Slit-Robo expression in the leech nervous system: insights into eyespot evolution

BACKGROUND

Slit and Robo are evolutionarily conserved ligand and receptor proteins, respectively, but the number of slit and robo gene paralogs varies across recent bilaterian genomes. Previous studies indicate that this ligand-receptor complex is involved in axon guidance. Given the lack of data regarding Slit/Robo in the Lophotrochozoa compared to Ecdysozoa and Deuterostomia, the present study aims to identify and characterize the expression of Slit/Robo orthologs in leech development.

RESULTS

We identified one slit (Hau-slit), and two robo genes (Hau-robo1 and Hau-robo2), and characterized their expression spatiotemporally during the development of the glossiphoniid leech Helobdella austinensis. Throughout segmentation and organogenesis, Hau-slit and Hau-robo1 are broadly expressed in complex and roughly complementary patterns in the ventral and dorsal midline, nerve ganglia, foregut, visceral mesoderm and/or endoderm of the crop, rectum and reproductive organs. Before yolk exhaustion, Hau-robo1 is also expressed where the pigmented eye spots will later develop, and Hau-slit is expressed in the area between these future eye spots. In contrast, Hau-robo2 expression is extremely limited, appearing first in the developing pigmented eye spots, and later in the three additional pairs of cryptic eye spots in head region that never develop pigment. Comparing the expression of robo orthologs between H. austinensis and another glossiphoniid leech, Alboglossiphonia lata allows to that robo1 and robo2 operate combinatorially to differentially specify pigmented and cryptic eyespots within the glossiphoniid leeches.

CONCLUSIONS

Our results support a conserved role in neurogenesis, midline formation and eye spot development for Slit/Robo in the Lophotrochozoa, and provide relevant data for evo-devo studies related to nervous system evolution.

Head transcriptome profiling of glossiphoniid leech (Helobdella austinensis) reveals clues about proboscis development

Cephalization refers to the evolutionary trend towards the concentration of neural tissues, sensory organs, mouth and associated structures at the front end of bilaterian animals. Comprehensive studies on gene expression related to the anterior formation in invertebrate models are currently lacking. In this study, we performed de novo transcriptional profiling on a proboscis-bearing leech (Helobdella austinensis) to identify differentially expressed genes (DEGs) in the anterior versus other parts of the body, in particular to find clues as to the development of the proboscis. Between the head and the body, 132 head-specific DEGs were identified, of which we chose 11 to investigate their developmental function during embryogenesis. Analysis of the spatial expression of these genes using in situ hybridization showed that they were characteristically expressed in the anterior region of the developing embryo, including the proboscis. Our results provide information on the genes related to head formation and insights into the function of proboscis-related genes during organogenesis with the potential roles of genes not yet characterized.

A tale of two leeches: Toward the understanding of the evolution and development of behavioral neural circuits

In the animal kingdom, behavioral traits encompass a broad spectrum of biological phenotypes that have critical roles in adaptive evolution, but an EvoDevo approach has not been broadly used to study behavior evolution. Here, we propose that, by integrating two leech model systems, each of which has already attained some success in its respective field, it is possible to take on behavioral traits with an EvoDevo approach. We first identify the developmental changes that may theoretically lead to behavioral evolution and explain why an EvoDevo study of behavior is challenging. Next, we discuss the pros and cons of the two leech model species, Hirudo, a classic model for invertebrate neurobiology, and Helobdella, an emerging model for clitellate developmental biology, as models for behavioral EvoDevo research. Given the limitations of each leech system, neither is particularly strong for behavioral EvoDevo. However, the two leech systems are complementary in their technical accessibilities, and they do exhibit some behavioral similarities and differences. By studying them in parallel and together with additional leech species such as Haementeria, it is possible to explore the different levels of behavioral development and evolution.

Reproductive differences among species, and between individuals and cohorts, in the leech genus Helobdella (Lophotrochozoa; Annelida; Clitellata; Hirudinida; Glossiphoniidae), with implications for reproductive resource allocation in hermaphrodites

Leeches and oligochaetes comprise a monophyletic group of annelids, the Clitellata, whose reproduction is characterized by simultaneous hermaphroditism. While most clitellate species reproduce by cross-fertilization, self-fertilization has been described within the speciose genus Helobdella. Here we document the reproductive life histories and reproductive capacities for three other Helobdella species. Under laboratory conditions, both H. robusta and H. octatestisaca exhibit uniparental reproduction, apparently reflecting self-fertility, and suggesting that this trait is ancestral for the genus. However, the third species, H. austinensis, seems incapable of reproduction by self-fertilization, so we inferred its reproductive life history by analyzing reproduction in breeding cohorts. Comparing the reproductive parameters for H. robusta reproducing in isolation and in cohorts revealed that reproduction in cohorts is dramatically delayed with respect to that of isolated individuals, and that cohorts of leeches coordinate their cocoon deposition in a manner that is not predicted from the reproductive parameters of individuals reproducing in isolation. Finally, our comparisons of reproductive capacity for individuals versus cohorts for H. robusta, and between different sizes of cohorts for H. austinensis, reveal differences in resource allocation between male and female reproductive roles that are consistent with evolutionary theory.

Behavioral analysis of substrate texture preference in a leech, Helobdella austinensis

Leeches in the wild are often found on smooth surfaces, such as vegetation, smooth rocks or human artifacts such as bottles and cans, thus exhibiting what appears to be a "substrate texture preference". Here, we have reproduced this behavior under controlled circumstances, by allowing leeches to step about freely on a range of silicon carbide substrates (sandpaper). To begin to understand the neural mechanisms underlying this texture preference behavior, we have determined relevant parameters of leech behavior both on uniform substrates of varying textures, and in a behavior choice paradigm in which the leech is confronted with a choice between rougher and smoother substrate textures at each step. We tested two non-exclusive mechanisms which could produce substrate texture preference: (1) a Differential Diffusion mechanism, in which a leech is more likely to stop moving on a smooth surface than on a rough one, and (2) a Smoothness Selection mechanism, in which a leech is more likely to attach its front sucker (prerequisite for taking a step) to a smooth surface than to a rough one. We propose that both mechanisms contribute to the texture preference exhibited by leeches.

Spatiotemporal expression of a twist homolog in the leech Helobdella austinensis

Genes of the twist family encode bHLH transcription factors known to be involved in the regulation and differentiation of early mesoderm. Here, we report our characterization of Hau-twist, a twist homolog from the leech Helobdella austinensis, a tractable lophotrochozoan representative. Hau-twist was expressed in segmental founder cells of the mesodermal lineage, in subsets of cells within the mesodermal lineage of the germinal plate, in circumferential muscle fibers of a provisional integument during segmentation and organogenesis stages and on the ventral side of the developing proboscis. Thus, consistent with other systems, our results suggest that twist gene of the leech Helobdella might function in mesoderm differentiation.

Leeches of the genus Helobdella as model organisms for Evo-Devo studies

Model organisms are important tools in modern biology and have been used elucidate mechanism underlying processes, such as development, heredity, neuronal signaling, and phototropism, to name but a few. In this context, the use of model organisms is predicated on uncovering evolutionarily conserved features of biological processes in the expectation that the findings will be applicable to organisms that are either inaccessible or intractable for direct experimentation. For the most part, particular species have been adapted as model organisms because they can be easily reared and manipulated in the laboratory. In contrast, a major goal in the field of evolutionary developmental biology (Evo-Devo) is to identify and elucidate the differences in developmental processes among species associated with the dramatic range of body plans among organisms, and how these differences have emerged over time in various branches of phylogeny. At first glance then, it would appear that the concept of model organisms for Evo-Devo is oxymoronic. In fact, however, laboratory-compatible, experimentally tractable species are of great use for Evo-Devo, subject to the condition that the ensemble of models investigated should reflect the range of taxonomic diversity, and for this purpose glossiphoniid leeches are useful. Four decades ago (1975), leeches of the species-rich genus Helobdella (Lophotrochozoa; Annelida; Clitellata; Hirudinida; Glossiphoniidae) were collected in Stow Lake, Golden Gate Park, San Francisco, CA (USA). These and other Helobdella species may be taken as Evo-Devo models of leeches, clitellate annelids, and the super-phylum Lophotrochozoa. Here we depict/discuss the biology/taxonomy of these Evo-Devo systems, and the challenges of identifying species within Helobdella. In addition, we document that H. austinensis has been established as a new model organism that can easily be cultivated in the laboratory. Finally, we provide an updated scheme illustrating the unique germ line/soma-differentiation during early development and speculate on the mechanisms of sympatric speciation in this group of aquatic annelids.

Differential expression of conserved germ line markers and delayed segregation of male and female primordial germ cells in a hermaphrodite, the leech helobdella

In sexually reproducing animals, primordial germ cells (PGCs) are often set aside early in embryogenesis, a strategy that minimizes the risk of genomic damage associated with replication and mitosis during the cell cycle. Here, we have used germ line markers (piwi, vasa, and nanos) and microinjected cell lineage tracers to show that PGC specification in the leech genus Helobdella follows a different scenario: in this hermaphrodite, the male and female PGCs segregate from somatic lineages only after more than 20 rounds of zygotic mitosis; the male and female PGCs share the same (mesodermal) cell lineage for 19 rounds of zygotic mitosis. Moreover, while all three markers are expressed in both male and female reproductive tissues of the adult, they are expressed differentially between the male and female PGCs of the developing embryo: piwi and vasa are expressed preferentially in female PGCs at a time when nanos is expressed preferentially in male PGCs. A priori, the delayed segregation of male and female PGCs from somatic tissues and from one another increases the probability of mutations affecting both male and female PGCs of a given individual. We speculate that this suite of features, combined with a capacity for self-fertilization, may contribute to the dramatically rearranged genome of Helobdella robusta relative to other animals.

Insights into bilaterian evolution from three spiralian genomes

Current genomic perspectives on animal diversity neglect two prominent phyla, the molluscs and annelids, that together account for nearly one-third of known marine species and are important both ecologically and as experimental systems in classical embryology^1–3. Here we describe the draft genomes of the owl limpet (Lottia gigantea), a marine polychaete (Capitella teleta) and a freshwater leech (Helobdella robusta), and compare them with other animal genomes to investigate the origin and diversification of bilaterians from a genomic perspective. We find that the genome organization, gene structure and functional content of these species are more similar to those of some invertebrate deuterostome genomes (for example, amphioxus and sea urchin) than those of other protostomes that have been sequenced to date (flies, nematodes and flatworms). The conservation of these genomic features enables us to expand the inventory of genes present in the last common bilaterian ancestor, establish the tripartite diversification of bilaterians using multiple genomic characteristics and identify ancient conserved long- and short-range genetic linkages across metazoans. Superimposed on this broadly conserved pan-bilaterian background we find examples of lineage-specific genome evolution, including varying rates of rearrangement, intron gain and loss, expansions and contractions of gene families, and the evolution of clade-specific genes that produce the unique content of each genome.

Regional differences in BMP-dependence of dorsoventral patterning in the leech Helobdella

In the leech Helobdella, the ectoderm exhibits a high degree of morphological homonomy between body segments, but pattern elements in lateral ectoderm arise via distinct cell lineages in the segments of the rostral and midbody regions. In each of the four rostral segments, a complete set of ventrolateral (O fate) and dorsolateral (P fate) ectodermal pattern elements arises from a single founder cell, op. In the 28 midbody and caudal segments, however, there are two initially indeterminate o/p founder cells; the more dorsal of these is induced to adopt the P fate by BMP5-8 emanating from the dorsalmost ectoderm, while the more ventral cell assumes the O fate. Previous work has suggested that the dorsoventral patterning of O and P fates differs in the rostral region, but the role of BMP signaling in those segments has not been investigated. We show here that suppression of dorsal BMP5-8 signaling (which effects a P-to-O fate change in the midbody) has no effect on the patterning of O and P fates in the rostral region. Furthermore, ectopic expression of BMP5-8 in the ventral ectoderm (which induces an O-to-P fate change in the midbody) has no effect in the rostral region. Finally, expression of a dominant-negative BMP receptor (which induces a P-to-O fate change in the midbody) fails to affect O/P patterning in the rostral region. Thus, the rostral segments appear to use some mechanism other than BMP signaling to pattern O and P cell fates along the dorsoventral axis. From a mechanistic standpoint, the OP lineage of the rostral segments and the O-P equivalence group of the midbody and caudal segments constitute distinct developmental modules that rely to differing degrees on positional cues from surrounding ectoderm in order to specify homonomous cell fates.

Intermediate filament genes as differentiation markers in the leech Helobdella

The intermediate filament (IF) cytoskeleton is a general feature of differentiated cells. Its molecular components, IF proteins, constitute a large family including the evolutionarily conserved nuclear lamins and the more diverse collection of cytoplasmic intermediate filament (CIF) proteins. In vertebrates, genes encoding CIFs exhibit cell/tissue type-specific expression profiles and are thus useful as differentiation markers. The expression of invertebrate CIFs, however, is not well documented. Here, we report a whole-genome survey of IF genes and their developmental expression patterns in the leech Helobdella, a lophotrochozoan model for developmental biology research. We found that, as in vertebrates, each of the leech CIF genes is expressed in a specific set of cell/tissue types. This allows us to detect earliest points of differentiation for multiple cell types in leech development and to use CIFs as molecular markers for studying cell fate specification in leech embryos. In addition, to determine the feasibility of using CIFs as universal metazoan differentiation markers, we examined phylogenetic relationships of IF genes from various species. Our results suggest that CIFs, and thus their cell/tissue-specific expression patterns, have expanded several times independently during metazoan evolution. Moreover, comparing the expression patterns of CIF orthologs between two leech species suggests that rapid evolutionary changes in the cell or tissue specificity of CIFs have occurred among leeches. Hence, CIFs are not suitable for identifying cell or tissue homology except among very closely related species, but they are nevertheless useful species-specific differentiation markers.

A new molecular logic for BMP-mediated dorsoventral patterning in the leech Helobdella

Bone morphogenetic protein (BMP) signaling is broadly implicated in dorsoventral (DV) patterning of bilaterally symmetric animals [1-3], and its role in axial patterning apparently predates the birth of Bilateria [4-7]. In fly and vertebrate embryos, BMPs and their antagonists (primarily Sog/chordin) diffuse and interact to generate signaling gradients that pattern fields of cells [8-10]. Work in other species reveals diversity in essential facets of this ancient patterning process, however. Here, we report that BMP signaling patterns the DV axis of segmental ectoderm in the leech Helobdella, a clitellate annelid (superphylum Lophotrochozoa) featuring stereotyped developmental cell lineages, but the detailed mechanisms of DV patterning in Helobdella differ markedly from fly and vertebrates. In Helobdella, BMP2/4s are expressed broadly, rather than in dorsal territory, whereas a dorsally expressed BMP5-8 specifies dorsal fate by short-range signaling. A BMP antagonist, gremlin, is upregulated by BMP5-8 in dorsolateral, rather than ventral territory, and yet the BMP-antagonizing activity of gremlin is required for normal ventral cell fates. Gremlin promotes ventral fates without disrupting dorsal fates by selectively inhibiting BMP2/4s, not BMP5-8. Thus, DV patterning in the development of the leech revealed unexpected evolutionary plasticity of the conserved BMP patterning system, presumably reflecting its adaptation to different modes of embryogenesis.

Lineage analysis of micromere 4d, a super-phylotypic cell for Lophotrochozoa, in the leech Helobdella and the sludgeworm Tubifex

The super-phylum Lophotrochozoa contains the plurality of extant animal phyla and exhibits a corresponding diversity of adult body plans. Moreover, in contrast to Ecdysozoa and Deuterostomia, most lophotrochozoans exhibit a conserved pattern of stereotyped early divisions called spiral cleavage. In particular, bilateral mesoderm in most lophotrochozoan species arises from the progeny of micromere 4d, which is assumed to be homologous with a similar cell in the embryo of the ancestral lophotrochozoan, more than 650 million years ago. Thus, distinguishing the conserved and diversified features of cell fates in the 4d lineage among modern spiralians is required to understand how lophotrochozoan diversity has evolved by changes in developmental processes. Here we analyze cell fates for the early progeny of the bilateral daughters (M teloblasts) of micromere 4d in the leech Helobdella sp. Austin, a clitellate annelid. We show that the first six progeny of the M teloblasts (em1-em6) contribute five different sets of progeny to non-segmental mesoderm, mainly in the head and in the lining of the digestive tract. The latter feature, associated with cells em1 and em2 in Helobdella, is seen with the M teloblast lineage in a second clitellate species, the sludgeworm Tubifex tubifex and, on the basis of previously published work, in the initial progeny of the M teloblast homologs in molluscan species, suggesting that it may be an ancestral feature of lophotrochozoan development.

High resolution cell lineage tracing reveals developmental variability in leech

Knowing the normal patterns of embryonic cell proliferation, migration, and differentiation is a cornerstone for understanding development. Yet for most species, the precision with which embryonic cell lineages can be determined is limited by technical considerations (the large numbers of cells, extended developmental times, opacity of the embryos), and these are exacerbated by the inherent variability of the lineages themselves. Here, we present an improved method of cell lineage tracing in the leech Helobdella, driving the expression of a nuclearly localized histone H2B:GFP (green fluorescent protein) fusion protein in selected lineages by microinjection of a plasmid vector. This construct generates a long lasting and minimally mosaic signal with single cell resolution, and does not disrupt the development of most lineages tested. We have validated this technique by elucidating details of cell lineages contributing to segmental and prostomial tissues that could not be observed with standard dextran lineage tracers.

Evolutionary dynamics of the wnt gene family: a lophotrochozoan perspective

The wnt gene family encodes a set of secreted glycoproteins involved in key developmental processes, including cell fate specification and regulation of posterior growth (Cadigan KM, Nusse R. 1997. Wnt signaling: a common theme in animal development. Genes Dev. 11:3286-3305.; Martin BL, Kimelman D. 2009. Wnt signaling and the evolution of embryonic posterior development. Curr Biol. 19:R215-R219.). As for many other gene families, evidence for expansion and/or contraction of the wnt family is available from deuterostomes (e.g., echinoderms and vertebrates [Nusse R, Varmus HE. 1992. Wnt genes. Cell. 69:1073-1087.; Schubert M, Holland LZ, Holland ND, Jacobs DK. 2000. A phylogenetic tree of the Wnt genes based on all available full-length sequences, including five from the cephalochordate amphioxus. Mol Biol Evol. 17:1896-1903.; Croce JC, Wu SY, Byrum C, Xu R, Duloquin L, Wikramanayake AH, Gache C, McClay DR. 2006. A genome-wide survey of the evolutionarily conserved Wnt pathways in the sea urchin Strongylocentrotus purpuratus. Dev Biol. 300:121-131.]) and ecdysozoans (e.g., arthropods and nematodes [Eisenmann DM. 2005. Wnt signaling. WormBook. 1-17.; Bolognesi R, Farzana L, Fischer TD, Brown SJ. 2008. Multiple Wnt genes are required for segmentation in the short-germ embryo of Tribolium castaneum. Curr Biol. 18:1624-1629.]), but little is known from the third major bilaterian group, the lophotrochozoans (e.g., mollusks and annelids [Prud'homme B, Lartillot N, Balavoine G, Adoutte A, Vervoort M. 2002. Phylogenetic analysis of the Wnt gene family. Insights from lophotrochozoan members. Curr Biol. 12:1395.]). To obtain a more comprehensive scenario of the evolutionary dynamics of this gene family, we exhaustively mined wnt gene sequences from the whole genome assemblies of a mollusk (Lottia gigantea) and two annelids (Capitella teleta and Helobdella robusta) and examined them by phylogenetic, genetic linkage, intron-exon structure, and embryonic expression analyses. The 36 wnt genes obtained represent 11, 12, and 9 distinct wnt subfamilies in Lottia, Capitella, and Helobdella, respectively. Thus, two of the three analyzed lophotrochozoan genomes retained an almost complete ancestral complement of wnt genes emphasizing the importance and complexity of this gene family across metazoans. The genome of the leech Helobdella reflects significantly more dynamism than those of Lottia and Capitella, as judged by gene duplications and losses, branch length, and changes in genetic linkage. Finally, we performed a detailed expression analysis for all the Helobdella wnt genes during embryonic development. We find that, although the patterns show substantial overlap during early cleavage stages, each wnt gene has a unique expression pattern in the germinal plate and during tissue morphogenesis. Comparisons of the embryonic expression patterns of the duplicated wnt genes in Helobdella with their orthologs in Capitella reveal extensive regulatory diversification of the duplicated leech wnt genes.

In situ hybridization of Helobdella (leech) embryos

The purpose of this protocol is to reveal the localization of transcripts in the Helobdella (leech) embryo. In situ hybridization protocols for Helobdella embryos are derived from those used for zebrafish and Xenopus. The protocols are different for early- and late-stage embryos (stages 1-8 and stages 9-11, respectively) due to a difference in tissue permeability.

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.

Arnolds' silver staining of Helbodella (leech) embryos

Arnolds' silver staining is used to visualize the boundaries of epidermal cells at the surface of embryos. As described in this article, silver staining can be performed on either living or fixed Helbodella (leech) embryos. To preserve the cell junction in fixed embryos, a special fixation procedure is used.

Devitellinization of living Helobdella (leech) embryos

Embryos of glossiphoniid leeches are enclosed in a thin vitelline envelope until "hatching" (stage 10). This protocol describes the procedure for removing the vitelline envelope from Helobdella robusta and H. sp. (Austin) embryos. This protocol is applicable to embryos of stages 1-9 but is probably more useful for early stages. With careful culturing, the devitellinized embryos can develop normally.

Whole-mount preparation of Helobdella (leech) embryos for microscopy

Due to the high yolk content and relatively large size of the Helobdella (leech) embryo, it is preferable to examine the embryos in a cleared whole-mount preparation after they have been stained. Three whole-mount procedures suitable for leech embryos are described here. The glycerol whole mount is quick and convenient because it does not require dehydration. However, prolonged incubation in buffered glycerol could cause the yolk to turn dark purple. Thus, it is not suitable for yolk-containing specimens that are intended for light microscopy. For fluorescence microscopy, anti-fading reagent can be added to the buffered glycerol. This article includes a recipe that works quite well for leech embryos. Also included are two alternative methods that require embryo dehydration, but have the advantage of clearing yolk-containing tissue. One method relies on the use of benzyl benzoate:benzyl alcohol (BBBA), a solution that provides the best clearing of yolk-containing tissue in the leech embryo. It is suitable for both fluorescence and light microscopy. Another alternative is the Epon whole-mount preparation. Epon is not quite as effective in clearing the yolk, but the viscosity of Epon facilitates orienting the specimens for imaging. As with BBBA, Epon is suitable for both light and fluorescence microscopy.

Handling of Helobdella (leech) embryos

Gravid Helobdella (leech) adults are identified by white egg masses that are visible through the ventral body wall. For convenience, gravid animals should be moved to smaller bowls so that those animals with newly laid embryos can be more readily identified. Zygotes (fertilized internally) turn pink when they are deposited into transparent cocoons on the ventral surface of the parent. A typical clutch consists of 20-100 embryos. Embryos can be collected after zygote deposition and before hatching (stage 10). This article describes how to collect and culture Helobdella embryos.

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.

D quadrant specification in the leech Helobdella: actomyosin contractility controls the unequal cleavage of the CD blastomere

The unequal division of the CD blastomere at second cleavage is critical in establishing the second embryonic axis in the leech Helobdella, as in other unequally cleaving spiralians. When CD divides, the larger D and smaller C blastomeres arise invariantly on the left and right sides of the embryo, respectively. Here we show that stereotyped cellular dynamics, including the formation of an intercellular blastocoel, culminate in a morphological left-right asymmetry in the 2-cell embryo, which precedes cytokinesis and predicts the chirality of the second cleavage. In contrast to the unequal first cleavage, the unequal second cleavage does not result from down-regulation of one centrosome, nor from an asymmetry within the spindle itself. Instead, the unequal cleavage of the CD cell entails a symmetric mitotic apparatus moving and anisotropically growing rightward in an actomyosin-dependent process. Our data reveal that mechanisms controlling the establishment of the D quadrant differ fundamentally even among the monophyletic clitellate annelids. Thus, while the homologous spiral cleavage pattern is highly conserved in this clade, it has diverged significantly at the level of cell biological mechanisms. This combination of operational conservation and mechanistic divergence begins to explain how the spiral cleavage program has remained so refractory to change while, paradoxically, accommodating numerous modifications throughout evolution.

Microinjection of Helobdella (leech) embryos

One advantage of using Helobdella (leech) embryos as an experimental system is their amenability for microinjection. Blastomeres ranging in size from the zygote (400 microm diameter) down to micromeres and primary blast cells (approximately 20 microm diameter) can be injected by pressure under a dissecting microscope. Smaller cells can be injected by iontophoresis under a compound microscope. Microinjection is useful for studying embryonic development. For example, developmental fates of a cell can be followed by injecting a lineage tracer. A specific cell can be killed by injecting a toxic substance. Furthermore, cells can be killed at a given developmental stage by directing intense fluorescence illumination or a blue laser beam on fluorescein-labeled cells. Finally, gene expression can be manipulated in leech embryos by injecting zygotes or selected blastomeres with synthetic mRNA, morpholino antisense oligo, or a plasmid construct. Molecules <1500 Da can diffuse freely among early blastomeres via gap junctions. When intercellular diffusion of an injected substance is undesirable, small molecules should be conjugated to larger molecules such as dextran (10 kDa). Different commercial microinjection setups can be adopted for Helobdella embryos. This article describes how to microinject embryos using a versatile homemade pressure injection system. Under a dissecting microscope, embryos are immobilized by suction in a custom-fabricated chamber with the target cell facing upward. Cells are visualized using transillumination via a long-working-distance, dark-field condenser. The tip of a micropipette is brought into the target cell with a micromanipulator, and the injectant is delivered into the cell by pressure.

Helobdella (leech): a model for developmental studies

Helobdella is a genus of freshwater leeches, several species of which have been used for developmental studies since the 1970s. Helobdella embryos have been used for cell-lineage tracing and dye-mediated photoablation, and they have also been very useful for studies in cellular neurobiology. In this article, we discuss the reasons that Helobdella is used for studying development and some of the questions that are addressed through the use of this organism.

And Lophotrochozoa makes three: Notch/Hes signaling in annelid segmentation

Segmentation is unquestionably a major factor in the evolution of complex body plans, but how this trait itself evolved is unknown. Approaching this problem requires comparing the molecular mechanisms of segmentation in diverse segmented and unsegmented taxa. Notch/Hes signaling is involved in segmentation in sequentially segmenting vertebrates and arthropods, as judged by patterns of expression of one or more genes in this network and by the disruption of segmental patterning when Notch/Hes signaling is disrupted. We have previously shown that Notch and Hes homologs are expressed in the posterior progress zone (PPZ), from which segments arise, in the leech Helobdella robusta, a sequentially segmenting lophotrochozoan (phylum Annelida). Here, we show that disrupting Notch/Hes signaling disrupts segmentation in this species as well. Thus, Notch/Hes functions in either the maintenance of the PPZ and/or the patterning processes of segmentation in representatives of all three superphyla of bilaterally symmetric animals. These results are consistent with two evolutionary scenarios. In one, segmentation was already present in the ancestor of all three superphyla. In the other, Notch/Hes signaling functioned in axial growth by terminal addition in an unsegmented bilaterian ancestor, and was subsequently exapted to function in segmentation as that process evolved independently in two or more taxa.

Factors specifying cell lineages in the leech

As in arthropods, several major organ systems in leeches, including body musculature, nervous system and nephridia, are organized into a fixed number of longitudinally iterated units called segments. Many cells, especially neurons, can be uniquely identified from segment to segment. Leech embryos comprise identified cells, which facilitates developmental analysis. So far as it is known, cell lineages in leech are largely determinate. Prior to first cleavage, cytoplasmic reorganization generates domains of yolk-deficient cytoplasm called teloplasm. In situ hybridization experiments suggest that teloplasm is enriched for polyadenylated RNAs. During the first three, unequal cell divisions, teloplasm is segregated to macromere D'; normally, this cell alone cleaves further to generate five bilateral pairs of embryonic stem cells, M, N, O/P and Q teloblasts. Centrifugation experiments have shown a causal link between inheritance of teloplasm and the cleavage pattern that generates teloblasts. Teloblasts undergo highly unequal divisions, generating a longitudinal array of segmental founder cells called m, n, o, p and q blast cells, from which the definitive segmental tissues arise via further stereotyped cell divisions. Microinjecting new-born teloblasts or their precursors with polyadenylic acid induces the formation of supernumerary teloblasts. This discovery permits further analyses of factors specifying the five cell lines generating segmental tissues of the leech.

Asymmetric cell divisions in the early embryo of the leech Helobdella robusta

The small glossiphoniid leech Helobdella robusta is among the best-studied representatives of the super-phylum Lophotrochozoa in terms of early development. The Helobdella embryo undergoes a modified version of spiral cleavage, characterized by stereotyped cell lineages comprising multiple examples of equal and unequal divisions, many of which are well-conserved with respect to those of other clitellate annelids, such as the oligochaete Tubifex. Here, we review the early development of Helobdella, focusing on the variety of unequal cell divisions. We then summarize an experimental analysis of the mechanisms underlying the unequal first cleavage in Helobdella, concluding that the unequal first cleavages in Helobdella and Tubifex proceed by different mechanisms. This result demonstrates the evolvability of the basic cell biological mechanisms underlying well-conserved developmental processes. Finally, we propose a model in which the unequal second cleavage in Helobdella may be regulated by the polarized distribution of PAR protein homologs, convergent with the unequal first cleavage of the nematode Caenorhabditis elegans (super-phylum Ecdysozoa).

Lessons from leeches: a call for DNA barcoding in the lab

Many evolution of development labs study organisms that must be periodically collected from the wild. Whenever this is the case, there is the risk that different field collections will recover genetically different strains or cryptic species. Ignoring this potential for genetic variation may introduce an uncontrolled source of experimental variability, leading to confusion or misinterpretation of the results. Leeches in the genus Helobdella have been a workhorse of annelid developmental biology for 30 years. Nearly all early Helobdella research was based on a single isolate, but in recent years isolates from multiple field collections and multiple sites across the country have been used. To assess the genetic distinctness of different isolates, we obtained specimens from most Helobdella laboratory cultures currently or recently in use and from some of their source field sites. From these samples, we sequenced part of the mitochondrial gene cytochrome oxidase I (COI). Sequence divergences and phylogenetic analyses reveal that, collectively, the Helobdella development community has worked on five distinct species from two major clades. Morphologically similar isolates that were thought to represent the same species (H. robusta) actually represent three species, two of which coexist at the same locality. Another isolate represents part of a species complex (the "H. triserialis" complex), and yet another is an invasive species (H. europaea). We caution researchers similarly working on multiple wild-collected isolates to preserve voucher specimens and to obtain from these a molecular "barcode," such as a COI gene sequence, to reveal genetic variation in animals used for research.

MAPK regulation of maternal and zygotic Notch transcript stability in early development

Spatiotemporal modulation of the evolutionarily conserved, intercellular Notch signaling pathway is important in the development of many animals. Examples include the regulation of neural-epidermal fate decisions in neurogenic ectoderm of Drosophila and somitogenesis in vertebrate presomitic mesoderm. In both these and most other cases, it appears that Notch-class transmembrane receptors are ubiquitously expressed. Modulation of the pathway is achieved primarily by the localized expression of the activating ligand or by alteration of receptor specificity through a glycosyl transferase. In contrast, we present this report of an instance where the abundance of the Notch-class mRNA itself is dynamically regulated. Taking advantage of the long cell cycle of the two-cell-stage embryo of the leech Helobdella robusta, we show that this regulation is achieved at the levels of both transcript stability and transcription. Moreover, MAPK signaling plays a significant role in regulating accumulation of the transcript by virtue of its effect on Hro-notch mRNA stability. Intracellular injection of heterologous reporter mRNAs shows that the Hro-notch 3' UTR, containing seven AU-rich elements, is key to regulating transcript stability. Thus, we show that regulation of the Notch pathway can occur at a previously underappreciated level, namely that of transcript stability. Given that AU-rich elements occur in the 3' UTR of Notch-class genes in Drosophila, human, and Caenorhabditis elegans, regulation of Notch signaling by modulation of mRNA levels may be operating in other animals as well.

Germline regeneration: the worms' turn

Asexual reproduction in the annelid Enchytraeus japonensis entails the regeneration of primordial germ cells from body parts that lack gonads. New primordial germ cells arise from piwi-expressing germline stem cells that are distinct from somatic stem cells.

Maternal expression of a NANOS homolog is required for early development of the leech Helobdella robusta

The gene nanos (nos) is a maternal posterior group gene required for normal development of abdominal segments and the germ line in Drosophila. Expression of nos-related genes is associated with the germ line in a broad variety of other taxa, including the leech Helobdella robusta, where zygotically expressed Hro-nos appears to be associated with primordial germ cells. The function of maternally inherited Hro-nos transcripts remains to be determined, however. Here, the function of maternal Hro-nos is examined using an antisense morpholino (MO) knockdown strategy, as confirmed by immunostaining and western blot analysis. HRO-NOS knockdown embryos exhibit abnormalities in the distribution of micromeres during cleavage. Subsequently, their germinal bands are positioned abnormally with respect to the embryonic midline and the micromere cap, epiboly fails, and the HRO-NOS knockdown embryos die. This lethality can be rescued by injection of mRNA encoding an eGFP::HRO-NOS fusion protein. HRO-NOS knockdown embryos make their normal complements of mesodermal and ectodermal teloblasts, and the progeny of these teloblasts segregate into distinct mesodermal and ectodermal layers. These results suggest that maternal Hro-nos is required for embryonic development. However, contrary to previous suggestions, maternal inherited Hro-nos does not appear necessary for ectoderm specification.

Asymmetrization of first cleavage by transient disassembly of one spindle pole aster in the leech Helobdella robusta

Unequal first cleavage is characteristic of a diverse group of protostome animals. In the nematode Caenorhabditis elegans, unequal first cleavage is achieved through the interaction of an apparently symmetric mitotic spindle apparatus with a clearly polarized cell cortex. In the clitellate annelid Tubifex tubifex, by contrast, the spindle is monastral and contains only one gamma-tubulin-reactive centrosome; this monastral spindle is inherently asymmetric throughout mitosis. Here, we have used immunostaining for beta- and gamma-tubulin to follow spindle dynamics during the unequal first cleavage in another clitellate annelid, the leech Helobdella robusta. We find that the mitotic spindle is diastral and symmetric through early metaphase, then becomes asymmetric following the transient down-regulation of one centrosome, as judged by gamma-tubulin immunofluorescence. Low levels of drugs that affect microtubule dynamics can symmetrize the first cleavage without affecting the gamma-tubulin dynamics. Our results provide a striking example of the evolvability of cellular mechanisms underlying an unambiguously homologous developmental process.

Characterization of Notch-class gene expression in segmentation stem cells and segment founder cells in Helobdella robusta (Lophotrochozoa; Annelida; Clitellata; Hirudinida; Glossiphoniidae)

To understand the evolution of segmentation, we must compare segmentation in all three major groups of eusegmented animals: vertebrates, arthropods, and annelids. The leech Helobdella robusta is an experimentally tractable annelid representative, which makes segments in anteroposterior progression from a posterior growth zone consisting of 10 identified stem cells. In vertebrates and some arthropods, Notch signaling is required for normal segmentation and functions via regulation of hes-class genes. We have previously characterized the expression of an hes-class gene (Hro-hes) during segmentation in Helobdella, and here, we characterize the expression of an H. robusta notch homolog (Hro-notch) during this process. We find that Hro-notch is transcribed in the segmental founder cells (blast cells) and their stem-cell precursors (teloblasts), as well as in other nonsegmental tissues. The mesodermal and ectodermal lineages show clear differences in the levels of Hro-notch expression. Finally, Hro-notch is shown to be inherited by newly born segmental founder cells as well as transcribed by them before their first cell division.

Applications of mRNA injections for analyzing cell lineage and asymmetric cell divisions during segmentation in the leech Helobdella robusta

Synthetic mRNAs can be injected to achieve transient gene expression even for `non-model' organisms in which genetic approaches are not feasible. Here,we have used this technique to express proteins that can serve as lineage tracers or reporters of cellular events in embryos of the glossiphoniid leech Helobdella robusta (phylum Annelida). As representatives of the proposed super-phylum Lophotrochozoa, glossiphoniid leeches are of interest for developmental and evolutionary comparisons. Their embryos are suitable for microinjection, but no genetic approaches are currently available. We have injected segmentation stem cells (teloblasts) with mRNAs encoding nuclear localized green fluorescent protein (nGFP) and its spectral variants, and have used tandem injections of nGFP mRNA followed by antisense morpholino oligomer (AS MO), to label single blast cell clones. These techniques permit high resolution cell lineage tracing in living embryos. We have applied them to the primary neurogenic (N) lineage, in which alternate segmental founder cells (nf and ns blast cells) contribute distinct sets of progeny to the segmental ganglia. The nf and ns blast cell clones exhibit strikingly different cell division patterns: the increase in cell number within the nf clone is roughly linear, while that in the ns clone is almost exponential. To analyze spindle dynamics in the asymmetric divisions of individual blast cells, we have injected teloblasts with mRNA encoding a tau::GFP fusion protein. Our results show that the asymmetric divisions of n blast cells result from a posterior shift of both the spindle within the cell and the midbody within the mitotic spindle, with differential regulation of these processes between nf and ns.

Cell interactions that affect axonogenesis in the leech Theromyzon rude

The leech nervous system comprises a relatively simple network of longitudinal (connective) and transverse (segmental) nerves. We have followed the normal pattern of axon development in the glossiphoniid leech Theromyzon rude by immunostaining embryonic preparations with antibody to acetylated alpha-tubulin. The dependence of the normal pattern of axon growth on cells in the mesodermal (M) and ectodermal (N, O, P and Q) lineages was examined by selectively ablating subsets of these lineages in developing embryos. We found that ablating mesoderm severely disrupted overall axonogenesis, while various ectodermal ablations induced a range of more specific phenotypes. In particular, formation of the posterior segmental nerve (PP) was abnormal in embryos deficient in primary neuroectoderm (N lineage). More specific ablations demonstrated that a subset of N-derived cells were required for establishing the PP nerve root. Previous studies have shown that the PP nerve root is normally pioneered by an O lineage-derived neuron (PD). Our results suggest that the role of the N lineage-derived cells is to induce the migration of neuron P(D) to its normal position in the posterior compartment of the hemiganglion.

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.

A hedgehog homolog regulates gut formation in leech (Helobdella)

Signaling by the hedgehog (hh)-class gene pathway is essential for embryogenesis in organisms ranging from Drosophila to human. We have isolated a hh homolog (Hro-hh) from a lophotrochozoan species, the glossiphoniid leech, Helobdella robusta, and examined its expression by reverse transcription polymerase chain reaction (RT-PCR) and whole-mount in situ hybridization. The peak of Hro-hh expression occurs during organogenesis (stages 10-11). No patterned expression was detected within the segmented portion of the germinal plate during the early stages of segmentation. In stage 10-11 embryos, Hro-hh is expressed in body wall, foregut, anterior and posterior midgut, reproductive organs and in a subset of ganglionic neurons. Evidence that Hro-hh regulates gut formation was obtained using the steroidal alkaloid cyclopamine, which specifically blocks HH signaling. Cyclopamine induced malformation of both foregut and anterior midgut in Helobdella embryos, and no morphologically recognizable gonads were seen. In contrast, no gross abnormalities were observed in the posterior midgut. Segmental ectoderm developed normally, as did body wall musculature and some other mesodermal derivatives, but the mesenchymal cells that normally come to fill most of the coelomic cavities failed to develop. Taken with data from Drosophila and vertebrates, our data suggest that the role of hh-class genes in gut formation and/or neural differentiation is ancestral to the bilaterians, whereas their role in segmentation evolved secondarily within the Ecdysozoa.

Expression and function of an even-skipped homolog in the leech Helobdella robusta

We have identified homologs of the Drosophila pair-rule gene even-skipped in the glossiphoniid leeches Helobdella robusta and Theromyzon trizonare. In leech embryos, segments arise sequentially from five pairs of embryonic stem cells (teloblasts) that undergo iterated divisions to generate columns (bandlets) of segmental founder cells (primary blast cells), which in turn generate segmentally iterated sets of definitive progeny. In situ hybridization revealed that Hro-eve is expressed in the teloblasts and primary blast cells, and that these transcripts appear to be associated with mitotic chromatin. In more advanced embryos, Hro-eve is expressed in segmentally iterated sets of cells in the ventral nerve cord. Lineage analysis revealed that neurons expressing Hro-eve arise from the N teloblast. To assess the function of Hro-eve, we examined embryos in which selected blastomeres had been injected with antisense Hro-eve morpholino oligonucleotide (AS-Hro-eve MO), concentrating on the primary neurogenic (N teloblast) lineage. Injection of AS-Hro-eve MO perturbed the normal patterns of teloblast and blast cell divisions and disrupted gangliogenesis. These results suggest that Hro-eve is important in regulating early cell divisions through early segmentation, and that it also plays a role in neuronal differentiation.

Maternal and zygotic expression of a nanos-class gene in the leech Helobdella robusta: primordial germ cells arise from segmental mesoderm

The nanos-class gene of the leech Helobdella robusta (Hro-nos) is present as a maternal transcript whose levels decay during cleavage; HRO-NOS protein is more abundant in the D quadrant cells relative to the A, B, and C quadrants; and HRO-NOS is more abundant in the ectodermal precursor cell (DNOPQ) than in its sister mesodermal precursor (DM) (Pilon and Weisblat, 1997). Here, using in situ hybridization, we show that Hro-nos mRNA is broadly distributed throughout the zygote, is concentrated in both animal and vegetal teloplasm during stage 1 and is at higher levels in DNOPQ than in DM at stage 4b. Hro-nos expression increases after stage 7, as judged by in situ hybridization, developmental RT-PCR, and western blots; this increase must therefore represent later zygotic expression. Of particular interest, during stages 9 and 10, each of 11 mid-body segments (M8-M18) has a pair of Hro-nos positive "spots" comprising of one or two large cells each. These spots later disappear in an anteroposterior progression. We find that these Hro-nos-expressing cells are of mesodermal origin, arising in a segmentally iterated manner from the M lineage, and correspond to cells previously proposed as primordial germ cells (PGCs; Bürger, 1891; Weisblat and Shankland, 1985). These results support the proposal that nanos-class genes functioned in the specification of germline cells in the ancestral bilaterian and possibly in a separate process related to embryonic polarity in the ancestral protostome.

Micromere lineages in the glossiphoniid leechHelobdella

In leech embryos, segmental mesoderm and ectoderm arise from teloblasts by lineages that are already relatively well characterized. Here, we present data concerning the early divisions and the definitive fate maps of the micromeres, a group of 25 small cells that arise during the modified spiral cleavage in leech (Helobdella robusta) and contribute to most of the nonsegmental tissues of the adult. Three noteworthy results of this work are as follows. (1) The c′′′ and dm′ clones (3d and 3c in traditional nomenclature) give rise to a hitherto undescribed network of fibers that run from one end of the embryo to the other. (2) The clones of micromeres b′′ and b′′′ (2b and 3b in traditional nomenclature) die in normal development; the b′′ clone can be rescued to assume the normal c′′ fate if micromere c′′ or its clone are ablated in early development. (3) Two qualitative differences in micromere fates are seen between H. robusta (Sacramento) and another Helobdella sp. (Galt). First, in Helobdella sp. (Galt), the clone of micromere b′′ does not normally die, and contributes a subset of the cells arising exclusively from c′′ in H. robusta (Sacramento). Second, in Helobdella sp. (Galt), micromere c′′′ makes no definitive contribution, whereas micromere dm′ gives rise to cells equivalent to those arising from c′′′ and dm′ in H. robusta (Sacramento).

Dorsal and snail homologs in leech development

As part of an examination of how developmental mechanisms such as axis specification, cell fate specification, and segmentation have evolved, we have cloned homologs of the Drosophila melanogaster genes dorsal and snail from the glossiphoniid leech Helobdella robusta. Sequences from one dorsal-class gene (Hro-dl) and two snail-class genes (Hro-sna1 and Hro-sna2) were identified. Polyclonal antibodies were raised against the most conserved domains of HRO-DL and HRO-SNA1. Nuclear staining appeared for both proteins in mid-embryogenesis, in mesodermal and ectodermal precursors. During segmentation, segmentally iterated stripes of cells with strong HRO-DL staining appeared. The stripes of HRO-DL staining were first concentrated in the cytoplasm of cells, and later in the nuclei. Around this time, HRO-SNA levels also appeared in nuclei in segmentally iterated stripes. The localization of HRO-DL and HRO-SNA proteins raise the possibility that these genes are part of a conserved genetic pathway that, instead of specifying the dorsoventral axis and the mesoderm as in flies, might play a role in the diversification of cell types within segment primordia during leech development.

An overview of glossiphoniid leech development

Dramatic advances in understanding the development of selected "model" organisms, coupled with the realization that genes which regulate development are often conserved between diverse taxa, have renewed interest in comparative development and evolution. Recent molecular phylogenies seem to be converging on a new consensus "tree," according to which higher bilaterians fall into three major groups, Deuterostoma, Ecdysozoa, and Lophotrochozoa. Commonly studied model systems for development fall almost exclusively within the first two of these groups. Glossiphoniid leeches (phylum Annelida) offer certain advantages for descriptive and experimental embryology per se, and can also serve to represent the lophotrochozoan clade. We present an overview of the development of glossiphoniid leeches, highlighting some current research questions and the potential for comparative cellular and molecular studies.