genes. During morphogenesis of the yolk-feeding embryo, beta-catenin-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 beta-catenin-1 is massively nuclearized, and genes involved in the morphogenesis and patterning of the adult tissues get activated.Our findings provide new insights into the early developmental events of Schmidtea polychroa, including cleavage, the involvement of beta-catenin-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.