Dorsal and snail homologs in leech development

Ectomesoderm and epithelial-mesenchymal transition-related genes in spiralian development

BACKGROUND

Spiralians (e.g., annelids, molluscs, and flatworms) possess two sources of mesoderm. One is from endodermal precursors (endomesoderm), which is considered to be the ancestral source in metazoans. The second is from ectoderm (ectomesoderm) and may represent a novel cell type in the Spiralia. In the mollusc Crepidula fornicata, ectomesoderm is derived from micromere daughters within the A and B cell quadrants. Their progeny lie along the anterolateral edges of the blastopore. There they undergo epithelial-mesenchymal transition (EMT), become rounded and undergo delamination/ingression. Subsequently, they assume the mesenchymal phenotype, and migrate beneath the surface ectoderm to differentiate various cell types, including muscles and pigment cells.

RESULTS

We examined expression of several genes whose homologs are known to regulate Type 1 EMT in other metazoans. Most of these genes were expressed within spiralian ectomesoderm during EMT.

CONCLUSIONS

We propose that spiralian ectomesoderm, which exhibits analogous cellular behaviors to other populations of mesenchymal cells, may be controlled by the same genes that drive EMT in other metazoans. Perhaps these genes comprise a conserved metazoan EMT gene regulatory network (GRN). This study represents the first step in elucidating the GRN controlling the development of a novel spiralian cell type (ectomesoderm). Developmental Dynamics 247:1097-1120, 2018. © 2018 Wiley Periodicals, Inc.

Deployment of regulatory genes during gastrulation and germ layer specification in a model spiralian mollusc Crepidula

BACKGROUND

During gastrulation, endoderm and mesoderm are specified from a bipotential precursor (endomesoderm) that is argued to be homologous across bilaterians. Spiralians also generate mesoderm from ectodermal precursors (ectomesoderm), which arises near the blastopore. While a conserved gene regulatory network controls specification of endomesoderm in deuterostomes and ecdysozoans, little is known about genes controlling specification or behavior of either source of spiralian mesoderm or the digestive tract.

RESULTS

Using the mollusc Crepidula, we examined conserved regulatory factors and compared their expression to fate maps to score expression in the germ layers, blastopore lip, and digestive tract. Many genes were expressed in both ecto- and endomesoderm, but only five were expressed in ectomesoderm exclusively. The latter may contribute to epithelial-to-mesenchymal transition seen in ectomesoderm.

CONCLUSIONS

We present the first comparison of genes expressed during spiralian gastrulation in the context of high-resolution fate maps. We found variation of genes expressed in the blastopore lip, mouth, and cells that will form the anus. Shared expression of many genes in both mesodermal sources suggests that components of the conserved endomesoderm program were either co-opted for ectomesoderm formation or that ecto- and endomesoderm are derived from a common mesodermal precursor that became subdivided into distinct domains during evolution.

Identification of 3'UTR sequence elements and a teloplasm localization motif sufficient for the localization of Hro-twist mRNA to the zygotic animal and vegetal poles

The early localization of mRNA transcripts is critical in sorting cell fate determinants in the developing embryo. In the glossiphoniid leech, Helobdella robusta, maternal mRNAs, such as Hro-twist, localize to the zygotic teloplasm. Ten seven nucleotide repeat elements (AAUAAUA) called ARE2 and a predicted secondary structural motif, called teloplasm localization motif (TLM), are present in the 3'UTR of Hro-twist mRNA. We used site-directed mutagenesis, deletions, and microinjection of labeled, exogenous transcripts to determine if ARE2 elements, and the TLM, play a role in Hro-twist mRNA localization. Deleting the poly-A tail and the cytoplasmic polyadenylation element (CPE) had no effect on Hro-twist mRNA localization. Site-directed mutagenesis of nucleotides that altered ARE2 element sequences or the TLM suggest that the ARE2 elements and the TLM are important for Hro-twist mRNA localization to the teloplasm of pre-cleavage zygotes. Hro-Twist protein expression data suggest that the localization of Hro-twist transcripts in zygotes and stage two embryos is not involved in ensuring mesoderm specification, as Hro-Twist protein is expressed uniformly in most cells before gastrulation. Our data may support a shared molecular mechanism for leech transcripts that localize to the teloplasm.

Immunostaining Helobdella (leech) embryos

The purpose of this staining protocol is to detect the localization of specific antigens in the Helobdella (leech) embryo. Immunostaining protocols must be optimized for each antibody and embryonic stage. Here, as a starting point, we present a general-purpose immunostaining protocol. Immunostaining of the leech embryo can be performed on the intact embryo or on dissected parts. For post-gastrulation embryos (stages 9 and 10), if the area of interest is in the germinal plate, immunostaining can be performed on the dissected germinal plate. It can also be performed on the ventral nerve cords dissected from late-stage embryos or juveniles.

Germ layer specification and axial patterning in the embryonic development of the freshwater planarian Schmidtea polychroa

Although patterning during regeneration in adult planarians has been studied extensively, very little is known about how the initial planarian body plan arises during embryogenesis. Herein, we analyze the process of embryo patterning in the species Schmidtea polychroa by comparing the expression of genes involved in the establishment of the metazoan body plan. Planarians present a derived ectolecithic spiralian development characterized by dispersed cleavage within a yolk syncytium and an early transient embryo capable of feeding on the maternally supplied yolk cells. During this stage of development, we only found evidence of canonical Wnt pathway, mostly associated with the development of its transient pharynx. At these stages, genes involved in gastrulation (snail) and germ layer determination (foxA and twist) are specifically expressed in migrating blastomeres and those giving rise to the temporary gut and pharyngeal muscle. After yolk ingestion, the embryo expresses core components of the canonical Wnt pathway and the BMP pathway, suggesting that the definitive axial identities are established late. These data support the division of planarian development into two separate morphogenetic stages: a highly divergent gastrulation stage, which segregates the three germ layers and establishes the primary organization of the feeding embryo; and subsequent metamorphosis, based on totipotent blastomeres, which establishes the definitive adult body plan using mechanisms that are similar to those used during regeneration and homeostasis in the adult.

Grandparental stem cells in leech segmentation: differences in CDC42 expression are correlated with an alternating pattern of blast cell fates

Embryonic segmentation in clitellate annelids (oligochaetes and leeches) is a cell lineage-driven process. Embryos of these worms generate a posterior growth zone consisting of 5 bilateral pairs of identified segmentation stem cells (teloblasts), each of which produces a column of segmental founder cells (blast cells). Each blast cell generates a lineage-specific clone via a stereotyped sequence of cell divisions, which are typically unequal both in terms of the relative size of the sister cells and in the progeny to which they give rise. In two of the five teloblast lineages, including the ventralmost, primary neurogenic (N) lineage, the blast cells adopt two different fates, designated nf and ns, in exact alternation within the blast cell column; this is termed a grandparental stem cell lineage. To lay groundwork for investigating unequal divisions in the leech Helobdella, we have surveyed the Helobdella robusta genome for genes encoding orthologs of the Rho family GTPases, including the rho, rac and cdc42 sub-families, which are known to be involved in multiple processes involving cell polarization in other systems. We find that, in contrast to most other known systems the Helobdella genome contains two cdc42 orthologs, one of which is expressed at higher levels in the ns blast cells than in nf blast cells. We also demonstrate that the asymmetric divisions of the primary nf and ns blast cells are regulated by the polarized distribution of the activated form of the Cdc42 protein, rather than by the overall level of expression. Our results provide the first molecular insights into the mechanisms of the grandparental stem cell lineages, a novel, yet evolutionarily ancient stem cell division pattern. Our results also provide an example in which asymmetries in the distribution of Cdc42 activity, rather than in the overall levels of Cdc42 protein, are important regulating unequal divisions in animal cells.

Insights into the evolution of the snail superfamily from metazoan wide molecular phylogenies and expression data in annelids

BACKGROUND

An important issue concerning the evolution of duplicated genes is to understand why paralogous genes are retained in a genome even though the most likely fate for a redundant duplicated gene is nonfunctionalization and thereby its elimination. Here we study a complex superfamily generated by gene duplications, the snail related genes that play key roles during animal development. We investigate the evolutionary history of these genes by genomic, phylogenetic, and expression data studies.

RESULTS

We systematically retrieved the full complement of snail related genes in several sequenced genomes. Through phylogenetic analysis, we found that the snail superfamily is composed of three ancestral families, snail, scratchA and scratchB. Analyses of the organization of the encoded proteins point out specific molecular signatures, indicative of functional specificities for Snail, ScratchA and ScratchB proteins. We also report the presence of two snail genes in the annelid Platynereis dumerilii, which have distinct expression patterns in the developing mesoderm, nervous system, and foregut. The combined expression of these two genes is identical to that of two independently duplicated snail genes in another annelid, Capitella spI, but different aspects of the expression patterns are differentially shared among paralogs of Platynereis and Capitella.

CONCLUSION

Our study indicates that the snail and scratchB families have expanded through multiple independent gene duplications in the different bilaterian lineages, and highlights potential functional diversifications of Snail and ScratchB proteins following duplications, as, in several instances, paralogous proteins in a given species show different domain organizations. Comparisons of the expression pattern domains of the two Platynereis and Capitella snail paralogs provide evidence for independent subfunctionalization events which have occurred in these two species. We propose that the snail related genes may be especially prone to subfunctionalization, and this would explain why the snail superfamily underwent so many independent duplications leading to maintenance of functional paralogs.

Toll-like receptors--taking an evolutionary approach

The Toll receptor was initially identified in Drosophila melanogaster for its role in embryonic development. Subsequently, D. melanogaster Toll and mammalian Toll-like receptors (TLRs) have been recognized as key regulators of immune responses. After ten years of intense research on TLRs and the recent accumulation of genomic and functional data in diverse organisms, we review the distribution and functions of TLRs in the animal kingdom. We provide an evolutionary perspective on TLRs, which sheds light on their origin at the dawn of animal evolution and suggests that different TLRs might have been co-opted independently during animal evolution to mediate analogous immune functions.

Characterization of twist and snail gene expression during mesoderm and nervous system development in the polychaete annelid Capitella sp. I

To investigate the evolutionary history of mesoderm in the bilaterian lineage, we are studying mesoderm development in the polychaete annelid, Capitella sp. I, a representative lophotrochozoan. In this study, we focus on the Twist and Snail families as candidate mesodermal patterning genes and report the isolation and in situ expression patterns of two twist homologs (CapI-twt1 and CapI-twt2) and two snail homologs (CapI-sna1 and CapI-sna2) in Capitella sp. I. CapI-twt1 is expressed in a subset of mesoderm derivatives during larval development, while CapI-twt2 shows more general mesoderm expression at the same stages. Neither twist gene is detected before the completion of gastrulation. The two snail genes have very distinct expression patterns. At cleavage and early gastrula stages, CapI-sna1 is broadly expressed in precursors of all three germ layers and becomes restricted to cells around the closing blastopore during late gastrulation; CapI-sna2 expression is not detected at these stages. After gastrulation, both snail genes are expressed in the developing central nervous system (CNS) at stages when neural precursor cells are internalized, and CapI-sna1 is also expressed laterally within the segmental mesoderm. Based on the expression patterns in this study, we suggest a putative function for Capitella sp. I twist genes in mesoderm differentiation and for snail genes in regulating CNS development and general cell migration during gastrulation.

Hau-Pax3/7A is an early marker of leech mesoderm involved in segmental morphogenesis, nephridial development, and body cavity formation

Two genes of the Pax III subfamily, Hau-Pax3/7A and -Pax3/7B, were identified from the leech Helobdella, and the expression and function of Hau-Pax3/7A in development are described. Leech embryos undergo spiral cleavage, then produce a set of teloblastic stem cells that generate segmented mesoderm and ectoderm. Hau-Pax3/7A is present as a maternal transcript in both ectodermal and mesodermal progenitors, but this pool of early RNA disappears and is replaced by a pattern of zygotic transcription restricted to the blast cell progeny of the mesodermal M teloblasts. Each mesodermal blast cell clone goes through multiple phases of Hau-Pax3/7A expression, the last of which is associated with the organogenesis of the nephridia and other segment-specific structures. Morpholino-mediated knockdown of Hau-Pax3/7A expression causes the mesodermal blast cell clones to undergo irregular patterns of morphogenesis that disrupt the segmental organization of the germinal plate, and interferes with both the specification and morphological differentiation of the mesodermal nephridia. Knockdown of Hau-Pax3/7A in the mesoderm can also lead to abnormalities in the formation of the dorsal cavities, possibly through indirect effects of this germ layer on neighboring tissues. This is the first report of broad mesodermal Pax III expression outside of chordates, and raises the possibility that such expression may be a primitive trait inherited from the last common ancestor of the bilaterian superphyla.

Differential expression of caudal and dorsal genes in the teloblast lineages of the oligochaete annelid Tubifex tubifex

We have cloned homologues (designated Ttu-cdx and Ttu-dl) of caudal and dorsal genes from the oligochaete annelid Tubifex tubifex. In situ hybridization revealed that Ttu-cdx begins to be expressed around the time of completion of ectodermal teloblastogenesis (i.e., the onset of gastrulation). At this time, Ttu-cdx expression is detected in a few cells that are located in the anteriormost part of the mesodermal germ bands (GBs). As development proceeds and the GBs elongate, the domain of Ttu-cdx-expressing cells in the GBs extends posteriorly. During this process, M teloblasts and primary blast cells remained negative for Ttu-cdx expression. This Ttu-cdx expression pattern is in sharp contrast to the posterior caudal expression in other organisms. Expression of Ttu-dl is detected from the one-cell stage through to the gastrula stage. Although Ttu-dl is expressed in most blastomeres, there are regional differences in Ttu-dl expression levels. During early cleavage, a large amount of Ttu-dl mRNA, which is supplied maternally, is inherited by D-cell line micromeres 2d and 4d; the remaining micromeres and macromeres inherit relatively small amounts of Ttu-dl transcripts. Another regional difference in Ttu-dl expression levels is seen in the ectodermal GB that is comprised of four bast-cell bandlets. The ventralmost bandlet (N lineage) exhibits the highest level of Ttu-dl expression with the lowest level in the two middle bandlets (O and P lineages) and an intermediate level in the dorsalmost bandlet (Q lineage).

Cell cycle-dependent expression of a hairy and Enhancer of split (hes) homolog during cleavage and segmentation in leech embryos

We have cloned genes related to hairy and Enhancer of split (hes) from glossiphoniid leeches, Helobdella robusta and Theromyzon rude. In leech, segments arise sequentially in anteroposterior progression from a posterior growth zone that consists of five bilaterally paired embryonic stem cells called teloblasts. Each teloblast gives rise to segmental founder cells (primary blast cells) that contribute iterated sets of definitive progeny in each segment. Thus, in leech, the "segmentation clock," is closely identified with the cell cycle clock of the teloblasts. We have characterized normal expression patterns of mRNA and protein for the H. robusta hes-class gene (Hro-hes). Semiquantitative RT-PCR revealed that Hro-hes mRNA levels peak while the teloblasts are actively producing primary blast cells. RT-PCR, in situ hybridization and immunostaining revealed that Hro-hes is expressed as early as the first zygotic mitosis and throughout early development. Hro-hes is expressed in macromeres, pro-teloblasts, teloblasts and primary blast cells. HRO-HES protein is localized in the nuclei of cells expressing HRO-HES during interphase; nuclear HRO-HES is reduced during mitosis. In contrast, Hro-hes is transcribed during mitosis and its transcripts are associated with mitotic apparatus (MA). Thus, Hro-hes transcription cycles in antiphase to the nuclear localization of HRO-HES protein. These results indicate that Hro-hes expression, and thus possibly its biological activity, is linked to the cell cycle.

Cg-Rel, the first Rel/NF-kappaB homolog characterized in a mollusk, the Pacific oyster Crassostrea gigas

We report here the identification and functional characterization of Cg-Rel, a gene encoding the Crassostrea gigas homolog of Rel/NF-kappaB transcription factors found in insects and mammals. Sequence and phylogenetic analysis showed that Cg-Rel shares the structural organization of Rel/NF-kappaB transcription factors of class II. It includes a Rel homology domain as well as a C-terminal transactivation domain (TD). Overexpression of Cg-Rel in the Drosophila S2 cell line activated the expression of a NF-kappaB-dependent reporter gene, whereas transfection with a Cg-Rel construct containing a C-terminal deletion of the TD or using a reporter gene with mutated kappaB binding sites failed to activate expression. These results suggest that Cg-Rel is a functional member of the Rel family of transcription factors, making this the sixth structurally homologous component of the Rel/NF-kappaB pathway characterized in C. gigas. Based on homology to other invertebrates' Rel/NF-kappaB cascade, the function of the oyster pathway may serve to regulate genes involved in innate defense and/or development. These findings serve to highlight a potentially important regulatory pathway to the study of oyster immunology, hence allowing comparison of the immune system in vertebrates and invertebrates, an important key issue to understand its evolution.

Embryonic development of the oligochaeteEnchytraeus coronatus: An SEM and histological study of embryogenesis from one-cell stage to hatching

We describe the embryonic development of the soil-living oligochaete Enchytraeus coronatus (Enchytraeidae, Oligochaeta, Annelida). Enchytraeus coronatus is a direct developer. It follows the typical spiral cleavage mode of development that is highly conserved among annelids and a large number of other lophotrochozoan taxa that are collectively named "Spiralia." Scanning electron microscopy (SEM) was combined with light microscopic analysis of wholemounted and sectioned embryos, differentially processed through histological stainings, to reconstruct and document cellular movements and organogenesis from early cleavage stages until hatching. With the help of these data we have established a scheme of morphologically defined stages in order to facilitate future studies on the molecular and histological level that will allow a detailed cross-species comparison among annelids and other phyla.

Conserved mechanism of dorsoventral axis determination in equal-cleaving spiralians

Many members of the spiralian phyla (i.e., annelids, echiurans, vestimentiferans, molluscs, sipunculids, nemerteans, polyclad turbellarians, gnathostomulids, mesozoans) exhibit early, equal cleavage divisions. In the case of the equal-cleaving molluscs, animal-vegetal inductive interactions between the derivatives of the first quartet micromeres and the vegetal macromeres specify which macromere becomes the 3D cell during the interval between fifth and sixth cleavage. The 3D macromere serves as a dorsal organizer and gives rise to the 4d mesentoblast. Even though it has been argued that this situation represents the ancestral condition among the Spiralia, these inductive events have only been documented in equal-cleaving molluscs. Embryos of the nemertean Cerebratulus lacteus also undergo equal, spiral cleavage, and the fate map of these embryos is similar to that of other spiralians. The role of animal first quartet micromeres in the establishment of the dorsal (D) cell quadrant was examined in C. lacteus by removing specific combinations of micromeres at the eight-cell stage. To follow the development of various cell quadrants, one quadrant was labeled with DiI at the four-cell stage, and specific first quartet micromeres were removed from discrete positions relative to the location of the labeled quadrant. The results indicate that the first quartet is required for normal development, as removal of all four micromeres prevented dorsoventral axis formation. In most cases, when either one or two adjacent first quartet micromeres were removed from one side of the embryo, the cell quadrant on the opposite side, with its macromere centered under the greatest number of the remaining animal micromeres, ultimately became the D quadrant. Twins containing duplicated dorsoventral axes were generated by removal of two opposing first quartet micromeres. Thus, any cell quadrant can become the D quadrant, and the dorsoventral axis is established after the eight-cell stage. While it is not yet clear exactly when key inductive interactions take place that establish the D quadrant in C. lacteus, contacts between the progeny of animal micromeres and vegetal macromeres are established during the interval between the fifth and sixth round of cleavage divisions (i.e., 32- to 64-cell stages). These findings argue that this mechanism of cell and axis determination has been conserved among equal-cleaving spiralians.

The snail superfamily of zinc-finger transcription factors

The Snail superfamily of zinc-finger transcription factors is involved in processes that imply pronounced cell movements, both during embryonic development and in the acquisition of invasive and migratory properties during tumour progression. Different family members have also been implicated in the signalling cascade that confers left right identity, as well as in the formation of appendages, neural differentiation, cell division and cell survival.