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

Transgenesis enables mapping of segmental ganglia in the leech Helobdella austinensis

The analysis of how neural circuits function in individuals and change during evolution is simplified by the existence of neurons identified as homologous within and across species. Invertebrates, including leeches, have been used for these purposes in part because their nervous systems comprise a high proportion of identified neurons, but technical limitations make it challenging to assess the full extent to which assumptions of stereotypy hold true. Here, we introduce Minos plasmid-mediated transgenesis as a tool for introducing transgenes into the embryos of the leech Helobdella austinensis (Spiralia; Lophotrochozoa; Annelida; Clitellata; Hirudinida; Glossiphoniidae). We identified an enhancer driving pan-neuronal expression of markers, including histone2B:mCherry, which allowed us to enumerate neurons in segmental ganglia. Unexpectedly, we found that the segmental ganglia of adult transgenic H. austinensis contain fewer and more variable numbers of neurons than in previously examined leech species.

Acceleration of genome rearrangement in clitellate annelids

Comparisons of multiple metazoan genomes have revealed the existence of ancestral linkage groups (ALGs), genomic scaffolds sharing sets of orthologous genes that have been inherited from ancestral animals for hundreds of millions of years (Simakov et al. 2022; Schultz et al. 2023) These ALGs have persisted across major animal taxa including Cnidaria, Deuterostomia, Ecdysozoa and Spiralia. Notwithstanding this general trend of chromosome-scale conservation, ALGs have been obliterated by extensive genome rearrangements in certain groups, most notably including Clitellata (oligochaetes and leeches), a group of easily overlooked invertebrates that is of tremendous ecological, agricultural and economic importance (Charles 2019; Barrett 2016). To further investigate these rearrangements, we have undertaken a comparison of 12 clitellate genomes (including four newly sequenced species) and 11 outgroup representatives. We show that these rearrangements began at the base of the Clitellata (rather than progressing gradually throughout polychaete annelids), that the inter-chromosomal rearrangements continue in several clitellate lineages and that these events have substantially shaped the evolution of the otherwise highly conserved Hox cluster.

Germ Line/Multipotency Genes Show Differential Expression during Embryonic Development of the Annelid Enchytraeus coronatus

Germ line development and the origin of the primordial germ cells (PGCs) are very variable and may occur across a range of developmental stages and in several developmental contexts. In establishing and maintaining germ line, a conserved set of genes is involved. On the other hand, these genes are expressed in multipotent/pluripotent cells that may give rise to both somatic and germline cells. To begin elucidating mechanisms by which the germ line is specified in Enchytraeus coronatus embryos, we identified twenty germline/multipotency genes, homologs of Vasa, PL10, Piwi, Nanos, Myc, Pumilio, Tudor, Boule, and Bruno, using transcriptome analysis and gene cloning, and characterized their expression by whole-mount in situ hybridization. To answer the question of the possible origin of PGCs in this annelid, we carried out an additional description of the early embryogenesis. Our results suggest that PGCs derive from small cells originating at the first two divisions of the mesoteloblasts. PGCs form two cell clusters, undergo limited proliferation, and migrate to the developing gonadal segments. In embryos and juvenile E. coronatus, homologs of the germline/multipotency genes are differentially expressed in both germline and somatic tissue including the presumptive germ cell precursors, posterior growth zone, developing foregut, and nervous system.

Peptidoglycan recognition proteins from the earthworm, Eisenia andrei: Differential inducibility and tissue-specific expression

Several pattern recognition receptors (PRRs) involved in innate immunity have been identified and characterized in earthworms. Peptidoglycan recognition proteins (PGRPs) are highly conserved PRRs that activate effector pathways such as prophenoloxidase cascade and Toll-like receptor pathway. In addition, PGRPs function as an enzyme, N-acetylmuramoyl-l-alanine amidase (NAMLAA), to directly hydrolyze peptidoglycan. We identified four full-length complementary DNA (cDNA) sequences, Ean-PGRP1/2/3/4, in Eisenia andrei, an earthworm. Sequence and phylogenetic analyses indicate that earthworm PGRP orthologs resemble short PGRP member proteins. The subcellular localizations of four Ean-PGRPs lacking the transmembrane domain are predicted to be extracellular or cytoplasmic. All Ean-PGRPs contain a highly conserved PGRP domain with a conserved Zn²⁺ binding site including a tyrosine residue essential for active amidase activity. Three highly conserved amino-acid residues (His, Trp, and Thr) necessary for amidase activity are also present, indicating that the Ean-PGRPs can be predicted to have amidase activity. Furthermore, we demonstrate that the Ean-PGRP genes are differentially induced by certain bacterial species, suggesting that the innate immune system of earthworms is likely to be somewhat specific rather than entirely non-specific. Tissue expression patterns indicate that Ean-PGRP mRNAs are primarily expressed in the immune-competent tissues and that their expression is tissue-specific according to Ean-PGRP types, particularly for Ean-PGRP1.

Glossiphoniid leeches as a touchstone for studies of development in clitellate annelids

My goals in this chapter are to share my enthusiasm for studying the biology of leeches, to place this work in context by presenting my rationale for studying non-traditional biological models in general, and to sample just three of the questions that intrigue me in leech biology, namely segmentation, genome evolution and neuronal fate specification. I first became excited about the idea of using leeches as a subject of investigation as an undergraduate in 1970 and have been engaged in this work since I arrived at Berkeley as a postdoc in 1976, intending to study leech neurobiology. Both my research interests and the rationale for the work have expanded greatly since then. What follows is a fragmentary personal and historical account-the interested reader may find more comprehensive treatments elsewhere (Kuo et al., 2020; Shankland & Savage, 1997; Shain, 2009; Weisblat & Huang, 2001; Weisblat & Kuo, 2009, 2014; Weisblat & Winchell, 2020).

Identification and Spatiotemporal Expression of Adenosine Deaminases Acting on RNA (ADAR) during Earthworm Regeneration: Its Possible Implication in Muscle Redifferentiation

Simple Summary

Among the animal species capable of regenerating missing body parts, a species of earthworm, Perionyx excavatus, has the most powerful regeneration capacity, which can completely and regenerate an amputated head and tail. Earthworm regeneration is a form of epimorphosis, a simple mode of development in adults that occurs around the sites of damage rather than throughout the body. In order to achieve this process, the earthworm must have molecular tools via which a variety of cell and tissue types can be precisely recovered from the pluripotent (or possibly totipotent) blastemal cells. Adenosine to inosine (A-to-I) RNA editing catalyzed by adenosine deaminases acting on RNA (ADAR) can generate substantial transcriptome and proteome variability and provide an ideal tool for cell and tissue re-specification. To understand the role of ADAR during earthworm regeneration, the molecular characteristics of an ADAR gene identified from P. excavatus (Pex-ADAR) were analyzed, and its spatial and temporal expression patterns were observed during regeneration. Domain analysis showed that Pex-ADAR is a member of the ADAR1 class. Its expression level primarily increases when and where muscle redifferentiation is actively taking place, suggesting that the RNA-editing enzyme Pex-ADAR is involved in muscle redifferentiation.

Abstract

Adenosine deaminases acting on RNA (ADAR) catalyze the hydrolytic deamination of adenosine (A) to produce inosine (I) in double-stranded RNA substrates. A-to-I RNA editing has increasingly broad physiological significance in development, carcinogenesis, and environmental adaptation. Perionyx excavatus is an earthworm with potent regenerative potential; it can regenerate the head and tail and is an advantageous model system to investigate the molecular mechanisms of regeneration. During RNA sequencing analysis of P. excavatus regenerates, we identified an ADAR homolog (Pex-ADAR), which led us to examine its spatial and temporal expression to comprehend how Pex-ADAR is linked to regeneration. At first, in domain analysis, we discovered that Pex-ADAR only has one double-stranded RNA-binding domain (dsRBD) and a deaminase domain without a Z-DNA-binding domain (ZBD). In addition, a comparison of the core deaminase domains of Pex-ADAR with those of other ADAR family members indicated that Pex-ADAR comprises the conserved three active-site motifs and a glutamate residue for catalytic activity. Pex-ADAR also shares 11 conserved residues, a characteristic of ADAR1, supporting that Pex-ADAR is a member of ADAR1 class. Its temporal expression was remarkably low in the early stages of regeneration before suddenly increasing at 10 days post amputation (dpa) when diverse cell types and tissues were being regenerated. In situ hybridization of Pex-ADAR messenger RNA (mRNA) indicated that the main expression was observed in regenerating muscle layers and related connective tissues. Taken together, the present results demonstrate that an RNA-editing enzyme, Pex-ADAR, is implicated in muscle redifferentiation during earthworm regeneration.

Spatiotemporal Expression of Anticoagulation Factor Antistasin in Freshwater Leeches

Antistasin, which was originally discovered in the salivary glands of the Mexican leech Haementeria officinalis, was newly isolated from Helobdella austinensis. To confirm the temporal expression of antistasin during embryogenesis, we carried out semi-quantitative RT-PCR. Hau-antistasin1 was uniquely expressed at stage 4 of the cleavage and was strongly expressed in the late stages of organogenesis, as were other antistasin members. In order to confirm the spatial expression of antistasin, we performed fluorescence in situ hybridization in the late stages of organogenesis. The expression of each antistasin in the proboscis showed a similar pattern and varied in expression in the body. In addition, the spatial expression of antistasin orthologs in different leeches showed the possibility of different function across leech species. Hau-antistasin1 was expressed in the same region as hedgehog, which is a known mediator of signal transduction pathway. Hau-antistasin1 is probably a downstream target of Hedgehog signaling, involved in segment polarity signal pathway.

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.

Sex Determination, Sexual Development, and Sex Change in Slipper Snails

Sex determination and sexual development are highly diverse and controlled by mechanisms that are extremely labile. While dioecy (separate male and female functions) is the norm for most animals, hermaphroditism (both male and female functions within a single body) is phylogenetically widespread. Much of our current understanding of sexual development comes from a small number of model systems, limiting our ability to make broader conclusions about the evolution of sexual diversity. We present the calyptraeid gastropods as a model for the study of the evolution of sex determination in a sequentially hermaphroditic system. Calyptraeid gastropods, a group of sedentary, filter-feeding marine snails, are sequential hermaphrodites that change sex from male to female during their life span (protandry). This transition includes resorption of the penis and the elaboration of female genitalia, in addition to shifting from production of spermatocytes to oocytes. This transition is typically under environmental control and frequently mediated by social interactions. Males in contact with females delay sex change to transition at larger sizes, while isolated males transition more rapidly and at smaller sizes. This phenomenon has been known for over a century; however, the mechanisms that control the switch from male to female are poorly understood. We review here our current understanding of sexual development and sex determination in the calyptraeid gastropods and other molluscs, highlighting our current understanding of factors implicated in the timing of sex change and the potential mechanisms. We also consider the embryonic origins and earliest expression of the germ line and the effects of environmental contaminants on sexual development.

On the origin of leeches by evolution of development

Leeches are a unique group of annelids arising from an ancestor that would be characterized as a freshwater oligochaete worm. Comparative biology of the oligochaetes and the leeches reveals that body plan changes in the oligochaete-to-leech transition probably occurred by addition or modification of the terminal steps in embryonic development and that they were likely driven by a change in the feeding behavior in the ancestor of leeches. In this review article, developmental changes that are associated with the evolution of several leech-specific traits are discussed. These include (1) the evolution of suckers, (2) the loss of chaetae, (3) the loss of septa, and (4) a fixed number of segments. An altered developmental fate of the teloblast is further proposed to be a key factor contributing to the fixation of the segment number, and the evolutionary change in teloblast development may also account for the loss of the ability to regenerate the lost body segments in the leech.

Temporal and spatial expression of the Fox gene family in the Leech Helobdella austinensis

The Forkhead box (Fox) gene family is an evolutionarily ancient gene family named after the Drosophila melanogaster forkhead gene (fkh). Fox genes are highly conserved transcription factors critical for embryogenesis and carcinogenesis. In the current study, we report a whole-genome survey of Fox genes and their expression patterns in the leech Helobdella austienesis. Phylogenetic analysis suggests that some Fox genes of leeches are correlated with other Lophotrochozoa and vertebrate Fox genes. Here we have performed semiquantitative reverse transcription polymerase chain reaction and whole-mount in situ hybridization of Fox genes throughout the embryonic development of H. austinensis. We found that each one of the leech Fox genes (FoxA1, FoxA3, FoxC, FoxL2, FoxO1, and FoxO2) is expressed in a specific set of cells or tissue type. From Stages 9-11, Hau-FoxA1 was expressed in the foregut of the anterior region, and Hau-FoxL2 was expressed in mesodermal muscle fiber. Hau-FoxA3 was temporally expressed in the ventral neuroectoderm. At Stage 11, Hau-FoxC was expressed in the foregut. Hau-FoxO genes have a ubiquitous expression. Our results provide more insight on the evolutionary linkage and role of the Fox gene function in Bilateria.

Duplicated FoxA genes in the leech Helobdella: Insights into the evolution of direct development in clitellate annelids

BACKGROUND

As an adaptation to the land, the clitellate annelid had reorganized its embryogenesis to develop "directly" without the ancestral planktonic larval stage. To study the evolution of gut development in the directly developing clitellates, we characterized the expression pattern of the conserved gut gene, FoxA, in the embryonic development of the leech.

RESULTS

The leech has three FoxA paralogs. Hau-FoxA1 is first expressed in a subset of endoderm cells and then in the foregut and the midgut. Hau-FoxA2 is expressed in the stomodeum, which is secondarily derived from the anterior ectoderm in the clitellates rather than the tissue around the blastopore, the ancestral site of mouth formation in Phylum Annelida. Hau-FoxA3 is expressed during the morphogenesis of segmental ganglia from the ectodermal teloblast lineages, a clitellate-specific trait. Hau-FoxA1 and Hau-FoxA2 are also expressed during the morphogenesis of the leech-specific front sucker.

CONCLUSIONS

The expression patterns suggested that Hau-FoxA1 carries out most of the conserved function in the endoderm and gut development, while the other two duplicates appear to have evolved unique novel functions in the directly developing clitellate embryos. Therefore, neofunctionalization and co-option of FoxA might have made a significant contribution to the evolution of direct development in Clitellata. Developmental Dynamics 247:763-778, 2018. © 2018 Wiley Periodicals, Inc.

Cell lineage and cell cycling analyses of the 4d micromere using live imaging in the marine annelid Platynereis dumerilii

Cell lineage, cell cycle, and cell fate are tightly associated in developmental processes, but in vivo studies at single-cell resolution showing the intricacies of these associations are rare due to technical limitations. In this study on the marine annelid Platynereis dumerilii, we investigated the lineage of the 4d micromere, using high-resolution long-term live imaging complemented with a live-cell cycle reporter. 4d is the origin of mesodermal lineages and the germline in many spiralians. We traced lineages at single-cell resolution within 4d and demonstrate that embryonic segmental mesoderm forms via teloblastic divisions, as in clitellate annelids. We also identified the precise cellular origins of the larval mesodermal posterior growth zone. We found that differentially-fated progeny of 4d (germline, segmental mesoderm, growth zone) display significantly different cell cycling. This work has evolutionary implications, sets up the foundation for functional studies in annelid stem cells, and presents newly established techniques for live imaging marine embryos.

Characterization of the piRNA pathway during development of the sea anemone Nematostella vectensis

PIWI-interacting RNAs (piRNAs) and associated proteins comprise a conserved pathway for silencing transposons in metazoan germlines. piRNA pathway components are also expressed in multipotent somatic stem cells in various organisms. piRNA functions have been extensively explored in bilaterian model systems, however, comprehensive studies in non-bilaterian phyla remain limited. Here we investigate the piRNA pathway during the development of Nematostella vectensis, a well-established model system belonging to Cnidaria, the sister group to Bilateria. To date, no population of somatic stem cells has been identified in this organism, despite its long life-span and regenerative capacities that require a constant cell-renewal. We show that Nematostella piRNA pathway components are broadly expressed in early developmental stages, while piRNAs themselves show differential expression, suggesting specific developmental roles of distinct piRNA families. In adults, piRNA associated proteins are enriched in the germline but also expressed in somatic cells, indicating putative stem cell properties. Furthermore, we provide experimental evidence that Nematostella piRNAs cleave transposable elements as well as protein-coding genes. Our results demonstrate that somatic expression of piRNA associated proteins as well as the roles of piRNAs in transposon repression and gene regulation are likely ancestral features that evolved before the split between Cnidaria and Bilateria.

A new earthworm cellulase and its possible role in the innate immunity

A new endogenous cellulase (Ean-EG) from the earthworm, Eisenia andrei and its expression pattern are demonstrated. Based on a deduced amino acid sequence, the open reading frame (ORF) of Ean-EG consisted of 1368 bps corresponding to a polypeptide of 456 amino acid residues in which is contained the conserved region specific to GHF9 that has the essential amino acid residues for enzyme activity. In multiple alignments and phylogenetic analysis, the deduced amino acid sequence of Ean- EG showed the highest sequence similarity (about 79%) to that of an annelid (Pheretima hilgendorfi) and could be clustered together with other GHF9 cellulases, indicating that Ean-EG could be categorized as a member of the GHF9 to which most animal cellulases belong. The histological expression pattern of Ean-EG mRNA using in situ hybridization revealed that the most distinct expression was observed in epithelial cells with positive hybridization signal in epidermis, chloragogen tissue cells, coelomic cell-aggregate, and even blood vessel, which could strongly support the fact that at least in the earthworm, Eisenia andrei, cellulase function must not be limited to digestive process but be possibly extended to the innate immunity.

On the role of the proventricle region in reproduction and regeneration in Typosyllis antoni (Annelida: Syllidae)

BACKGROUND

Syllids are a species rich annelid family possessing remarkable regenerative ability, which is not only the response after traumatic injury, but also a key step during the life cycle of several syllid taxa. In these animals the posterior part of the body becomes an epitoke and is later detached as a distinct unit named stolon. Such a sexual reproductive mode is named schizogamy or stolonization. The prostomium and the proventricle, a modified foregut structure, have been proposed to have a control function during this process, though the concrete mechanisms behind it have never been elucidated.

RESULTS

By using different experimental set-ups, histology and immunohistochemistry combined with subsequent cLSM analyzes, we investigate and document the regeneration and stolonization in specimens of Typosyllis antoni that were amputated at different levels throughout the antero-posterior body axis. The removal of the anterior end including the proventricle implies an incomplete anterior regeneration as well as severe deviations from the usual reproductive pattern, i.e. accelerated stolonization, masculinization and the occurrence of aberrant stolons. The detailed anatomy of aberrant stolons is described. A histological study of the proventricle revealed no signs of glandular or secretory structures. The ventricle and the caeca are composed of glandular tissue but they are not involved in the reproductive and regenerative processes.

CONCLUSIONS

As in other investigated syllids, the proventricle region has a significant role during stolonization and reproduction processes in Typosyllis antoni. When the proventricle region is absent, anterior and posterior regeneration are considerably deviated from the general patterns. However, proventricle ultrastructure does not show any glandular component, thereby questioning a direct involvement of this organ itself in the control of reproduction and regeneration. Our findings offer a comprehensive starting point for further studies of regeneration and reproductive control in syllids as well as annelids in general.

Developmental and molecular biology of annelid regeneration: a comparative review of recent studies

Studies of annelid regeneration have greatly increased in frequency in recent years, providing new insights into the developmental basis and evolution of regeneration. In this review, we summarize recent findings related to regeneration in annelids, focusing on molecular and developmental studies of epimorphic (blastema-based) regeneration, morphallactic (tissue-remodeling based) regeneration, and development and regeneration of putative stem cells of the posterior growth zone and germline. Regeneration is being investigated in a broad range of annelids spanning the phylum, and comparing findings among species reveals both widely conserved features that may be ancestral for the phylum as well as features that are variable across the group.

Developmental expression of "germline"- and "sex determination"-related genes in the ctenophore Mnemiopsis leidyi

Background

An essential developmental pathway in sexually reproducing animals is the specification of germ cells and the differentiation of mature gametes, sperm and oocytes. The “germline” genes vasa, nanos and piwi are commonly identified in primordial germ cells, suggesting a molecular signature for the germline throughout animals. However, these genes are also expressed in a diverse set of somatic stem cells throughout the animal kingdom leaving open significant questions for whether they are required for germline specification. Similarly, members of the Dmrt gene family are essential components regulating sex determination and differentiation in bilaterian animals, but the functions of these transcription factors, including potential roles in sex determination, in early diverging animals remain unknown. The phylogenetic position of ctenophores and the genome sequence of the lobate Mnemiopsisleidyi motivated us to determine the compliment of these gene families in this species and determine expression patterns during development.

Results

Our phylogenetic analyses of the vasa, piwi and nanos gene families show that Mnemiopsis has multiple genes in each family with multiple lineage-specific paralogs. Expression domains of Mnemiopsis nanos, vasa and piwi, during embryogenesis from fertilization to the cydippid stage, were diverse, with little overlapping expression and no or little expression in what we think are the germ cells or gametogenic regions. piwi paralogs in Mnemiopsis had distinct expression domains in the ectoderm during development. We observed overlapping expression domains in the apical organ and tentacle apparatus of the cydippid for a subset of “germline genes,” which are areas of high cell proliferation, suggesting that these genes are involved with “stem cell” specification and maintenance. Similarly, the five Dmrt genes show diverse non-overlapping expression domains, with no clear evidence for expression in future gametogenic regions of the adult. We also report on splice variants for two Mnemiopsis Dmrt genes that impact the presence and composition of the DM DNA binding domain for these transcription factors.

Conclusion

Our results are consistent with a potential role for vasa, piwi and nanos genes in the specification or maintenance of somatic stem cell populations during development in Mnemiopsis. These results are similar to previous results in the tentaculate ctenophore Pleurobrachia, with the exception that these genes were also expressed in gonads and developing gametes of adult Pleurobrachia. These differences suggest that the Mnemiopsis germline is either specified later in development than hypothesized, the germline undergoes extensive migration, or the germline does not express these classic molecular markers. Our results highlight the utility of comparing expression of orthologous genes across multiple species. We provide the first description of Dmrt expression in a ctenophore, which indicates that Dmrt genes are expressed in distinct structures and regions during development but not in future gametogenic regions, the only sex-specific structure for this hermaphroditic species.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-016-0051-9) contains supplementary material, which is available to authorized users.

Integrated gene mapping and synteny studies give insights into the evolution of a sex proto-chromosome in Solea senegalensis

The evolution of genes related to sex and reproduction in fish shows high plasticity and, to date, the sex determination system has only been identified in a few species. Solea senegalensis has 42 chromosomes and an XX/XY chromosome system for sex determination, while related species show the ZZ/ZW system. Next-generation sequencing (NGS), multi-color fluorescence in situ hybridization (mFISH) techniques, and bioinformatics analysis have been carried out, with the objective of revealing new information about sex determination and reproduction in S. senegalensis. To that end, several bacterial artificial chromosome (BAC) clones that contain candidate genes involved in such processes (dmrt1, dmrt2, dmrt3, dmrt4, sox3, sox6, sox8, sox9, lh, cyp19a1a, amh, vasa, aqp3, and nanos3) were analyzed and compared with the same region in other related species. Synteny studies showed that the co-localization of dmrt1-dmrt2-drmt3 in the largest metacentric chromosome of S. senegalensis is coincident with that found in the Z chromosome of Cynoglossus semilaevis, which would potentially make this a sex proto-chromosome. Phylogenetic studies show the close proximity of S. senegalensis to Oryzias latipes, a species with an XX/XY system and a sex master gene. Comparative mapping provides evidence of the preferential association of these candidate genes in particular chromosome pairs. By using the NGS and mFISH techniques, it has been possible to obtain an integrated genetic map, which shows that 15 out of 21 chromosome pairs of S. senegalensis have at least one BAC clone. This result is important for distinguishing those chromosome pairs of S. senegalensis that are similar in shape and size. The mFISH analysis shows the following co-localizations in the same chromosomes: dmrt1-dmrt2-dmrt3, dmrt4-sox9-thrb, aqp3-sox8, cyp19a1a-fshb, igsf9b-sox3, and lysg-sox6.

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.

The ontogeny of nanos homologue expression in the oligochaete annelid Tubifex tubifex

We have cloned and characterized the expression of a nanos homologue (designated Ttu-nos) from the oligochaete annelid Tubifex tubifex. Ttu-nos mRNA is distributed broadly throughout the early cleavage stages. Ttu-nos is expressed in most if not all of the early blastomeres, in which Ttu-nos RNA associates with pole plasms. Ttu-nos transcripts are concentrated to 2d and 4d cells. Shortly after 2d(111) (derived from 2d cell) divides into a bilateral pair of NOPQ proteloblasts, Ttu-nos RNA vanishes from the embryo, which is soon followed by the resumption of Ttu-nos expression in nascent primary blast cells produced by teloblasts. The resumption of Ttu-nos expression occurs only in a subset of teloblast lineages (viz., M, N and Q). After Ttu-nos expression is retained in the germ band for a while, it disappears in anterior-to-posterior progression. At the end of embryogenesis, there is no trace of Ttu-nos expression. Thereafter, growing juveniles do not show any sign of Ttu-nos expression, either. The first sign of Ttu-nos expression is detected in oocytes in the ovary of young adults (ca 40 days after hatching), and its expression continues in growing oocytes that undergo yolk deposition and maturation in the ovisac.

Gonad establishment during asexual reproduction in the annelid Pristina leidyi

Animals that can reproduce by both asexual agametic reproduction and sexual reproduction must transmit or re-establish their germ line post-embryonically. Although such a dual reproductive mode has evolved repeatedly among animals, how asexually produced individuals establish their germ line remains poorly understood in most groups. We investigated germ line development in the annelid Pristina leidyi, a species that typically reproduces asexually by paratomic fission, intercalating a new tail and head in the middle of the body followed by splitting. We found that in fissioning individuals, gonads occur in anterior segments in the anterior-most individual as well as in new heads forming within fission zones. Homologs of the germ line/multipotency genes piwi, vasa, and nanos are expressed in the gonads, as well as in proliferative tissues including the posterior growth zone, fission zone, and regeneration blastema. In fissioning animals, certain cells on the ventral nerve cord express a homolog of piwi, are abundant near fission zones, and sometimes make contact with gonads. Such cells are typically undetectable near the blastema and posterior growth zone. Time-lapse imaging provides direct evidence that cells on the ventral nerve cord migrate preferentially towards fission zones. Our findings indicate that gonads form routinely in fissioning individuals, that a population of piwi-positive cells on the ventral nerve cord is associated with fission and gonads, and that cells resembling these piwi-positive cells migrate along the ventral nerve cord. We suggest that the piwi-positive ventral cells are germ cells that transmit the germ line across asexually produced individuals via migration along the ventral nerve cord.

vasa and piwi are required for mitotic integrity in early embryogenesis in the spider Parasteatoda tepidariorum

Studies in vertebrate and invertebrate model organisms on the molecular basis of primordial germ cell (PGC) specification have revealed that metazoans can specify their germ line either early in development by maternally transmitted cytoplasmic factors (inheritance), or later in development by signaling factors from neighboring tissues (induction). Regardless of the mode of PGC specification, once animal germ cells are specified, they invariably express a number of highly conserved genes. These include vasa and piwi, which can play essential roles in any or all of PGC specification, development, or gametogenesis. Although the arthropods are the most speciose animal phylum, to date there have been no functional studies of conserved germ line genes in species of the most basally branching arthropod clade, the chelicerates (which includes spiders, scorpions, and horseshoe crabs). Here we present the first such study by using molecular and functional tools to examine germ line development and the roles of vasa and piwi orthologues in the common house spider Parasteatoda (formerly Achaearanea) tepidariorum. We use transcript and protein expression patterns of Pt-vasa and Pt-piwi to show that primordial germ cells (PGCs) in the spider arise during late embryogenesis. Neither Pt-vasa nor Pt-piwi gene products are localized asymmetrically to any embryonic region before PGCs emerge as paired segmental clusters in opisthosomal segments 2-6 at late germ band stages. RNA interference studies reveal that both genes are required maternally for egg laying, mitotic progression in early embryos, and embryonic survival. Our results add to the growing body of evidence that vasa and piwi can play important roles in somatic development, and provide evidence for a previously hypothesized conserved role for vasa in cell cycle progression.

Developmental biology of the leech Helobdella

Glossiphoniid leeches of the genus Helobdella provide experimentally tractable models for studies in evolutionary developmental biology (Evo-Devo). Here, after a brief rationale, we will summarize our current understanding of Helobdella development and highlight the near term prospects for future investigations, with respect to the issues of: D quadrant specification; the transition from spiral to bilaterally symmetric cleavage; segmentation, and the connections between segmental and non-segmental tissues; modifications of BMP signaling in dorsoventral patterning and the O-P equivalence group; germ line specification and genome rearrangements. The goal of this contribution is to serve as a summary of, and guide to, published work.