Figure 3

Schematic illustration of early development in Cnidaria, Ctenophora, and Bilateria. In bilaterian eggs (1 to 3a), cytasters (cyts) are present in the oocyte (during and at the end of oogenesis; 1c). When the egg is activated by fertilization (1a), cytasters reorganize the ectoplasm around them forming the cortical cytoplasmic modules (CCM, dashed circle) and mitosis starts. Immediately following mitosis (3a), cytasters begin to lose their distinct radial configurations, gradually reverting back to small astral areas that diminish considerably in size (dashed line), possibly becoming reduced centrioles (RC). Bilaterian cytasters have a similar role to that of sperm pronuclei in ctenophores. In Ctenophora (1 to 3b), previous to the entry of sperm (1b) no cytoplasmic movements are observed. When supernumerary sperm (sns) enter the egg (2b), cytoplasm reorganizes into several sperm pronuclear zones (SPZ, dashed circle) and the female pronucleus (fp) chooses one sperm (double arrow). In this moment cell division starts and supernumerary sperm asters diminish in size and become integrated to the microtubule network (dashed line; 3b). In Cnidaria, normal development (1 to 3c) requires a microtubule network (mn) exclusively organized by the maternal centrosome (mc; 1c), and after fertilization (2c), by the sperm-derived centrosome. All maternal determinants necessary for normal development are transported towards the animal pole (arrows in 1c), where the nucleus and centrosome are found. Fertilized eggs of cnidarians contain a residual microtubule network at the time of first mitosis (3c).