Abstract
A key event in early embryonic development is activation of zygotic gene expression. The mechanisms of this process have been well studied in only a few model organisms, which do not fully reflect the diversity of developmental patterns and cell fate determination strategies. Among bilaterian animals, representatives of the Spiralia clade, which exhibit remarkable conservation and determinative specification of cell lineages, remain largely unexplored in terms of genome activation. In this study, we used transcriptomic analysis to investigate zygotic genome activation in the White Sea annelid Ophelia limacina, which exhibits homoquadrant (equal) spiral cleavage. We demonstrate that zygotic transcription begins as early as the 8-cell stage, leading to the upregulation of thousands of genes, including components of the Wnt and TGF-β signaling pathways, as well as transcription factors such as Sox2—a conserved regulator of pluripotency and genome activation in vertebrates. These findings broaden our understanding of the variability of molecular mechanisms underlying zygotic genome activation and raise new questions regarding the potential evolutionary conservation of key factors involved in this process.