ECB-ART-41845BMC Biol 2010 Nov 30;8:143. doi: 10.1186/1741-7007-8-143.
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Uncoupling of complex regulatory patterning during evolution of larval development in echinoderms.
BACKGROUND: Conservation of orthologous regulatory gene expression domains, especially along the neuroectodermal anterior-posterior axis, in animals as disparate as flies and vertebrates suggests that common patterning mechanisms have been conserved since the base of Bilateria. The homology of axial patterning is far less clear for the many marine animals that undergo a radical transformation in body plan during metamorphosis. The embryos of these animals are microscopic, feeding within the plankton until they metamorphose into their adult forms. RESULTS: We describe here the localization of 14 transcription factors within the ectoderm during early embryogenesis in Patiria miniata, a sea star with an indirectly developing planktonic bipinnaria larva. We find that the animal-vegetal axis of this very simple embryo is surprisingly well patterned. Furthermore, the patterning that we observe throughout the ectoderm generally corresponds to that of "head/anterior brain" patterning known for hemichordates and vertebrates, which share a common ancestor with the sea star. While we suggest here that aspects of head/anterior brain patterning are generally conserved, we show that another suite of genes involved in retinal determination is absent from the ectoderm of these echinoderms and instead operates within the mesoderm. CONCLUSIONS: Our findings therefore extend, for the first time, evidence of a conserved axial pattering to echinoderm embryos exhibiting maximal indirect development. The dissociation of head/anterior brain patterning from "retinal specification" in echinoderm blastulae might reflect modular changes to a developmental gene regulatory network within the ectoderm that facilitates the evolution of these microscopic larvae.
PubMed ID: 21118544
PMC ID: PMC3002323
Article link: BMC Biol
Species referenced: Echinodermata
Genes referenced: foxd4 foxe3l foxg1 foxj1 gata6 klf13 lhx9l LOC100887844 LOC100893907 Nk1 nkx1 nkx2-1 onecut2 pax6 pole six6 tbr1 unpgl zic1
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|Figure 1. Comparison of orthologous neuroectodermal gene expression domains among the deuterostomes. (A-E) Indirectly developing echinoderms. (F) Directly developing hemichordate. (G) Generalized vertebrate. Sea stars (Figures 1A-1C) and sea urchins (Figures 1D and 1E) are viewed laterally; animal pole is up and oral side is right. Figures 1F and 1G are dorsal views; anterior is up. Genes are listed beside their cognate expression domains. Vertical bars in Figures 1A, 1B, 1F and 1G approximate domain boundaries. The orange to yellow gradient in Figures 1A, 1F and 1G reflects a general conservation of anterior (animal)-most axial patterning among the three phyla. (A) Nested, concentric expression domains pattern the animal-vegetal (AV) axis of blastulae; asterisks denote previously reported expression [18,19]. (B) Concentric domains of zic, foxq2, rx and six3 persist in gastrulae (orange to peach gradient); additional oral (for example, foxg, foxd and gbx; light orange) and aboral (for example, lhx2; purple) domains are evident. Genes (left) are broadly expressed. (C) Expression in larval animal pole domain (orange to peach) and/or ciliary bands (gold). (D) Sea urchin animal pole (orange and light orange), ciliary band (gold), aboral ectoderm (turquoise) and oral ectoderm (foxg; gray) are molecularly distinct territories in blastulae. (E) Expression is maintained in gastrulae animal pole (orange) and ciliary band (gold). (D and E) Pink circles represent skeletogenic mesoderm. See references [20,22,23,41-49]. (F) Orthologs expressed in hemichordate anterior, middle and posterior body segments show corresponding expression in the vertebrate forebrain, midbrain and hindbrain, respectively; data are summarized from Lowe et al. . (G) Expression in generalized vertebrate centralized nervous system. F, forebrain; M, midbrain; R1-R8, rhombomeres of hindbrain. zic ; pea3 ; hnf-6/onecut ; and tbr ; foxj1 ; hox genes . See references  and [26-32]. Echinoderm gene names (quotations) are substituted for simplicity in Figures 1F and 1G.|
|Figure 2. Nested concentric expression domains pattern the axial ectoderm of sea star, P. miniata, blastulae. Embryos are oriented with the animal pole up. (A-G) Whole mount in situ hybridization (WMISH). (A) zic, (B) foxq2, (C) rx, and (D) nk2.1 expression is restricted to the animal-most ectoderm. Transcripts of (E) six3 and (F) klf13 are detected in the ectoderm and in the vegetal plate endomesoderm. Arrows in (E) and (F) point to a clearing above the vegetal pole where no or few transcripts are detected. (G) nk1 transcripts are localized to a ring above the vegetal pole. (H) The boundary between the vegetal-most ectoderm (nk1, red) and the endoderm (gatae, green) as visualized by fluorescence in situ hybridization (FISH). Colocalization is in yellow. (I and J) WMISH. Transcripts of (I) foxj1 and (J) pea3 are detected throughout the entire ectoderm. pea3 is weakly detected in the vegetal plate endomesoderm. Arrows in Figure 1J point to the limits of foxj1 expression. (K) Schematic shows the patterns described above as five nested domains of expression along the AV axis. For simplicity, nk2.1 is grouped here with the concentric domains of foxq2 and rx expression. Gene names are listed next to their cognate expression domains. Vertical bars approximate the expression boundaries of associated genes. The color gradient spans the animal (orange) to vegetal (yellow) limits of the ectoderm.|
|Figure 3. Heterogeneous regulatory patterning of the larval ciliary bands as visualized by WMISH. (A) Schematic describes the position of the two larval ciliary bands (red) from oral (left) and lateral (right) views. A, anus; CB, ciliary band; M, mouth. (B-F) WMISH. Expression of (B and C) foxj1 and (D and E) klf13 is initially broad throughout (B and D) the ectoderm of gastrulae, then later is restricted to (C and E) the larval ciliary bands. Arrows in Figure 3B show the vegetal limits of foxj1 expression. Arrows in Figure 3D point to a clearing above the vegetal pole where transcripts of klf13 were detected. klf13 transcripts are additionally detected in an ectodermal territory near the mouth (arrows in Figure 3E). (F) foxg is first expressed within two ectodermal domains on the oral side of gastrulae. (G) FISH of nk2.1 (green) and ciliary band marker foxg (red) highlights nk2.1 expression in only the transverse preoral ciliary band. Colocalization is shown in yellow. (H-M) WMISH. foxd is expressed within a single domain in (H) the oral side ectoderm of gastrulae and (I) in the transverse, preoral larval ciliary band. gbx is expressed in one domain in (J) the oral side ectoderm in gastrulae and in (K) the transverse postoral larval ciliary band. (L) nk1 is expressed in the transverse postoral ciliary band in the larva. (M) A two-probe WMISH shows lhx2 expression in a spotted pattern in the aboral ectoderm (arrows, left) opposite of foxg expression (arrowheads, right). Embryos are oriented with the animal pole up and laterally, except in Figures 3E, 3G, 3I and 3K, which are oral views. In lateral views, the oral side is to the right.|
|Figure 4. Gene expression molecularly defines the animal pole domain in the sea star. Embryos are shown laterally, with the animal pole up and oral side to the right. (A-F) WMISH. Expression of (A) foxq2, (B) pax6, (C) pea3, (D) zic, (E) rx and (F) six3 within the apical-most ectoderm defines the animal pole domain within late gastrulae (Figures 4A, 4D and 4F) and early larvae (Figures 4B, 4C and 4E). The vegetal limits of this domain are variable (see dotted lines in Figures 4E and 4F). Transcripts of pea3 additionally localize within the ectoderm of the larval ciliary bands (arrows in Figure 4C). pax6 expression in mesodermally derived coelom (arrow in Figure 4B). (G) FISH demonstrates that the ciliary bands, as marked by foxg (red), run through the ectoderm of the animal pole domain, as marked by pax6 (green). Colocalization is shown in yellow.|
|Figure 5. Retinal determination orthologs are expressed within sea urchin, (Strongylocentrotus purpuratus, Sp), and sea star mesoderm. (A-N) WMISH. (A and B) SpEya, (C and D) SpPax6 and (E and F) SpSix1.2 are expressed at the tip of the archenteron in gastrulae (Figures 5A, 5C and 5E) and in a mesodermally derived coelom in early pluteus larvae (Figures 5B, 5D and 5F). PmEya (Figures 5G-5I) and PmPax6 (Figures 5J-5L) expression is first detected in the mesoderm of the archenteron bulb in midgastrulae (Figures 5H and 5K; arrows) and then more prominently in a mesodermally derived coelom in late gastrulae (Figures 5I and 5L; arrow). PmPax6 transcripts are also found broadly throughout the ectoderm (Figure 5K), with more pronounced expression in the apical ectoderm (Figure 5L). SpOpsin1 (Figure 5M) and SpOpsin4 (Figure 5N) are expressed in 1-week-old larvae. (O-R) FISH in 1-week-old larvae. SpEya (Figure 5O) is shown relative to the skeletal marker SpSm50, which was used to orient the embryo. Figures 5P-5R show transcripts of SpOpsin1 (green) colocalizing with those of SpEya (red). Colocalization is shown in yellow. Arrows in Figures 5P, 5Q and 5R point to expression. Additional areas of expression within the 1-week-old larvae may be a result of nonspecific staining.|
References [+] :
Aamar, Isolation and expression analysis of foxj1 and foxj1.2 in zebrafish embryos. 2009, Pubmed