βcatenin-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 dispersed 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.

The first localization of βcatenin-1 in the nucleus occurs in yolk cells surrounding the embryonic syncytium. By 24 h post-deposition, βcatenin-1 starts to be nuclear in blastomeres, coinciding with the activation of signaling and cell motility genes. During morphogenesis of the yolk-feeding embryo, βcatenin-1 is first localized in the nucleus at one pole (gut and pharynx progenitors) and in epidermal progenitors, and afterward 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 and 7 days post-deposition, βcatenin-1 is massively located in the nucleus, 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 βcatenin-1 in forming the embryonic tissues, and the morphogenesis of two distinct body plans. These findings are significant to understanding the evolution of the peculiar mode of planarian development.