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Genomic insights into Wnt signaling in an early diverging metazoan, the ctenophore Mnemiopsis leidyi.

Pang K, Ryan JF, NISC Comparative Sequencing ProgramMullikin JC, Baxevanis AD, Martindale MQ - Evodevo (2010)

Bottom Line: In situ hybridization of the four Wnt ligands showed that they are expressed in discrete regions associated with the aboral pole, tentacle apparati and apical organ.Furthermore, it is difficult to compare the Mnemiopsis Wnt expression patterns with those of other metazoans. mRNA expression of Wnt pathway components appears later in development than expected, and zygotic gene expression does not appear to play a role in early axis specification.Notably absent in the Mnemiopsis genome are most major secreted antagonists, which suggests that complex regulation of this secreted signaling pathway probably evolved later in animal evolution.

View Article: PubMed Central - HTML - PubMed

Affiliation: Kewalo Marine Laboratory, Pacific Biosciences Research Center, University of Hawaii at Manoa, Honolulu, HI, USA. mqmartin@hawaii.edu.

ABSTRACT

Background: Intercellular signaling pathways are a fundamental component of the integrating cellular behavior required for the evolution of multicellularity. The genomes of three of the four early branching animal phyla (Cnidaria, Placozoa and Porifera) have been surveyed for key components, but not the fourth (Ctenophora). Genomic data from ctenophores could be particularly relevant, as ctenophores have been proposed to be one of the earliest branching metazoan phyla.

Results: A preliminary assembly of the lobate ctenophore Mnemiopsis leidyi genome generated using next-generation sequencing technologies were searched for components of a developmentally important signaling pathway, the Wnt/β-catenin pathway. Molecular phylogenetic analysis shows four distinct Wnt ligands (MlWnt6, MlWnt9, MlWntA and MlWntX), and most, but not all components of the receptor and intracellular signaling pathway were detected. In situ hybridization of the four Wnt ligands showed that they are expressed in discrete regions associated with the aboral pole, tentacle apparati and apical organ.

Conclusions: Ctenophores show a minimal (but not obviously simple) complement of Wnt signaling components. Furthermore, it is difficult to compare the Mnemiopsis Wnt expression patterns with those of other metazoans. mRNA expression of Wnt pathway components appears later in development than expected, and zygotic gene expression does not appear to play a role in early axis specification. Notably absent in the Mnemiopsis genome are most major secreted antagonists, which suggests that complex regulation of this secreted signaling pathway probably evolved later in animal evolution.

No MeSH data available.


Expression of Wnt pathway components. Whole-mount in situ hybridization of other members of the Wnt pathway, including (A) MlFzdA, (B) MlFzdB, (C) MlSfrp, (D) MlDsh, (E) MlBcat and (F) MlTcf. The timeline above the images denotes the different stages of embryos below, from 0-3 hours post-fertilization (if applicable) to 24 HPF or the cydippid stage. Unless noted, all images are lateral views, with the asterisk marking the blastopore or mouth. Blue/purple staining shows where the genes are expressed. (A) MlFzdA is detected uniformly from egg, through early cleavage stages and gastrulation. From 9 HPF onward, it is expressed mainly in the tentacle bulb (arrows) and pharynx (ph). (B) MlFzdB is not detected until 3-4 HPF in cells of the ectoderm (ec). At 5-6 HPF, it is expressed in the tentacle bulb and around the blastopore, in cells that will invaginate to form the pharynx. Later, it is additionally expressed in the trans-tentacular muscle (white arrow), which connects the two tentacles. (C) MlSfrp is expressed in the invaginating pharynx and in the presumptive mesoderm (mes). This mesodermal expression becomes confined to two regions of the tentacle bulb, which becomes barely detectable in the cydippids. The pharyngeal expression is also not detected in cydippid stages. (D) MlDsh is expressed uniformly from egg to cydippid stages. (E) MlBcat is first detected after gastrulation (about 4 HPF) in ectodermal cells around the blastopore. This blastoporal expression continues however, at 6 HPF there is MlBcat expression everywhere, except in the cells that form the comb plates (arrowheads). (F) MlTcf is expressed primarily in the ectoderm after gastrulation but excluded from cells that form the comb plates. At cydippid stages, it is expressed in discrete regions of the apical organ floor (ao) and in the tentacle bulbs.
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Figure 7: Expression of Wnt pathway components. Whole-mount in situ hybridization of other members of the Wnt pathway, including (A) MlFzdA, (B) MlFzdB, (C) MlSfrp, (D) MlDsh, (E) MlBcat and (F) MlTcf. The timeline above the images denotes the different stages of embryos below, from 0-3 hours post-fertilization (if applicable) to 24 HPF or the cydippid stage. Unless noted, all images are lateral views, with the asterisk marking the blastopore or mouth. Blue/purple staining shows where the genes are expressed. (A) MlFzdA is detected uniformly from egg, through early cleavage stages and gastrulation. From 9 HPF onward, it is expressed mainly in the tentacle bulb (arrows) and pharynx (ph). (B) MlFzdB is not detected until 3-4 HPF in cells of the ectoderm (ec). At 5-6 HPF, it is expressed in the tentacle bulb and around the blastopore, in cells that will invaginate to form the pharynx. Later, it is additionally expressed in the trans-tentacular muscle (white arrow), which connects the two tentacles. (C) MlSfrp is expressed in the invaginating pharynx and in the presumptive mesoderm (mes). This mesodermal expression becomes confined to two regions of the tentacle bulb, which becomes barely detectable in the cydippids. The pharyngeal expression is also not detected in cydippid stages. (D) MlDsh is expressed uniformly from egg to cydippid stages. (E) MlBcat is first detected after gastrulation (about 4 HPF) in ectodermal cells around the blastopore. This blastoporal expression continues however, at 6 HPF there is MlBcat expression everywhere, except in the cells that form the comb plates (arrowheads). (F) MlTcf is expressed primarily in the ectoderm after gastrulation but excluded from cells that form the comb plates. At cydippid stages, it is expressed in discrete regions of the apical organ floor (ao) and in the tentacle bulbs.

Mentions: To understand to which cells these Wnt ligands are signaling, we also looked at the expression patterns of the Frizzled-related genes and other intracellular components (Figure 7). MlFzdA is expressed maternally, in cleavage stages, and through gastrulation in a uniform manner (Figure 7A). After gastrulation and through cydippid formation, expression becomes concentrated primarily in the pharynx, tentacle bulb, and two ectodermal domains between the comb rows in the sagittal plane. MlFzdB, which lacks the intracellular motif, is initially expressed after gastrulation in the ectoderm (Figure 7B). However, later in development, most of the ectodermal expression is downregulated (except in the pharynx), and there is an additional expression domain in the muscle cells that connects the two tentacle apparati. The Secreted Frizzled-related gene, MlSfrp, is expressed after gastrulation in the pharynx and also in the mesoderm, which becomes two diffuse regions of the tentacle bulb (Figure 7C). By the cydippid stage, only faint tentacle bulb staining can be detected.


Genomic insights into Wnt signaling in an early diverging metazoan, the ctenophore Mnemiopsis leidyi.

Pang K, Ryan JF, NISC Comparative Sequencing ProgramMullikin JC, Baxevanis AD, Martindale MQ - Evodevo (2010)

Expression of Wnt pathway components. Whole-mount in situ hybridization of other members of the Wnt pathway, including (A) MlFzdA, (B) MlFzdB, (C) MlSfrp, (D) MlDsh, (E) MlBcat and (F) MlTcf. The timeline above the images denotes the different stages of embryos below, from 0-3 hours post-fertilization (if applicable) to 24 HPF or the cydippid stage. Unless noted, all images are lateral views, with the asterisk marking the blastopore or mouth. Blue/purple staining shows where the genes are expressed. (A) MlFzdA is detected uniformly from egg, through early cleavage stages and gastrulation. From 9 HPF onward, it is expressed mainly in the tentacle bulb (arrows) and pharynx (ph). (B) MlFzdB is not detected until 3-4 HPF in cells of the ectoderm (ec). At 5-6 HPF, it is expressed in the tentacle bulb and around the blastopore, in cells that will invaginate to form the pharynx. Later, it is additionally expressed in the trans-tentacular muscle (white arrow), which connects the two tentacles. (C) MlSfrp is expressed in the invaginating pharynx and in the presumptive mesoderm (mes). This mesodermal expression becomes confined to two regions of the tentacle bulb, which becomes barely detectable in the cydippids. The pharyngeal expression is also not detected in cydippid stages. (D) MlDsh is expressed uniformly from egg to cydippid stages. (E) MlBcat is first detected after gastrulation (about 4 HPF) in ectodermal cells around the blastopore. This blastoporal expression continues however, at 6 HPF there is MlBcat expression everywhere, except in the cells that form the comb plates (arrowheads). (F) MlTcf is expressed primarily in the ectoderm after gastrulation but excluded from cells that form the comb plates. At cydippid stages, it is expressed in discrete regions of the apical organ floor (ao) and in the tentacle bulbs.
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Related In: Results  -  Collection

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Figure 7: Expression of Wnt pathway components. Whole-mount in situ hybridization of other members of the Wnt pathway, including (A) MlFzdA, (B) MlFzdB, (C) MlSfrp, (D) MlDsh, (E) MlBcat and (F) MlTcf. The timeline above the images denotes the different stages of embryos below, from 0-3 hours post-fertilization (if applicable) to 24 HPF or the cydippid stage. Unless noted, all images are lateral views, with the asterisk marking the blastopore or mouth. Blue/purple staining shows where the genes are expressed. (A) MlFzdA is detected uniformly from egg, through early cleavage stages and gastrulation. From 9 HPF onward, it is expressed mainly in the tentacle bulb (arrows) and pharynx (ph). (B) MlFzdB is not detected until 3-4 HPF in cells of the ectoderm (ec). At 5-6 HPF, it is expressed in the tentacle bulb and around the blastopore, in cells that will invaginate to form the pharynx. Later, it is additionally expressed in the trans-tentacular muscle (white arrow), which connects the two tentacles. (C) MlSfrp is expressed in the invaginating pharynx and in the presumptive mesoderm (mes). This mesodermal expression becomes confined to two regions of the tentacle bulb, which becomes barely detectable in the cydippids. The pharyngeal expression is also not detected in cydippid stages. (D) MlDsh is expressed uniformly from egg to cydippid stages. (E) MlBcat is first detected after gastrulation (about 4 HPF) in ectodermal cells around the blastopore. This blastoporal expression continues however, at 6 HPF there is MlBcat expression everywhere, except in the cells that form the comb plates (arrowheads). (F) MlTcf is expressed primarily in the ectoderm after gastrulation but excluded from cells that form the comb plates. At cydippid stages, it is expressed in discrete regions of the apical organ floor (ao) and in the tentacle bulbs.
Mentions: To understand to which cells these Wnt ligands are signaling, we also looked at the expression patterns of the Frizzled-related genes and other intracellular components (Figure 7). MlFzdA is expressed maternally, in cleavage stages, and through gastrulation in a uniform manner (Figure 7A). After gastrulation and through cydippid formation, expression becomes concentrated primarily in the pharynx, tentacle bulb, and two ectodermal domains between the comb rows in the sagittal plane. MlFzdB, which lacks the intracellular motif, is initially expressed after gastrulation in the ectoderm (Figure 7B). However, later in development, most of the ectodermal expression is downregulated (except in the pharynx), and there is an additional expression domain in the muscle cells that connects the two tentacle apparati. The Secreted Frizzled-related gene, MlSfrp, is expressed after gastrulation in the pharynx and also in the mesoderm, which becomes two diffuse regions of the tentacle bulb (Figure 7C). By the cydippid stage, only faint tentacle bulb staining can be detected.

Bottom Line: In situ hybridization of the four Wnt ligands showed that they are expressed in discrete regions associated with the aboral pole, tentacle apparati and apical organ.Furthermore, it is difficult to compare the Mnemiopsis Wnt expression patterns with those of other metazoans. mRNA expression of Wnt pathway components appears later in development than expected, and zygotic gene expression does not appear to play a role in early axis specification.Notably absent in the Mnemiopsis genome are most major secreted antagonists, which suggests that complex regulation of this secreted signaling pathway probably evolved later in animal evolution.

View Article: PubMed Central - HTML - PubMed

Affiliation: Kewalo Marine Laboratory, Pacific Biosciences Research Center, University of Hawaii at Manoa, Honolulu, HI, USA. mqmartin@hawaii.edu.

ABSTRACT

Background: Intercellular signaling pathways are a fundamental component of the integrating cellular behavior required for the evolution of multicellularity. The genomes of three of the four early branching animal phyla (Cnidaria, Placozoa and Porifera) have been surveyed for key components, but not the fourth (Ctenophora). Genomic data from ctenophores could be particularly relevant, as ctenophores have been proposed to be one of the earliest branching metazoan phyla.

Results: A preliminary assembly of the lobate ctenophore Mnemiopsis leidyi genome generated using next-generation sequencing technologies were searched for components of a developmentally important signaling pathway, the Wnt/β-catenin pathway. Molecular phylogenetic analysis shows four distinct Wnt ligands (MlWnt6, MlWnt9, MlWntA and MlWntX), and most, but not all components of the receptor and intracellular signaling pathway were detected. In situ hybridization of the four Wnt ligands showed that they are expressed in discrete regions associated with the aboral pole, tentacle apparati and apical organ.

Conclusions: Ctenophores show a minimal (but not obviously simple) complement of Wnt signaling components. Furthermore, it is difficult to compare the Mnemiopsis Wnt expression patterns with those of other metazoans. mRNA expression of Wnt pathway components appears later in development than expected, and zygotic gene expression does not appear to play a role in early axis specification. Notably absent in the Mnemiopsis genome are most major secreted antagonists, which suggests that complex regulation of this secreted signaling pathway probably evolved later in animal evolution.

No MeSH data available.