Identification of the Zebrafish nanog-like Gene
(A) Alignment of the homeodomain sequence of fish and tetrapod Nanog proteins. The percentage similarity of each to zebrafish Nanog-like is shown at the right.
(B) Intron/exon structure of the zebrafish, mouse, and human Nanog genes. The 60-aa homeodomain is encoded by the last 132 bp of the second exon and the first 48 bp of the third exon in all three organisms.
(C) Whole-mount in situ hybridization of nanog-like identified high maternal (1-cell) and blastula (dome) expression, and rapidly diminishing expression during gastrulation (shield and tailbud stages).
(D) Blastomeres of nanog-like-knockdown embryos, injected with one of two distinct morpholinos targeting the translational start site, failed to thin at the germ ring stage and constricted to squeeze out the yolk cell by the 80% epiboly stage.
(E) Time-lapse microscopy of the nanog-like-MO2-injected embryo showing the yolk burst process.
(F) Rescue of the nanog-like-MO1-injected embryos by coinjection of 100 pg zebrafish nanog-like or human NANOG mRNA. Embryos shown are at 24 hpf.
(G) Statistics for the rescue of nanog-like-MO1-injected embryos by zebrafish nanog-like or human NANOG mRNA. A total of 100 GFP mRNA-injected, 115 nanog-like mRNA-injected, and 209 human NANOG mRNA-injected embryos were analyzed. Phenotype classes were counted at 24 hpf and are presented as a percentage of the whole.

Nanog-like Regulates Endoderm Formation by Activating Nodal
(A) nanog-like knockdown impairs ventrolateral endoderm formation. The endoderm genes mixer, gata5, sox32, and sox17 are absent at the ventrolateral margin in nanog-like-MO1-injected embryos. All views are shown as an animal view with the dorsal side to the right.
(B) ndr1 and ndr2 expression is restricted to the dorsal area in the nanog-like-MO1-injected embryo; ventrolateral expression of both genes can be restored by coinjection of nanog-like mRNA. All views are lateral with the presumptive dorsal side to the right.
(C) Ventrolateral sox32 and sox17 expression can be rescued by ndr1 mRNA. All views are shown as an animal view with the presumptive dorsal side to the right.

Lack of YSL Transcription in nanog-like Morphants Leads to Epiboly and Endoderm Defects
(A) Microarray analysis of gene expression in nanog-like morphants revealed an absence of YSL gene transcription. YSL genes downregulated in three separate experiments are displayed as a heat map.
(B) The expression of gata6, mxtx2, camsap1l1, hnf4a, and slc26a1 is absent in the YSL of nanog-like morphants. All views are lateral with the presumptive dorsal side to the right.
(C) The YSN of the wild-type and nanog-like knockdown embryos at the sphere stage and the 50% epiboly stage labeled with SYTOX green. All views are shown as an animal view with the dorsal side to the right.
(D) Time-lapse microscopy of the YSNs of a nanog-like-MO2-injected embryo revealed the YSN aggregation process. Three YSNs undergo the 12^th division at t = 16 min. At t = 22 min, the upper left YSNs separate normally while the lower YSNs fail to completely separate. At t = 36 min, the resulting six daughter YSNs start to aggregate.
(E) Higher magnitude confocal microscopy revealed that the aggregated YSNs in Figure 3D remained separated.
(F) Nanog-like is required autonomously for YSL transcription. To achieve YSL-specific expression, 300 pg nanog-like-Myc mRNA was injected into the yolk at 2.5 hpf. The expression of mxtx2, gata6, and camsap1l1 was rescued by YSL-specific nanog-like-Myc expression.

Mxtx2 Is Required for and Sufficient for YSL Induction
(A) Lateral view of embryos injected with 500 pg control morpholino (con-MO), 500 pg mxtx2 morpholino (mxtx2-MO), or 100 pg mxtx2 mRNA at the one-cell stage. At the 80% epiboly stage, mxtx2 morphants develop a yolk burst phenotype, and the mxtx2 mRNA-injected embryo failed to undergo epiboly. The YSL markers gata6, camsap1l1, hnf4a, and slc26a1 were absent in the mxtx2 morphant. YSL genes were ectopically expressed in the embryos injected with mxtx2 mRNA.
(B) Defective Nodal signaling and endoderm formation in the mxtx2 morphant. ndr1 and ndr2 are shown as a lateral view with the presumptive dorsal side to the right. gata5 and sox32 staining is shown as an animal view with the dorsal side to the right.
(C) By overexpressing nanog-like-Myc, expression of mxtx2, gata6, and camsap1l1 in nanog-like morphants can be rescued. CHX was added at 2 hpf to allow for the translation of injected nanog-like-Myc mRNA but to prevent translation of the earliest zygotic transcripts. When CHX is added at 2 hpf, expression of mxtx2, but not gata6 or camsap1l1, can still be rescued, indicating that mxtx2 is the direct target of Nanog-like.

mxtx2 Expression Rescues the nanog-like Morphant
(A) The YSL lineage was restored by mxtx2 expression in the nanog-like morphant. Lateral view of embryos injected with 10 ng control morpholino (con-MO) or 10 ng nanog-like morpholino (nanog-like-MO1). nanog-like-MO1-injected embryos were injected with either 100 pg GFP mRNA or 100 pg mxtx2 mRNA and 25 pg GFP mRNA at the four-cell stage. About one third of injected embryos had GFP expression in the YSL and were collected for further analysis. gata6 expression was rescued in 46% (13/28) of mxtx2-injected embryos; camsap1l1 expression was rescued in 63% (12/19) of mxtx2-injected embryos; hnf4a expression was rescued in 36% (8/22) of the mxtx2-injected embryos; and slc26a expression was rescued in 52% (14/27) of mxtx2-injected embryos. mxtx2 expression also rescued the yolk burst phenotype in the nanog-like morphant. At the 80% epiboly stage, 96% (65/68) GFP-injected nanog-like morphants showed the yolk burst phenotype, and 45% (35/78) nanog-like morphants having mxtx2 and GFP expression in the YSL showed normal epiboly progress. At 24 hpf, 99% (67/68) of GFP-injected nanog-like morphants were dead, and 17% (13/78) of nanog-like morphants having mxtx2 and GFP expression in the YSL showed axes rescue with defective head formation and body elongation.
(B) mxtx2 expression rescued the endoderm defect in the nanog-like morphant. Lateral view of embryos with dorsal side to the right. Ventrolateral ndr1 expression was rescued in 56% (18/32) of the embryos; ndr2 expression was rescued in 45% (16/35) of the embryos; mixer expression was rescued in 74% (23/31) of the embryos; gata5 expression was rescued in 62% (13/21) of the embryos; and sox32 expression was rescued in 47% (16/34) of the embryos.
(C) Model for endoderm and YSL induction. During early blastula development, the YSL precursor is formed by the collapse of marginal cells into the yolk cell. Acquisition of the YSL fate is mediated by Nanog-like through activation of the YSL master regulator Mxtx2. Stabilized β-catenin activates Nodal signals for endoderm induction in the dorsal region. The YSL and marginal blastomeres produce Nodal signals to induce the ventrolateral endoderm formation.

nanog-like morphants can express ndr1 and ndr2 upon receiving Nodal signals, related to Figure 2

(A) 4-cell stage embryos were injected with 20pg GFP mRNA at the vegetal pole of the yolk cell and imaged at the dome stage. Embryos were classified based on their GFP expression pattern.

(B) 107 GFP mRNA-injected embryos were sorted into three categories based on their GFP expression pattern.

(C) 1-cell stage embryos were injected with 10ng control morpholino (con-MO), or 10ng nanog-like-MO1. To activate the Nodal signaling, 100pg Tar* mRNA was co-injected. Expression of ndr1 and ndr2 was examined by whole mount in situ hybridization.

Lack of YSL transcription in nanog-like morphants, related to Figure 3

(A) The expression of gata6, mxtx2, camsap1l1, hnf4a, and slc26a1 is absent in the YSL of nanog-like morphants. The shield structure is formed in nanog-like morphants, as indicated by the expression of gsc. All views are lateral with the presumptive dorsal side to the right.

(B) The F-actin ring is absent in nanog-like morphants. F-factin was stained with rhodamine phalloidin (RP). Nuclei were stained with DAPI. White brackets indicate the F-actin ring in the YSL.

(C) Injection of mRNA into the yolk of 2.5 hpf embryos enables YSL-specific expression. GFP mRNA was injected into the yolk of a 2.5 hpf embryo. The embryo was imaged at the dome stage. Left: white field; right: GFP channel.

Mxtx2 binds to YSL genes, related to Figure 6

(B) Mxtx2 binding profiles at gpr137bb, mxtx1, otop1, znf503, gata3, cebpa, hnf1ba, and slc40a1 loci. ChIP-Seq data are shown in reads per million with the y-axis floor set to 2 reads per million.

(C) Binding profiles for Nanog-like and Mxtx2 at the nanog-like and snai1a loci. Many loci are co-occupied by both transcription factors.