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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.
Figure 1. PCA of profiled blastomeres collected from early developmental stages of Xenopus laevis.Triangles indicate blastomeres from the 8-cell stage; squares from the 16-cell stage, and circles from the 32-cell stage. Blastomeres originating from the animal hemisphere are shown in red and those from the vegetal hemisphere are shown in blue. Left and right graphs are data from two different females (left: four embryos from 8-cell stage, four embryos from 16-cell stage, and one 32-cell stage embryo, in total 128 blastomeres; right: two embryos from 8-cell stage, three from 16-cell stage, and one from 32-cell stage, in total 96 blastomeres).
Figure 2. Hierarchical clustering of blastomeres and mRNAs of Xenopus laevis early embryos presented as heatmaps.One embryo from each developmental stage was arbitrarily chosen for the cluster analysis. (A) 8-cell, (B) 16-cell and (C) 32-cell embryo. Green color indicates low expression and red high expression. The dendrograms clustering blastomeres and genes are shown in the top and left side of the heatmap, respectively. In the dendrograms similarity between blastomeres/genes is indicated by the height at which they are joined.
Figure 3. Principal component analysis of maternal genes.(A) 8-cell stage (four embryos/32 cells analyzed), (B) 16-cell stage (four embryos/64 cells analyzed), and (C) 32-cell stage (32 cells analyzed) embryos. Three clusters are seen in all the developmental stages. First cluster comprises 18S rRNA, 5S rRNA, and cyc1(mitochondrial cytochrome c); second cluster vg1, cdx1 (xcad2), wnt11, dazl, vegt, ddx25 (deadsouth), otx1, and trim36; third cluster (red squares) fzd7, par1, bmp2, pias1, dvl2, dvl3, lrp6, foxr1, fart1, mapk8, odc1, axin1, est1, U3 snoRNA, apc, gapdh, acta, eef1a1, tcf3, zpc, RNA polymerase II, gsk3b, maml1, tubb, ctnnb1 (β-catenin), mos, oct60, foxh1, stat3, and pax6. The animal-vegetal distinction is found along the y-axis (PC2), while differences in expression level of the maternal transcripts is reflected along the x-axis (PC1).
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