We have measured the expression of 41 maternal mRNAs in individual blastomeres collected from the 8 to 32-cell Xenopus laevis embryos to determine when and how asymmetry in the body plan is introduced. We demonstrate that the asymmetry along the animal-vegetal axis in the oocyte is transferred to the daughter cells during early cell divisions. All studied mRNAs are distributed evenly among the set of animal as well as vegetal blastomeres. We find no asymmetry in mRNA levels that might be ascribed to the dorso-ventral specification or the left-right axis formation. We hypothesize that while the animal-vegetal asymmetry is a consequence of mRNA gradients, the dorso-ventral and left-right axes specifications are induced by asymmetric distribution of other biomolecules, probably proteins.

Laboratory of Gene Expression Institute of Biotechnology Academy of Sciences of the Czech Republic Prague Czech Republic

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