Boyle MJ et al. (2014), Molecular conservation of metazoan gut formatio...

ECB-ART-43938

Evodevo 2014 Jun 17;5:39. doi: 10.1186/2041-9139-5-39.

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Molecular conservation of metazoan gut formation: evidence from expression of endomesoderm genes in Capitella teleta (Annelida).

Boyle MJ , Yamaguchi E , Seaver EC .

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BACKGROUND: Metazoan digestive systems develop from derivatives of ectoderm, endoderm and mesoderm, and vary in the relative contribution of each germ layer across taxa and between gut regions. In a small number of well-studied model systems, gene regulatory networks specify endoderm and mesoderm of the gut within a bipotential germ layer precursor, the endomesoderm. Few studies have examined expression of endomesoderm genes outside of those models, and thus, it is unknown whether molecular specification of gut formation is broadly conserved. In this study, we utilize a sequenced genome and comprehensive fate map to correlate the expression patterns of six transcription factors with embryonic germ layers and gut subregions during early development in Capitella teleta. RESULTS: The genome of C. teleta contains the five core genes of the sea urchin endomesoderm specification network. Here, we extend a previous study and characterize expression patterns of three network orthologs and three additional genes by in situ hybridization during cleavage and gastrulation stages and during formation of distinct gut subregions. In cleavage stage embryos, Ct-otx, Ct-blimp1, Ct-bra and Ct-nkx2.1a are expressed in all four macromeres, the endoderm precursors. Ct-otx, Ct-blimp1, and Ct-nkx2.1a are also expressed in presumptive endoderm of gastrulae and later during midgut development. Additional gut-specific expression patterns include Ct-otx, Ct-bra, Ct-foxAB and Ct-gsc in oral ectoderm; Ct-otx, Ct-blimp1, Ct-bra and Ct-nkx2.1a in the foregut; and both Ct-bra and Ct-nkx2.1a in the hindgut. CONCLUSIONS: Identification of core sea urchin endomesoderm genes in C. teleta indicates they are present in all three bilaterian superclades. Expression of Ct-otx, Ct-blimp1 and Ct-bra, combined with previously published Ct-foxA and Ct-gataB1 patterns, provide the most comprehensive comparison of these five orthologs from a single species within Spiralia. Each ortholog is likely involved in endoderm specification and midgut development, and several may be essential for establishment of the oral ectoderm, foregut and hindgut, including specification of ectodermal and mesodermal contributions. When the five core genes are compared across the Metazoa, their conserved expression patterns suggest that ''gut gene'' networks evolved to specify distinct digestive system subregions, regardless of species-specific differences in gut architecture or germ layer contributions within each subregion.

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Species referenced: Echinodermata
Genes referenced: foxa1 gscl LOC100887844 LOC100893907 LOC115919910 LOC575170 nkx2-1 otx2 pole prdm1 tbxt

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	Figure 1. Schematics of embryonic and larval development in Capitella teleta . (A) Ten stages of development representing a linear series of events from fertilization through emergence of the stage 9 metatrochophore larva prior to settlement and metamorphosis to a juvenile worm. Stages 1 to 9 each represent approximately 24 hrs at 18 to 19°C. (B) Schematics of selected embryonic and larval stages depicting endoderm development and gut formation. Stage 3 schematics are shown with anterior to the left. Stage 4 and stage 6 larvae are oriented in pairs of ventral and lateral views with anterior to the left. Gray shading represents endodermal precursor cells prior to gastrulation and during gastrulation and as definitive endoderm after gastrulation during larval development. View orientations are shown below each embryo and larva. Abbreviations: ant, anterior; bp, blastopore; br, brain; ec, ectoderm; en, endoderm; fg, foregut; hg, hindgut; mo, mouth; pos, posterior; St, stage; vcf, vegetal cross-furrow; vnc, ventral nerve cord; yo, yolk; 1 m, 1st quartet micromere; 1 M, 1st quartet macromere; 3 m, 3rd quartet micromeres; 3 M, 3rd quartet macromeres. Dotted line in stage 3 gastrula indicates the position of the blastopore. Thin solid lines internal to the contour of the body in stages 3, 4 and 6 indicate the inner boundary of regions of ectoderm distinguished by low yolk content.
	Figure 2. Expression of Otx, Blimp1, Bra, Nkx2.1a, FoxAB and Gsc during embryogenesis in Capitella teleta . Each row shows expression (blue color) for the single gene listed at the left margin. The approximate stage of development for the embryos in each column is listed at the top margin. The orientation of each embryo is listed at the bottom right corner of every panel (anm, animal view; veg, vegetal view; vent, ventral view). In embryos with approximately 28 to 33 cells, the D quadrant is toward the bottom of each panel. Yellow dotted lines indicate the position of the blastopore. M is the animal view of L; R is the animal view of Q. Anterior is to the left in all Stage 3 embryos. (A-E) Ct-otx is expressed in each macromere (black arrows) and micromere (white arrows) of 8-cell embryos (A), in micromeres (black arrows) encircling the animal pole (B-D), in both anterior (black arrowheads) and posterior (white arrowheads) ectoderm and within and around the blastopore (white arrows) of gastrulae (E). (F-J) Ct-blimp1 is not detected in macromeres (dashed black arrow) or micromeres (dashed white arrow) of 8-cell embryos (F); Ct-blimp1 is expressed in macromeres (black arrow) and 4q micromeres (white arrow) of 28- to 33-cell embryos (G), in vegetal cells (white arrows) of stage 2 and stage 3 embryos (H-I), and in the endoderm (J). (K-O) Ct-bra is expressed in each macromere (black arrows) and micromere (white arrows) of 8-cell embryos (K), in macromeres (black arrows) and micromeres (white arrows) in each quadrant of 28- to 33-cell embryos (L-M), anterior to the blastopore (short white arrows) and around the blastopore (dashed black arrows) of gastrulae (N), and on the ventral anterior surface of late gastrulae (O). (P-T) Ct-nkx2.1a is expressed in each macromere (black arrows) and micromere (white arrows) of 8-cell embryos (P), in macromeres (black arrows) and micromeres (white arrows) of 28- to 33-cell embryos (Q-R), along the left and right sides of the blastopore (S), and in both anterior ectoderm (black arrowheads) and endoderm (white arrows) of late gastrulae (T). (U-Y) Ct-foxAB is not detected in macromeres (dashed black arrow) or micromeres (dashed white arrow) of 8-cell embryos (U); Ct-foxAB is expressed in a single D-quadrant cell (white arrow) at the 28-cell stage (V), in each quadrant (white arrows) on the animal hemisphere of stage 2 (W), outside the blastopore (white arrows) in vegetal micromeres (X), and in anterior surface cells (white arrows) encircling the site of stomodeum formation (Y). (A’-E’) Ct-gsc is not detected in macromeres (dashed black arrow) or micromeres (dashed white arrow) of 8-cell embryos (A’); Ct-gsc is expressed in two animal micromeres (white arrows) within embryos of approximately 28 cells (B’), in each quadrant (white arrows) on the animal hemisphere of stage 2 (C’), in several micromeres (white arrows) anterior to the blastopore (D’) and in micromeres (white arrows) on the anterior surface of late gastrulae (E’). bp, blastopore; St, stage. The image in each panel was created by combining micrographs from a series of focal planes.
	Figure 3. Expression of Otx, Blimp1, Bra, Nkx2.1a, FoxAB and Gsc during larval development in Capitella teleta . Each row shows expression (blue color) for the single gene listed at the left margin. The stage of development for the larvae in each column is listed at the top margin. The orientation of each larva is listed at the bottom right corner of every panel (vent, ventral view; lat, lateral view). For each row, columns 1 and 2 are different views of the same stage 4 larva; columns 3 and 4 are different views of the same stage 6 larva. Anterior is to the left in all panels; ventral is down for each lateral view. A black or white asterisk marks the position of the mouth. (A-B) Ct-otx is expressed in anterior ectoderm, including the brain (white arrowheads), in the stomodeum (white arrows) endoderm (black arrows) and hindgut primordia (black arrowheads) at stage 4. (C-D) Ct-otx is expressed in the brain (white arrowheads) foregut (white arrows) posterior segments (dashed black arrows) and surface cells on the ventral midline (dashed white arrow) at stage 6. (E-H) Ct-blimp1 is expressed in endoderm (black arrow) at stage 4, and in the brain (white arrowheads) foregut (white arrows) most segments of the trunk (black arrows) and the posterior growth zone (dashed black arrows) at stage 6. (I-L) Ct-bra is expressed on the posterior side of the foregut (white arrows) and in ectoderm at the posterior-most end of the body (black arrowheads) of stage 4 larvae, and in the brain (white arrowheads) foregut (white arrows) and anus (black arrowheads) at stage 6. (M-P) Ct-nkx2.1a is expressed in the brain (white arrowheads) foregut (white arrows) endoderm (black arrow) and hindgut (black arrowheads) in stage 4 and stage 6 larvae. (Q-T) Ct-foxAB is expressed primarily in surface cells around the mouth (white arrows) and also lateral to the mouth (black arrows, S) at stage 4 and stage 6. (U-V) Ct-gsc is expressed in surface cells around the mouth (white arrows) at stage 4. (W-X) Ct-gsc is expressed in ectoderm cells anterior to the brain (white arrowheads) and in several anterior cells flanking each side of the brain and foregut (white arrows) in stage 6 larvae. The image in each panel was created by combining micrographs from a series of focal planes.
	Figure 4. Comparative summary of gene expression in cell and organ territories during embryonic and larval development in Capitella teleta. (A) Schematic illustrations of early embryonic stages and a table comparing the presence or absence of detectable gene expression in selected blastomere and germ layer regions. (B) Schematic illustrations of post-gastrula and larval stages and a table comparing the presence or absence of gene expression in selected tissues and organs associated with gut formation. Schematic stages are oriented in ventral view with anterior to the left. The colored names and ovals identify C. teleta orthologs of the five core genes in the endomesoderm GRN ‘kernel’ of the sea urchin. The five orthologs in (A) and (B) are separated by a dashed line from three additional C. teleta genes characterized in this study. The gray shading represents endoderm. Abbreviations: ant, anterior; bp, blastopore; br, brain; cns, central nervous system; ec, ectoderm; en, endoderm; fg, foregut; hg, hindgut; mg, midgut; mo, mouth; pos, posterior; St, stage; 1 m, 1st quartet micromere; 1 M, 1st quartet macromere; 3 m, 3rd quartet micromeres; 3 M, 3rd quartet macromeres. Dotted line in stage 3 gastrula indicates the position of the blastopore. Thin solid lines internal to the contour of the body in stages 3, 4 and 6 indicate the inner boundary of regions of ectoderm distinguished by low yolk content.
	Figure 5. Comparative gene expression within digestive organ systems of the Metazoa. Gene orthologs of five endomesoderm transcription factors (Otx, Blimp1, Brachyury, FoxA, Gata4/5/6) are designated by colored ovals; additional transcription factors (Nkx2.1, Goosecoid) are designated by black and white ovals, respectively. Each oval represents gene expression by in situ hybridization. For each taxon, gene-specific expression was confirmed in one or more subregions along the digestive organ system [see Additional file9: Table S1]. ‘oral (ectoderm)’ is the future site of mouth formation, or a definitive stomodeum; ‘foregut’ is internal to the mouth opening or described as the foregut, pharynx or esophagus; ‘midgut (endoderm)’ represents endoderm precursor cells in an embryo or gastrula, or is described as the midgut, endoderm or intestine; ‘hindgut’ is the posterior terminus of the gut system, it may include the anus and is described as hindgut, proctodeum or anus. All gene expression patterns were described during a period of development that included gut formation. Where gene-specific expression in a particular gut subregion(s) was detected, it has been indicated in the table. Where no gene expression data are listed for a gene, or gut subregion, then either no expression was detected in that subregion, or expression data do not exist for that particular gene in that taxon. Expression of Nk2.1 in C. teleta is for the Ct-nkx2.1a ortholog. A supplementary table and list of references for all expression data in Figure 5 are available in Additional file9: Table S1.

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