Meeting Abstract

S9.5  Wednesday, Jan. 6  Symmetry makers and symmetry breakers: the transition of a spiral cellular arrangement to bilateral symmetry in early embryos of Platynereis dumerilii SCHNEIDER, Stephan Q; Iowa State University sqs@iastate.edu

Embryonic development shapes the size of subsequent morphological features e.g. of larvae and/or adults by specifiying areas and/or progenitor cells of different size and fates. Spiralian embryos accomplish these by highly stereotypic and invariant asymmetric cell divisions. In Platynereis embryos each embryonic cell can be already identified by its size and position within the embryo. Here we report the cell lineages of the four animal-pole daughter cells from an eight cell stage embryo, the micromeres 1a, 1b, 1c, and 1d until the ~ 200 cell stage. We define the stereotyped sister cell asymmetries (as observed by different cell sizes, cell cycle times, and beta-catenin activation patterns) in each cell division cycle within this period. The patterns of spindle orientation are tightly regulated on the right and left side to generate invariant embryonic symmetries as well as asymmetries. Bilateral symmetrical patterns of progenitor cells arise within most of the cell lineages e.g. that form pairs of eyes and brains. However, symmetries get sometimes broken to generate asymmetric single embryonic progenitors that may form the apical organ and the dorsal midline. Our analyses demonstrate how modules of sister cell asymmetries form animal-vegetal, dorsal-ventral, and left and right global embryonic axes. The identification of cell lineage relationships of each cell of the ~200 cell stage embryo enabled us to identify each cell that expresses certain developmental regulators including the homeodomain protein Pdum Distal-less defining novel potentially ancestral roles for this transcription factor in dorsal-ventral axis formation.