The asymmetric cell division machinery in early annelid embryogenesis: Maternal and embryonic contributions
About 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. The spiral arrangement of cells must depend on highly choreographed molecular mechanisms by recognizing polarity cues, orienting mitotic spindles, and generating cell size asymmetries between sister cells with astonishing precision and predictability. Although conserved molecular mechanisms for ACD have been unraveled in several genetic model systems over the last two decades, spiral-cleaving embryos have not been studied. Here we identify, for the first time, cohorts of factors that may contribute to early embryonic ACDs in a spiralian embryo.
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 exhibits high levels of transcripts in the fertilized single-celled embryo, with progressively lower levels at later stages. This expression pattern suggests high maternal contribution and slow decrease of transcripts that encode the cellular machinery for ACD during spiral embryogenesis. Many candidates were cloned, and their maternal expression validated by in situ hybridization in mature oocytes, zygotes, and early P. dumerilii embryos.
Spiralian embryos are champions of ACD generating embryonic cells of different size with astonishing accuracy. Our results suggest that the molecular machinery is already stored as maternal transcripts in the oocyte. Thus, the spiralian egg can be viewed as a 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 sets the stage for a functional dissection of ACD in a spiral-cleaving embryo.