Figure 5

Early cleavage and model of POC action. a-c, Simplified early lineage trees of three nematode species with different cleavage types. A series of unequal cleavages attributed to the action of POCs results in cells arranged in tandem along the a-p axis. For symbols and color codes of POCs, see Figure 2. Note that cells performing l-r divisions belong to different generations. These cells or their descendants are shown in grey boxes to indicate lineage branches involved in the establishment of bilateral symmetry. Circles, blastomeres performing symmetric cleavages; hexagons, blastomeres performing asymmetric cleavages. Orientation of cleavage spindles: asterisk, variable; arrow, longitudinal, pointing toward smaller daughter cell; horizontal bar, longitudinal with daughters of equal size, vertical bar, left-right; circle, dorsal-ventral; 'X' somewhat variable generating imperfect symmetric clones; '+', germline cell giving rise to two mirror image gonadal arms in the anterior and posterior half of the adult.