Abstract
BACKGROUND: 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. RESULTS: 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. CONCLUSIONS: 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.