Fig. 8. Summary of evidence for the multiple mechanisms that regulate SoxB1 accumulation along the animal-vegetal axis. (A) Uniformly distributed SoxB1 is asymmetrically partitioned at fourth cleavage among different sized blastomeres in proportion to their cytoplasmic volume. [Image of an embryo doubly stained with SoxB1 antibody and DAPI between 16 and 32-cell stages reproduced, with permission, from Kenny et al. (Kenny et al., 1999)]. (B,C) SoxB1 is selectively degraded in micromeres and macromeres via different mechanisms, both of which depend on nuclear ß-catenin function. SoxB1 peptides lacking the NLS sequences or the 3'-terminal region are eliminated from micromeres but not macromres (B), whereas SoxB1 variants retaining the NLSs and at least one out of three regions in the C-terminal domain clear from both micromere and macromere derivatives (C). (D) SoxB1 mRNA levels remain high in vegetal blastomeres in the absence of nuclear ß-catenin. (E) SoxB1 mRNA concentrations are elevated in animal blastomeres in embryos lacking SoxB1 protein.