1 Maternal Cytoplasmic Factors for Generation of Unique Cleavage Patterns in Animal Embryos

This chapter focuses on the maternal control of the generation of unique cleavage patterns. In ascidian embryos, experimental transplantation of egg cytoplasm has revealed the presence of localized factors in the egg cytoplasm. In gastropods and nematodes, maternal-effect variants and mutants show genetic evidence for the maternal contribution in controlling the pattern of cleavage. In nematodes, the products of such genes, the PAR proteins, show clear localization in the egg cortex. These studies indicate the importance of egg organization and localized factors for generation of unique cleavage patterns. To create divergence of the cleavage pattern from the “default” equal and orthogonal pattern, various mechanisms for positioning the mitotic apparatus are used in different animal systems. In sea urchin, ascidian, and nematode embryos, movements of centrosomes during interphase play key roles in the orientation and asymmetric positioning of the mitotic apparatus. The movements of the centrosome involve the cortical site, the centrosome, and the microtubules between them. In the nematode embryo, and in the second and third cleavages of oligochaete eggs, the mitotic spindle migrates during the mitotic phase. Microfilaments are required for the migration in the latter case. In the first cleavage of oligochaete, a difference in the size of the asters causes unequal cleavage.