Additional figure data.

Embryogenesis requires coordinated gene regulatory activities early on that establish the trajectory of subsequent development, during a period called the maternal-to-zygotic transition (MZT). The MZT comprises transcriptional activation of the embryonic genome and post-transcriptional regulation of egg-inherited maternal mRNA. Investigation into the MZT in animals has focused almost exclusively on bilaterians, which include all classical models such as flies, worms, sea urchin, and vertebrates, thus limiting our capacity to understand the gene regulatory paradigms uniting the MZT across all animals. Here, we elucidate the MZT of a non-bilaterian, the cnidarian Hydractinia symbiolongicarpus. Using parallel poly(A)-selected and non poly(A)-dependent RNA-seq approaches, we find that the Hydractinia MZT is composed of regulatory activities similar to many bilaterians, including cytoplasmic readenylation of maternally contributed mRNA, delayed genome activation, and separate phases of maternal mRNA deadenylation and degradation that likely depend on both maternally and zygotically encoded clearance factors, including microRNAs. But we also observe massive upregulation of histone genes and an expanded repertoire of predicted H4K20 methyltransferases, aspects thus far particular to the Hydractinia MZT and potentially underlying a novel mode of early embryonic chromatin regulation. Thus, similar regulatory strategies with taxon-specific elaboration underlie the MZT in both bilaterian and non-bilaterian embryos, providing insight into how an essential developmental transition may have arisen in ancestral animals.胚胎发生早期需要协同的基因调控活动，以确立后续发育的轨迹，这一时期被称为母源-合子转换（maternal-to-zygotic transition, MZT）。母源-合子转换包含胚胎基因组的转录激活以及卵源性母源mRNA的转录后调控。此前针对动物母源-合子转换的研究几乎仅聚焦于两侧对称动物，涵盖果蝇、线虫、海胆、脊椎动物等所有经典模式生物，这限制了我们理解贯穿所有动物类群的母源-合子转换基因调控范式的能力。本研究阐明了非两侧对称动物——刺胞动物Hydractinia symbiolongicarpus的母源-合子转换过程。本研究采用平行的聚腺苷酸化RNA富集（poly(A)-selected）与非聚腺苷酸依赖的RNA测序技术，发现Hydractinia symbiolongicarpus的母源-合子转换所包含的调控活动与多数两侧对称动物相似，包括母源mRNA的胞质再腺苷酸化、延迟的基因组激活，以及分阶段进行的母源mRNA脱腺苷酸化与降解——这一过程可能同时依赖母源与合子编码的清除因子，其中包括微小RNA（microRNAs）。但本研究同时观察到组蛋白基因的大规模上调，以及预测得到的H4K20甲基转移酶家族的扩增；这些特征迄今为止为Hydractinia symbiolongicarpus的母源-合子转换所特有，可能为早期胚胎染色质调控的新型模式奠定了基础。综上，两侧对称与非两侧对称动物胚胎的母源-合子转换均以相似的调控策略为基础，同时伴随类群特异性的精细化调控，这为理解这一关键发育转变在祖先动物中的起源演化提供了新的视角。