Foregut Formation of the Nemerteans and Its Role in Nemertean Systematics

Iwata, Fumio

doi:10.1093/icb/25.1.23

Cited by 44 publications

(14 citation statements)

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“…Through proliferation and cell rearrangement, the embryo begins to lengthen along the anterior-posterior axis. Coincidently, the vegetal pole and future stomatoblasts become relocated anteriorly along the ventral side and the animal-vegetal axis becomes curved by approximately 90 degrees (Iwata, 1985;van den Biggelaar and Dictus, 2004 Fig. 7).…”

Section: Axial Elongation and Anterior Displacement Of The Mouthmentioning

confidence: 99%

“…In species that gastrulate by invagination, the primary quartet micromeres are typically roughly equal to, or in some cases larger than, the macromeres (Pilger, 1997). Following cleavage stages, a coeloblastula (hollow cluster of cells with a central fluid-filled lumen) is often formed (Iwata, 1985;Henry and Martindale, 1997;Pilger, 1997), and these species tend to be small and exhibit indirect development, and make planktotrophic larvae.The details of invagination have been examined in only a few species. Prior to invagination in Owenia, the vegetal cells of the blastula become columnar and the vegetal plate flattens.…”

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confidence: 99%

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Ins and outs of Spiralian gastrulation

Lyons¹,

Henry²

2014

Int. J. Dev. Biol.

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Gastrulation is a critical stage of metazoan development during which endodermal and mesodermal tissues are internalized, and morphogenesis transforms the early embryo into each animal's unique body-plan. While gastrulation has been studied extensively in classic model systems such as flies, worms, and vertebrates, less is known about gastrulation at a mechanistic level in other taxa. Surprisingly, one particularly neglected group constitutes a major branch of animals: the Spiralia. A unique feature of spiralian development is that taxa with diverse adult body-plans, such as annelids, molluscs, nemerteans and platyhelminths all share a highly stereotyped suite of characters during embryogenesis called spiral cleavage. The spiral cleavage program makes it possible to compare distantly related embryos using not only morphological features, and gene expression patterns, but also homologous cell lineages. Having all three criteria available for comparison is especially critical for understanding the evolution of a complex process like gastrulation. Thus studying gastrulation in spiralians is likely to lead to novel insights about the evolution of body-plans, and the evolution of morphogenesis itself. Here we review relevant literature about gastrulation in spiralians and frame questions for future studies. We describe the internalization of the endoderm, endomesoderm and ectomesoderm; where known, we review data on the cellular and molecular control of those processes. We also discuss several morphogenetic events that are tied to gastrulation including: axial elongation, origins of the mouth and anus, and the fate of the blastopore. Since spiral cleavage is ancestral for a major branch of bilaterians, understanding gastrulation in spiralians will contribute more broadly to ongoing debates about animal body-plan divergence, such as: the origin of the through-gut, the emergence of indirect versus direct development, and the evolution of gene-regulatory networks that specify endomesoderm. We emphasize the fact that spiralian gastrulation provides the unique opportunity to connect well-defined embryonic cell lineages to variation in cell fate and cell behavior, making it an exceptional case study for evo-devo. KEY WORDS: spiralia, endomesoderm, ectomesoderm, blastopore, axial elongation, epiboly, invagination Gastrulation is a critical embryonic event during which presumptive endodermal and mesodermal cells are internalized (Stern, 2004). Gastrulation is closely tied to the development of key axial properties, and to the patterning of certain organ systems, such as the digestive tract; the openings of the mouth and/or anus often arise at or close to the site of gastrulation, called the blastopore (see Technau and Scholz, 2003;Hejnol and Martindale, 2009). Gastrulation also holds a pivotal role in evolutionary theories about the emergence and divergence of bilaterian body-plans (Arendt and Nübler-Jung,1997;Nielsen, 2001;Martindale and Hejnol, 2009 As evo-devo questions go, tracing the evolutionary history o...

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Section: Axial Elongation and Anterior Displacement Of The Mouthmentioning

confidence: 99%

mentioning

confidence: 99%

Ins and outs of Spiralian gastrulation

Lyons¹,

Henry²

2014

Int. J. Dev. Biol.

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“…Furthermore, the axis of the larva (and adult) relative to the polarity of the egg may change as a function of foregut morphogenesis in the embryo (25). During foregut formation the mouth moves to various degrees from the vegetal pole to the ventral side of the body, e.g., in Lineus torquatus the stomodaeum moves toward the opposite side of the ptilidium larva (25). This suggests that nemerteans may be in an evolutionary transition zone between proto-and deuterostomes, a point that is worth reexamination.…”

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confidence: 99%

Homeobox genes in the ribbonwormLineus sanguineus: Evolutionary implications

Kmita-Cunisse

Loosli²,

Bièrne

et al. 1998

Proc. Natl. Acad. Sci. U.S.A.

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From our current understanding of the genetic basis of development and pattern formation in Drosophila and vertebrates it is commonly thought that clusters of Hox genes sculpt the morphology of animals in specific body regions. Based on Hox gene conservation throughout the animal kingdom it is proposed that these genes and their role in pattern formation evolved early during the evolution of metazoans. Knowledge of the history of Hox genes will lead to a better understanding of the role of Hox genes in the evolution of animal body plans. To infer Hox gene evolution, reliable data on lower chordates and invertebrates are crucial. Among the lower triploblasts, the body plan of the ribbonworm Lineus (nemertini) appears to be close to the common ancestral condition of protostomes and deuterostomes. In this paper we present the isolation and identification of Hox genes in Lineus sanguineus. We find that the Lineus genome contains a single cluster of at least six Hox genes: two anterior-class genes, three middle-class genes, and one posterior-class gene. Each of the genes can be definitely assigned to an ortholog group on the basis of its homeobox and its f lanking sequences. The most closely related homeodomain sequences are invariably found among the mouse or Amphioxus orthologs, rather than Drosophila and other invertebrates. This suggests that the ribbonworms have diverged relatively little from the last common ancestors of protostomes and deuterostomes, the urbilateria.

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“…In C. ochracea the development from the gastrula to the larva is similar to that described in other palaeonemertean species and includes the translocation of the mouth opening to the ventral side of the body [17,19,28]. Likewise, an apical tuft of elongated cilia housed in a shallow epidermal depression is present after the first day of development [30].…”

Section: Comparative Aspects Of Larvae In Paleonemerteamentioning

confidence: 91%

First record on the development of the larva of the basally branching nemertean species Carinina ochracea (Palaeonemertea)

Döhren

2016

Helgol Mar Res

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Nemertea are a clade of unsegmented, vermiform, mostly marine Spiralia. While it is generally accepted that Nemertea develop via a pelagic larval stage there is considerable uncertainty about its characteristic, ancestral features. The uncertainty is mainly due to highly disparate larval types in the derived clade Neonemertea and the scarce information on the larval types of Palaeonemertea, the basal-most branching clade of Nemertea. To remedy this shortage of data the early larval development of a member of the palaeonemertean genus Carinina, Carinina ochracea, is described for the first time. Congruently to what has been described in other palaeonemertean species the larva of C. ochracea is uniformly ciliated along with an apical tuft of elongated cilia housed in an epidermal depression, the apical pit. A pair of frontal epidermal invaginations present bilaterally of the apical pit has been shown in other palaeonemertean larvae as well. Following gastrulation the blastopore is shifted to the ventral side of the body to become the mouth opening. A pair of simple unbranched nephridia that is situated on both sides of the mouth opening completes the list of structures documented for palaeonemertean larvae. Formation of the anal opening was not observed during this study. The presence of a single pigmented ventral eye was hitherto reported from the larva of Carinoma species only. The presented results complement the record of larval types found in Palaeonemertea and will serve as a reference for subsequent studies on the development of internal structures in palaeonemertean species.

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Foregut Formation of the Nemerteans and Its Role in Nemertean Systematics

Cited by 44 publications

References 0 publications

Ins and outs of Spiralian gastrulation

Ins and outs of Spiralian gastrulation

Homeobox genes in the ribbonwormLineus sanguineus: Evolutionary implications

First record on the development of the larva of the basally branching nemertean species Carinina ochracea (Palaeonemertea)

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