Castro Colabianchi AM , González Pérez NG , Franchini LF , López SL .

	Figure 1. (A) Classic model which considers that the Xenopus unfertilized egg is radially symmetric around the animal-vegetal axis. Radial symmetry is broken shortly after fertilization by cortical rotation through relocation of maternal dorsal determinants from the vegetal pole to the future dorsal side. (B) Xenopus unfertilized eggs bear an intrinsic bilateral symmetry related to the position of the eccentric EMP, which can direct dorsoventral axialization if the SEP-oriented cues fail. Diagram representing the results obtained by Brown et al. (1994) (Brown et al., 1994), who analyzed the direction of yolk mass rotation in eggs with the cortex artificially immobilized, here illustrated as cortical rotation for simplicity. Only in eggs in which the sperm entered the animal hemisphere on the opposite side of the EMP, the later can override the SEP to direct rotation. Global frequencies for either SEP- or EMP- directed rotation were calculated from the results presented on Figure 6 of (Brown et al., 1994) and are shown as percentage of the total number of eggs analyzed (n = 31) in the right column. From the same dataset we also calculated the frequencies for SEP- or EMP- directed rotation for the eggs with the EMP and SEP on the same or on opposite sides of the animal hemisphere (percentages between brackets).
	Figure 2. Notch1 protein and mRNA are asymmetrically distributed in the animal hemisphere of Xenopus laevis zygotes before the onset of cortical rotation. Albino embryos at s1 fixed before (t0, t0′) or at the end (t1) of cortical rotation; mpf, minutes post-fertilization. (A’–E’) are the same embryos shown in (A–E), respectively. (A–C’) combined Notch1 immunofluorescence (A–C) and gdf1 (vg1) ISH (A’–C’). Red and white arrows respectively point to the highest and lowest immunofluorescence signal, indicating that Notch1 protein is asymmetrically distributed in the animal hemisphere before and after cortical rotation, while gdf1 (vg1) mRNA is uniformly distributed in the vegetal region (yellow arrows), as expected. (D–E’) double ISH for notch1 (blue) and gdf1 (vg1) (turquoise). Blue arrows point to the asymmetric distribution of notch1 mRNA in the animal hemisphere before (D,D’) and at the end of cortical rotation (E,E’). Embryo’s orientation was verified by gdf1 (vg1) mRNA location (turquoise arrows) as reference since it is uniformly distributed in the vegetal cortex. An, animal pole; Veg, vegetal pole; broken line, animal-vegetal axis.
	Figure 3. Asymmetric distribution of Notch1 protein in the animal hemisphere of unfertilized Xenopus laevis eggs. (A–B’) Pigmented animal hemispheres were dissected by cutting them away through the equatorial plane, bleached, and directly processed for Notch1 immunofluorescence (A,A’) or cryosections followed by Notch1 immunofluorescence combined with GAPDH immunofluorescence as ubiquitous reference protein (B,B’). (C,C’) Whole albino egg, Notch1 immunofluorescence (C’) combined with wnt11b ISH as reference to verify the orientation of the animal-vegetal axis (C’), since wnt11b mRNA is uniformly distributed in the vegetal region. Yellow and white arrows respectively point to the highest and lowest Notch1 immunofluorescence signal, indicating that Notch1 protein is asymmetrically distributed in the animal hemisphere in unfertilized eggs. An, animal pole; Veg, vegetal pole; IF, immunofluorescence; cyan double arrow, animal-vegetal axis. (D–F) Quantification of Notch1 and GAPDH immunofluorescence in cryosections of animal hemispheres obtained from pigmented, unfertilized eggs, bleached before immunofluorescence. The images shown in (D) are the same as in (B,B’), but here, the selected ROIs are demarcated. (E) Dispersion graph comparing the relative mean pixel intensity (mpi) between ROIa and ROIb for Notch1 and GAPDH immunofluorescence in animal hemisphere cryosections from 15 independent unfertilized eggs. The mean ROIa/ROIb ratio was significantly higher for Notch1 than for GAPDH immunofluorescence, which was close to 1, as expected for a protein of homogeneous distribution (p = 0.0008, two-tailed paired t-test). (F) Comparison of relative Notch1/GAPDH mpi levels between ROIa and ROIb in the same set of eggs shown in (E). The Notch1/GAPDH ratio was significantly higher in ROIa than in ROIb (p = 0.0016, two-tailed paired t-test, n = 15). Since each sample always comprised a pair of measurements (as defined by ROIa and ROIb), paired t-tests (two-tailed) were performed; the difference between means was considered statistically significant when p < 0.05.
	Figure 4. Distribution of notch1 mRNA and other transcripts in the animal hemisphere of unfertilized Xenopus laevis eggs. (A–A”), (G–J′) Pigmented eggs were cut through the equatorial plane. Animal hemispheres were processed for ISH for the following markers: notch1 (A–A”), bmp4 (G,G′), dll1 (H,H′), hes4 (I,I′), and pou5f3.1 (J,J′). Animal hemispheres in (A,G–J) were photographed in animal view, then turned 180° to the right of the observer to photograph their internal side (equatorial face of the animal hemisphere; internal views in (A′,G′-J′). The animal hemisphere processed for notch1 ISH was photographed before (A,A′) and after bleaching (A″) in animal (A,A′) and internal views (A′). (B,B′) Albino egg processed for double ISH for notch1 (blue) and wnt11b (turquoise) shown in lateral (B) and animal views (B′). Dark blue and white arrowheads respectively point to the highest and lowest notch1 ISH signal, indicating that its transcripts are asymmetrically distributed in the animal hemisphere. The egg’s orientation was verified by the location of wnt11b mRNA as reference (turquoise arrowheads), which is uniformly distributed in the vegetal cortex (B). (C–F) Whole albino eggs processed for double ISH for notch1 (magenta) and pou5f3.1 (turquoise). Animal (C,D) and internal views of hemisections cut in the animal-vegetal plane (E,F). Magenta and gray arrowheads respectively point to the highest and lowest notch1 ISH signal, indicating that its transcripts are enriched on one side of the animal hemisphere, while pou5f3.1 does not show such asymmetry. An, animal pole; Veg, vegetal pole; broken line, animal-vegetal axis. Dark blue and white arrowheads in (A′-A″,G′–I′) respectively point to the highest and lowest levels of notch1, bmp4, dll1, and hes4 transcripts in the animal hemisphere. Notice the central location of pou5f3.1 transcripts in the animal hemisphere, most likely related to accumulation in the nuclear region. (K,K′) Absence of non-specific staining in whole albino eggs (upper row) and animal hemispheres from pigmented eggs (lower row) that were processed for ISH but without adding probes, in parallel with albino eggs processed for double ISH (as those shown in B,B′) and revealed with BCIP and with eggs processed for notch1 ISH (as shown in A-A″) and revealed with NBT + BCIP. After the ISH procedure, albino eggs were bisected to show the internal face (K′, upper row).
	Figure 5. Morphometric analysis showing that the distribution of notch1 mRNA is biased towards the EMP side in unfertilized Xenopus laevis eggs. (A–E″) Examples of animal hemispheres obtained from pigmented eggs that were analyzed by morphometry. (A–A″) pou5f3.1 ISH. (B–E″) notch1 ISH in four different animal hemispheres. The first column shows the external, animal views; pink arrowheads point to the EMP. The second column shows the same animal views labeled with the EMP (e, pink circles) and GAPc (g, white circles) locations, and a salmon line corresponding to the egg’s outline drawn by the Analize Particle tools with FIJI. In the third column, these landmarks were overlapped as an overlay to the inverted internal views (equatorial face) showing the ISH domains of the corresponding eggs. Axes and quadrants were labeled as described in the text. Dotted black lines demarcate the ISH ROIs; their centroids are labeled with a yellow circle for pou5f3.1 (p) and a green circle for notch1 (n). The white dotted arrow lines correspond to the radius containing the ISH centroids (ISHc). Eggs in (A–A″,D–D″) are the same as Figure 4F, F′ and A-A″, respectively, but here, their images are oriented for the morphometric analysis and labeled with the landmarks. (F) Polar coordinates graph showing the distribution of ISH centroids for pou5f3.1 and notch1 (color-coded circles, see references in the upper right corner of the figure) for all the analyzed eggs, according to their relative radial distances to the GAPc and their deviation angle from the EMP-GAPc axis (y-axis). Inside the circles, numbers indicate the individual egg# (see Supplementary Table S4 for the corresponding measurements). For notch1 ISH centroids, the mean deviation angle Ө from the EMP-GAPc axis is indicated. The pink circle shows the mean EMP relative radial distance to the GAPc for all analyzed eggs; pink arrowheads show the range of EMP relative positions of the analyzed eggs. Notice that the inferior range limit is the cut-off level assumed for performing the morphometric analysis. The inset in the lower right corner shows a dispersion diagram representing the EMP relative radial distance to the GAPc for all the analyzed eggs. Mean±SEM values are indicated (black and grey lines, respectively). Their spatial distribution is displayed in the polar coordinates graphs of Supplementary Figures S2–S4 and measurements are shown in Supplementary Table S4. See Supplementary Movie S1 showing the transitions between images. (G) ISH centroids for notch1 mRNA are significantly eccentric in relation to the central marker pou5f3.1 (unpaired, two-tailed t-test; p < 0.0001). Bars indicate mean±SEM. (H) Spatial composition of the notch1 ISH domain. Both quadrants on the same side of the EMP (q1, q4) show a significantly higher contribution to the ISH area than the opposite quadrants (q2, q3) in the analyzed eggs (two-tailed, paired t-test; q1,q2: p = 0.0052; q1,q3: p = 0.0083; q1,q4: p = 0.5519; q2,q3: p = 0.7671; q2,q4: p = 0.0089; q3,q4: p = 0.0003); n.s., non-significant differences. Bars indicate mean±SEM. notch1 ISH, n = 15, two independent females; pou5f3.1 ISH, n = 7, one female.
	Figure 6. Notch1 protein becomes asymmetrically distributed in an orthogonal axis with respect to the animal-vegetal axis during Xenopus laevis oogenesis. Defolliculated oocytes obtained from pigmented females were fixed, bleached, and processed for combined immunofluorescence for Notch1 protein and ISH for wnt11b mRNA as reference marker of the vegetal pole to orient the oocytes in their animal-vegetal axis. From left to right, successive stages of oogenesis are shown in two representative series. Oocytes were classified according to the stages (s) described by (Dumont, 1972). For each series, the upper tier shows Notch1 immunofluorescence. The same oocytes are shown in bright field in the lower tier, with wnt11b mRNA located in the vegetal pole throughout oogenesis and also on the cytoplasm of early sI oocytes, as previously described (Kloc and Etkin, 1995). An, animal pole; cyan double arrow, animal-vegetal axis, as determined by wnt11b mRNA location; the white dotted line demarcates the contour of sI and sII oocytes and the wnt11b domain from sIII to sVI. Yellow and white arrows on the sVI oocyte point to the vegetal boundary of Notch1 protein expression in the animal hemisphere, which is more vegetal on one side (yellow arrow) than on the other side of the picture (white arrow), thus showing an asymmetric domain in the animal hemisphere. Scale bar: 200 µm. See data in Supplementary Table S5.
	Figure 7. Zebrafish notch1a mRNA is asymmetrically distributed during early embryogenesis in the animal hemisphere from the 1-cell stage. Black and grey arrows in (A–G) respectively point to the regions with the highest and lowest notch1a ISH signal. The dotted ellipse in (A) demarcates the blastodisc. (H) ISH performed in parallel with the same notch1a probe in a 24 hpf embryo, showing typical expression for this transcript.
	Figure 8. (A) Classic model of axis formation in Xenopus, which only considers relocation of maternal dorsal determinants after fertilization. (B) New hypothesis for axis formation in Xenopus, which considers both relocation of maternal dorsal determinants after fertilization and an intrinsic bilaterality of unfertilized eggs harboring an asymmetric distribution of ventralizing molecules in the animal hemisphere. See main text for additional details.
	FIGURE 1. (A) Classic model which considers that the Xenopus unfertilized egg is radially symmetric around the animal-vegetal axis. Radial symmetry is broken shortly after fertilization by cortical rotation through relocation of maternal dorsal determinants from the vegetal pole to the future dorsal side. (B) Xenopus unfertilized eggs bear an intrinsic bilateral symmetry related to the position of the eccentric EMP, which can direct dorsoventral axialization if the SEP-oriented cues fail. Diagram representing the results obtained by Brown et al. (1994) (Brown et al., 1994), who analyzed the direction of yolk mass rotation in eggs with the cortex artificially immobilized, here illustrated as cortical rotation for simplicity. Only in eggs in which the sperm entered the animal hemisphere on the opposite side of the EMP, the later can override the SEP to direct rotation. Global frequencies for either SEP- or EMP- directed rotation were calculated from the results presented on Figure 6 of (Brown et al., 1994) and are shown as percentage of the total number of eggs analyzed (n = 31) in the right column. From the same dataset we also calculated the frequencies for SEP- or EMP- directed rotation for the eggs with the EMP and SEP on the same or on opposite sides of the animal hemisphere (percentages between brackets).
	FIGURE 2. Notch1 protein and mRNA are asymmetrically distributed in the animal hemisphere of Xenopus laevis zygotes before the onset of cortical rotation. Albino embryos at s1 fixed before (t0, t0′) or at the end (t1) of cortical rotation; mpf, minutes post-fertilization. (A’–E’) are the same embryos shown in (A–E), respectively. (A–C’) combined Notch1 immunofluorescence (A–C) and gdf1 (vg1) ISH (A’–C’). Red and white arrows respectively point to the highest and lowest immunofluorescence signal, indicating that Notch1 protein is asymmetrically distributed in the animal hemisphere before and after cortical rotation, while gdf1 (vg1) mRNA is uniformly distributed in the vegetal region (yellow arrows), as expected. (D–E’) double ISH for notch1 (blue) and gdf1 (vg1) (turquoise). Blue arrows point to the asymmetric distribution of notch1 mRNA in the animal hemisphere before (D,D’) and at the end of cortical rotation (E,E’). Embryo’s orientation was verified by gdf1 (vg1) mRNA location (turquoise arrows) as reference since it is uniformly distributed in the vegetal cortex. An, animal pole; Veg, vegetal pole; broken line, animal-vegetal axis.
	FIGURE 3. Asymmetric distribution of Notch1 protein in the animal hemisphere of unfertilized Xenopus laevis eggs. (A–B’) Pigmented animal hemispheres were dissected by cutting them away through the equatorial plane, bleached, and directly processed for Notch1 immunofluorescence (A,A’) or cryosections followed by Notch1 immunofluorescence combined with GAPDH immunofluorescence as ubiquitous reference protein (B,B’). (C,C’) Whole albino egg, Notch1 immunofluorescence (C’) combined with wnt11b ISH as reference to verify the orientation of the animal-vegetal axis (C’), since wnt11b mRNA is uniformly distributed in the vegetal region. Yellow and white arrows respectively point to the highest and lowest Notch1 immunofluorescence signal, indicating that Notch1 protein is asymmetrically distributed in the animal hemisphere in unfertilized eggs. An, animal pole; Veg, vegetal pole; IF, immunofluorescence; cyan double arrow, animal-vegetal axis. (D–F) Quantification of Notch1 and GAPDH immunofluorescence in cryosections of animal hemispheres obtained from pigmented, unfertilized eggs, bleached before immunofluorescence. The images shown in (D) are the same as in (B,B’), but here, the selected ROIs are demarcated. (E) Dispersion graph comparing the relative mean pixel intensity (mpi) between ROIa and ROIb for Notch1 and GAPDH immunofluorescence in animal hemisphere cryosections from 15 independent unfertilized eggs. The mean ROIa/ROIb ratio was significantly higher for Notch1 than for GAPDH immunofluorescence, which was close to 1, as expected for a protein of homogeneous distribution (p = 0.0008, two-tailed paired t-test). (F) Comparison of relative Notch1/GAPDH mpi levels between ROIa and ROIb in the same set of eggs shown in (E). The Notch1/GAPDH ratio was significantly higher in ROIa than in ROIb (p = 0.0016, two-tailed paired t-test, n = 15). Since each sample always comprised a pair of measurements (as defined by ROIa and ROIb), paired t-tests (two-tailed) were performed; the difference between means was considered statistically significant when p < 0.05.
	FIGURE 4. Distribution of notch1 mRNA and other transcripts in the animal hemisphere of unfertilized Xenopus laevis eggs. (A–A”), (G–J′) Pigmented eggs were cut through the equatorial plane. Animal hemispheres were processed for ISH for the following markers: notch1 (A–A”), bmp4 (G,G′), dll1 (H,H′), hes4 (I,I′), and pou5f3.1 (J,J′). Animal hemispheres in (A,G–J) were photographed in animal view, then turned 180° to the right of the observer to photograph their internal side (equatorial face of the animal hemisphere; internal views in (A′,G′-J′). The animal hemisphere processed for notch1 ISH was photographed before (A,A′) and after bleaching (A″) in animal (A,A′) and internal views (A′). (B,B′) Albino egg processed for double ISH for notch1 (blue) and wnt11b (turquoise) shown in lateral (B) and animal views (B′). Dark blue and white arrowheads respectively point to the highest and lowest notch1 ISH signal, indicating that its transcripts are asymmetrically distributed in the animal hemisphere. The egg’s orientation was verified by the location of wnt11b mRNA as reference (turquoise arrowheads), which is uniformly distributed in the vegetal cortex (B). (C–F) Whole albino eggs processed for double ISH for notch1 (magenta) and pou5f3.1 (turquoise). Animal (C,D) and internal views of hemisections cut in the animal-vegetal plane (E,F). Magenta and gray arrowheads respectively point to the highest and lowest notch1 ISH signal, indicating that its transcripts are enriched on one side of the animal hemisphere, while pou5f3.1 does not show such asymmetry. An, animal pole; Veg, vegetal pole; broken line, animal-vegetal axis. Dark blue and white arrowheads in (A′-A″,G′–I′) respectively point to the highest and lowest levels of notch1, bmp4, dll1, and hes4 transcripts in the animal hemisphere. Notice the central location of pou5f3.1 transcripts in the animal hemisphere, most likely related to accumulation in the nuclear region. (K,K′) Absence of non-specific staining in whole albino eggs (upper row) and animal hemispheres from pigmented eggs (lower row) that were processed for ISH but without adding probes, in parallel with albino eggs processed for double ISH (as those shown in B,B′) and revealed with BCIP and with eggs processed for notch1 ISH (as shown in A-A″) and revealed with NBT + BCIP. After the ISH procedure, albino eggs were bisected to show the internal face (K′, upper row).
	FIGURE 5. Morphometric analysis showing that the distribution of notch1 mRNA is biased towards the EMP side in unfertilized Xenopus laevis eggs. (A–E″) Examples of animal hemispheres obtained from pigmented eggs that were analyzed by morphometry. (A–A″) pou5f3.1 ISH. (B–E″) notch1 ISH in four different animal hemispheres. The first column shows the external, animal views; pink arrowheads point to the EMP. The second column shows the same animal views labeled with the EMP (e, pink circles) and GAPc (g, white circles) locations, and a salmon line corresponding to the egg’s outline drawn by the Analize Particle tools with FIJI. In the third column, these landmarks were overlapped as an overlay to the inverted internal views (equatorial face) showing the ISH domains of the corresponding eggs. Axes and quadrants were labeled as described in the text. Dotted black lines demarcate the ISH ROIs; their centroids are labeled with a yellow circle for pou5f3.1 (p) and a green circle for notch1 (n). The white dotted arrow lines correspond to the radius containing the ISH centroids (ISHc). Eggs in (A–A″,D–D″) are the same as Figure 4F, F′ and A-A″, respectively, but here, their images are oriented for the morphometric analysis and labeled with the landmarks. (F) Polar coordinates graph showing the distribution of ISH centroids for pou5f3.1 and notch1 (color-coded circles, see references in the upper right corner of the figure) for all the analyzed eggs, according to their relative radial distances to the GAPc and their deviation angle from the EMP-GAPc axis (y-axis). Inside the circles, numbers indicate the individual egg# (see Supplementary Table S4 for the corresponding measurements). For notch1 ISH centroids, the mean deviation angle Ө from the EMP-GAPc axis is indicated. The pink circle shows the mean EMP relative radial distance to the GAPc for all analyzed eggs; pink arrowheads show the range of EMP relative positions of the analyzed eggs. Notice that the inferior range limit is the cut-off level assumed for performing the morphometric analysis. The inset in the lower right corner shows a dispersion diagram representing the EMP relative radial distance to the GAPc for all the analyzed eggs. Mean±SEM values are indicated (black and grey lines, respectively). Their spatial distribution is displayed in the polar coordinates graphs of Supplementary Figures S2–S4 and measurements are shown in Supplementary Table S4. See Supplementary Movie S1 showing the transitions between images. (G) ISH centroids for notch1 mRNA are significantly eccentric in relation to the central marker pou5f3.1 (unpaired, two-tailed t-test; p < 0.0001). Bars indicate mean±SEM. (H) Spatial composition of the notch1 ISH domain. Both quadrants on the same side of the EMP (q1, q4) show a significantly higher contribution to the ISH area than the opposite quadrants (q2, q3) in the analyzed eggs (two-tailed, paired t-test; q1,q2: p = 0.0052; q1,q3: p = 0.0083; q1,q4: p = 0.5519; q2,q3: p = 0.7671; q2,q4: p = 0.0089; q3,q4: p = 0.0003); n.s., non-significant differences. Bars indicate mean±SEM. notch1 ISH, n = 15, two independent females; pou5f3.1 ISH, n = 7, one female.
	FIGURE 6. Notch1 protein becomes asymmetrically distributed in an orthogonal axis with respect to the animal-vegetal axis during Xenopus laevis oogenesis. Defolliculated oocytes obtained from pigmented females were fixed, bleached, and processed for combined immunofluorescence for Notch1 protein and ISH for wnt11b mRNA as reference marker of the vegetal pole to orient the oocytes in their animal-vegetal axis. From left to right, successive stages of oogenesis are shown in two representative series. Oocytes were classified according to the stages (s) described by (Dumont, 1972). For each series, the upper tier shows Notch1 immunofluorescence. The same oocytes are shown in bright field in the lower tier, with wnt11b mRNA located in the vegetal pole throughout oogenesis and also on the cytoplasm of early sI oocytes, as previously described (Kloc and Etkin, 1995). An, animal pole; cyan double arrow, animal-vegetal axis, as determined by wnt11b mRNA location; the white dotted line demarcates the contour of sI and sII oocytes and the wnt11b domain from sIII to sVI. Yellow and white arrows on the sVI oocyte point to the vegetal boundary of Notch1 protein expression in the animal hemisphere, which is more vegetal on one side (yellow arrow) than on the other side of the picture (white arrow), thus showing an asymmetric domain in the animal hemisphere. Scale bar: 200 µm. See data in Supplementary Table S5.
	FIGURE 7. Zebrafish notch1a mRNA is asymmetrically distributed during early embryogenesis in the animal hemisphere from the 1-cell stage. Black and grey arrows in (A–G) respectively point to the regions with the highest and lowest notch1a ISH signal. The dotted ellipse in (A) demarcates the blastodisc. (H) ISH performed in parallel with the same notch1a probe in a 24 hpf embryo, showing typical expression for this transcript.
	FIGURE 8. (A) Classic model of axis formation in Xenopus, which only considers relocation of maternal dorsal determinants after fertilization. (B) New hypothesis for axis formation in Xenopus, which considers both relocation of maternal dorsal determinants after fertilization and an intrinsic bilaterality of unfertilized eggs harboring an asymmetric distribution of ventralizing molecules in the animal hemisphere. See main text for additional details.

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