The Asymmetric Cell Division Machinery in the Spiral-Cleaving Egg and Embryo of the Marine Annelid Platynereis Dumerilii Aron B
Total Page：16
File Type：pdf, Size：1020Kb
Nakama et al. BMC Developmental Biology (2017) 17:16 DOI 10.1186/s12861-017-0158-9 RESEARCH ARTICLE Open Access The asymmetric cell division machinery in the spiral-cleaving egg and embryo of the marine annelid Platynereis dumerilii Aron B. Nakama1, Hsien-Chao Chou1,2 and Stephan Q. Schneider1* Abstract 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. Keywords: Maternal contribution, Spiral cleavage, Cortical domains, PAR genes, Spindle orientation, Cell polarity, Crumbs complex, Planar cell polarity, Neural adhesion complexes, Transcriptional profiles * Correspondence: [email protected] 1Department of Genetics, Development and Cell Biology, Iowa State University, 503 Science Hall II, Ames, IA 50011, USA Full list of author information is available at the end of the article © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Nakama et al. BMC Developmental Biology (2017) 17:16 Page 2 of 22 Background [12]. Although cellular aspects of ACD, including the Asymmetric cell division (ACD) is the fundamental role of the cytoskeleton, have been studied in annelid process that subdivides a mother cell into two daughter embryos by the Shimizu and Weisblat laboratories [13– cells that exhibit differences in cell fate, cell size, and/or 17], surprisingly little is known about the molecular ma- cell position. As such, it constitutes a central developmen- chinery that executes ACD in spiralians, even though tal mechanism essential for cell lineage diversification dur- these embryos exhibit some of the most readily observ- ing early embryogenesis, formation of organs, and division able and extreme cell size asymmetries in all metazoans. of stem cells (reviewed in [1–4]). Much of our current Developing P. dumerilii embryos produce the typical knowledge about ACD has been established over the last hallmarks of spiral cleavage. Every cell division, beginning two decades exploiting two genetic invertebrate model or- with the one-celled zygote, is asymmetric. ACD continues ganisms, the nematode worm Caenorhabditis elegans (C. through early cleavage stages and beyond generating elegans) and the arthropod fruit fly Drosophila melanoga- daughter cells of different sizes [18, 19]. For example, the ster (Drosophila). In both systems, a detailed understand- first mitotic division always generates the smaller AB and ing of embryonic and postembryonic processes on a the larger CD blastomeres [10, 19, 20]. This first division cellular level, and the use of forward and reverse genetic defines the D/V axis. As in other spiral cleaving embryos, tools, has led to the identification of mostly conserved the developmental program in P. dumerilii is recognizable cohorts of genes. These genes execute ACDs in various through the pattern of cell divisions based upon the developmental contexts including the formation of diver- stereotypic orientation of the mitotic spindle. Starting with sified cell lineages within embryos, the nervous system, the 3rd cell division the alternating mitotic spindles are sensory organs, wing development, and epithelia. oriented either clockwise or counter clockwise, with re- The significance of ACD becomes unmistakable when spect to the animal pole, generating animal-pole and considering the earliest steps during metazoan embryo- vegetal-pole sister cell pairs [10, 18–20]. Thus, the embry- genesis. Mitotic cell divisions subdivide the fertilized egg onic cells of differing size and position are assembled in a into embryonic cells with different fates and defined po- stereotypic and invariant pattern in each stage. sitions that foreshadow the three emerging body axes of The generation of the spiral cleavage pattern along the the embryo. This process has been studied on the molecu- animal-vegetal axis requires an asymmetric cue, cellular lar level extensively, in C. elegans, whereallthreebodyaxes polarization, and/or some initial symmetry-breaking event anterior/posterior (A/P), dorsal/ventral (D/V), and right/left in oocyte or zygote that leads to subsequent ACDs. In (R/L) are determined by the third round of embryonic cell some organisms, the unfertilized egg/oocyte is polarized divisions [2, 5]. Complementary studies in Drosophila,and as observed in the intrinsically well-defined polarity axes in various vertebrate models including frog and mouse em- within the Drosophila egg [21]. In other organisms, early bryos, have identified similarities on the molecular level [4, asymmetric cues are introduced through fertilization e.g. 6, 7]. However, comparisons are complicated by the derived the sperm entry point defines the future A/P axis in C. nature of the molecular machinery for ACD in C. elegans elegans [22, 23], and the D/V axis in Xenopus laevis [24]. and/or the divergence of early developmental processes Currently, it is not known if internal polarity of the oocyte between metazoan models. Although studies on ACD in exists in P. dumerilii. However, dramatic reorganization of various new models are emerging [8, 9], there are still large the egg upon sperm contact, termed ooplasmic segrega- gaps in our understanding of the nature, utilization, conser- tion, has been described [18, 19]. vation, and evolution of the ACD mechanisms; especially in After sperm contacts the egg, cortical streaming pushes regards to establishing cellular asymmetries and cell yolk granules and lipid droplets towards the future vegetal lineages during metazoan embryogenesis. pole. At the same time, an area of clear cytoplasm forms at Here we introduce the marine annelid Platynereis the future animal pole, harboring the oocyte nucleus. About dumerilii as a model to study ACD in a spiral-cleaving one-hour post fertilization (hpf), the oocyte nucleus com- embryo [10]. Spiral cleavage is a widespread develop- pletes meiotic cell divisions, via two extreme ACDs, that mental mode among approximately one-third of all ani- lead to the extrusion of two polar bodies. The site of polar mal phyla including mollusks (snails and clams), and body extrusion defines the animal pole and determines the annelids (leeches and earthworms) [11]. This mode is animal/vegetal (A/V) axis [10, 19] (Fig. 1). Subsequently, characterized by a series of asymmetric cell divisions the male pronucleus migrates towards the female pro- that subdivide the fertilized egg into embryonic cells of nucleus (now located at the animal pole). Pronuclear fusion different size and fates with an invariant, stereotypic and duplication of the male centrioles mark the beginning spiral arrangement. Indeed, early observations of spiral of the first mitotic cell division of the zygote [25]. The mi- cleaving embryos and their cell size asymmetries were totic spindle pole located