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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.
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