Fig. 2

Early embryonic development of the marine annelid P. dumerilii. a 2-cell stage: The first mitotic ACD generates the larger CD and smaller AB blastomeres (CD is dorsal side, AB is ventral). b 4-cell stage: The second round of mitotic ACDs produces the four founder cells: A, B, C, and D. The D blastomere, the largest, contains the largest quantity of clear cytoplasm. c 8-cell stage: The third set of ACDs produces the smaller animal-pole sister cells, micromeres 1a, 1b, 1c, 1d, and the larger vegetal-pole sister cells, macromeres 1A, 1B, 1C, and 1D. Polar view of the spindle orientations (arrows) shows the counterclockwise offset of the micromeres from the macromeres generating the spiral arrangement of cells by the spiral cleavage. Upper row in (a), (b), and (c) shows view from animal pole, and lower row shows side views. Arrows in (b) and (c) indicate displacement angles of mitotic spindles with respect to viewing direction. The arrowheads point to the sister cell located closer to the animal pole. Arrow bases indicate the sister cell located closer to the vegetal pole. Polar bodies (teal spheres) mark the animal pole. Lipid droplets (yellow spheres) are localized at vegetal pole. Nuclei (purple) are located within the clear cytoplasm (lighter yellow) closer to the animal pole, surrounded by yolk-rich cytoplasm (light brown). d Developmental timeline of P. dumerilii from zygote to the ~330-cell stage (modified from Pruitt et al. 2014). The first mitotic cell division occurs shortly after 2 hpf, and subsequent stereotypic ACDs generate the spiral arrangement of cells during the next 5 h. After 7 hpf, cell divisions become more bilaterally symmetric in P. dumerilii embryos. Dashed lines indicate approximate age of various cell stages. Developmental expression profiles for each gene were generated by RNA-seq collected from embryos at seven time points (2, 4, 6, 8, 10, 12, and 14 hpf)