BACKGROUND Over one third of all animal phyla utilize a mode of early embryogenesis called 'spiral cleavage' to divide the fertilized egg into embryonic cells with different cell fates. This mode is characterized by a series of invariant, stereotypic, asymmetric cell divisions (ACDs) that generates cells of different size and defined position within the early embryo. Astonishingly, very little is known about the underlying molecular machinery to orchestrate these ACDs in spiral-cleaving embryos. Here we identify, for the first time, cohorts of factors that may contribute to early embryonic ACDs in a spiralian embryo. RESULTS To do so we analyzed stage-specific transcriptome data in eggs and early embryos of the spiralian annelid Platynereis dumerilii for the expression of over 50 candidate genes that are involved in (1) establishing cortical domains such as the partitioning defective (par) genes, (2) directing spindle orientation, (3) conveying polarity cues including crumbs and scribble, and (4) maintaining cell-cell adhesion between embryonic cells. In general, each of these cohorts of genes are co-expressed exhibiting high levels of transcripts in the oocyte and fertilized single-celled embryo, with progressively lower levels at later stages. Interestingly, a small number of key factors within each ACD module show different expression profiles with increased early zygotic expression suggesting distinct regulatory functions. In addition, our analysis discovered several highly co-expressed genes that have been associated with specialized neural cell-cell recognition functions in the nervous system. The high maternal contribution of these 'neural' adhesion complexes indicates novel general adhesion functions during early embryogenesis. CONCLUSIONS Spiralian embryos are champions of ACD generating embryonic cells of different size with astonishing accuracy. Our results suggest that the molecular machinery for ACD is already stored as maternal transcripts in the oocyte. Thus, the spiralian egg can be viewed as a totipotent yet highly specialized cell that evolved to execute fast and precise ACDs during spiral cleaving stages. Our survey identifies cohorts of factors in P. dumerilii that are candidates for these molecular mechanisms and their regulation, and sets the stage for a functional dissection of ACD in a spiral-cleaving embryo.

中文翻译：

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海洋环节动物 Platynereis dumerilii 螺旋分裂卵和胚胎中的不对称细胞分裂机制。

背景技术超过三分之一的动物门利用一种称为“螺旋卵裂”的早期胚胎发生模式将受精卵分裂成具有不同细胞命运的胚胎细胞。这种模式的特点是一系列不变的、刻板的、不对称的细胞分裂（ACD），在早期胚胎中产生不同大小和确定位置的细胞。令人惊讶的是，人们对在螺旋分裂胚胎中协调这些 ACD 的潜在分子机制知之甚少。在这里，我们首次确定了可能导致螺旋状胚胎中早期胚胎 ACD 的一组因素。结果 为此，我们分析了螺旋环节动物 Platynereis dumerilii 的卵和早期胚胎的阶段特异性转录组数据，以了解 50 多个候选基因的表达，这些基因参与 (1) 建立皮质域，例如分区缺陷 (par) 基因， （2）指导纺锤体方向，（3）传递极性线索，包括面包屑和涂鸦，以及（4）维持胚胎细胞之间的细胞间粘附。一般来说，这些基因群中的每一个都是共表达的，在卵母细胞和受精单细胞胚胎中表现出高水平的转录本，而在后期阶段水平逐渐降低。有趣的是，每个 ACD 模块内的少数关键因子显示出不同的表达谱，早期合子表达增加表明不同的调节功能。此外，我们的分析发现了几种高度共表达的基因，这些基因与神经系统中专门的神经细胞识别功能相关。这些“神经”粘附复合物的高母体贡献表明早期胚胎发生期间新的一般粘附功能。结论 螺旋胚胎是 ACD 的冠军，能够以惊人的准确性生成不同大小的胚胎细胞。我们的结果表明，ACD 的分子机制已经作为母体转录本存储在卵母细胞中。因此，螺旋卵可以被视为一种全能但高度特化的细胞，其进化为在螺旋分裂阶段执行快速且精确的 ACD。我们的调查确定了 P. dumerilii 中的一组因子，这些因子是这些分子机制及其调节的候选因子，并为螺旋分裂胚胎中 ACD 的功能解剖奠定了基础。