Expression of AMD 1, a gene for a MyoD 1-related factor in the ascidian Halocynthia roretzi

A myogenic regulatory factor is required for myogenesis during limb regeneration in the Chinese mitten crab

Myogenic regulatory factors (MRFs) are a group of transcription factors that regulate the activity of skeletal muscle cells during embryonic development and postnatal myogenesis in various vertebrate species. However, the role of MRFs in limb regeneration remains poorly understood in crustaceans. In this study, we identified a full-length cDNA encoding a myogenic regulatory factor from Eriocheir sinensis (EsMRF) and evaluated its mRNA expression profile during muscle development, growth, and regeneration. The expression of EsMRF was found to correlate with the onset of muscle formation during development and with the regeneration process following limb autotomy. To elucidate the function of MRF during limb regeneration in E. sinensis, we assessed regenerative efficiency using RNA interference (RNAi) targeting EsMRF. Our findings revealed that the blockade of MRF delayed limb regeneration by disrupting the proliferation and myogenesis of blastema cells at the basal growth stage. Furthermore, luciferase assays results demonstrated that EsMRF can transcriptionally activate target myogenic genes, either through direct binding to their promoters or by interacting with co-regulators such as EsHEB or EsMEF2. This study identifies a novel MRF in E. sinensis and elucidates its function during limb regeneration, thereby contributing to our understanding of muscle growth and regeneration mechanisms in crustaceans.

Protein kinases and protein phosphatases encoded in the Ciona robusta genome

Protein kinases (PKs) and protein phosphatases (PPs) regulate the phosphorylation of proteins that are involved in a variety of biological processes. To study such biological processes systematically, it is important to know the whole repertoire of PKs and PPs encoded in a genome. In the present study, we surveyed the genome of an ascidian (Ciona robusta or Ciona intestinalis type A) to comprehensively identify the genes that encoded PKs and PPs. Because ascidians belong to the sister group of vertebrates, a comparison of the whole repertoire of PKs and PPs of ascidians with those of vertebrates may help to delineate the complements of these proteins that were present in the last common ancestor of these two groups of animals. Our results show that the repertory of PPs was much more expanded in vertebrates than the repertory of PKs. We also showed that approximately 75% of PKs and PPs were expressed during development from eggs to larvae. Thus, the present study provides catalogs for PKs and PPs encoded in the ascidian genome. These catalogs will be useful for systematic studies of biological processes that involve phosphorylation and for evolutionary studies of the origin of vertebrates.

Tbx15/18/22 shares a binding site with Tbx6-r.b to maintain expression of a muscle structural gene in ascidian late embryos

The ascidian larval tail contains muscle cells for swimming. Most of these muscle cells differentiate autonomously. The genetic program behind this autonomy has been studied extensively and the genetic cascade from maternal factors to initiation of expression of a muscle structural gene, Myl.c, has been uncovered; Myl.c expression is directed initially by transcription factor Tbx6-r.b at the 64-cell stage and then by the combined actions of Tbx6-r.b and Mrf from the gastrula to early tailbud stages. In the present study, we showed that transcription of Myl.c continued in late tailbud embryos and larvae, although a fusion protein of Tbx6-r.b and GFP was hardly detectable in late tailbud embryos. A knockdown experiment, reporter assay, and in vitro binding assay indicated that an essential cis-regulatory element of Myl.c that bound Tbx6-r.b in early embryos bound Tbx15/18/22 in late embryos to maintain expression of Myl.c. We also found that Tbx15/18/22 was controlled by Mrf, which constitutes a regulatory loop with Tbx6-r.b. Therefore, our data indicated that Tbx15/18/22 was activated initially under control of this regulatory loop as in the case of Myl.c, and then Tbx15/18/22 maintained the expression of Myl.c after Tbx6-r.b had disappeared. RNA-sequencing of Tbx15/18/22 morphant embryos revealed that many muscle structural genes were regulated similarly by Tbx15/18/22. Thus, the present study revealed the mechanisms of maintenance of transcription of muscle structural genes in late embryos in which Tbx15/18/22 takes the place of Tbx6-r.b.

Functional analysis of two MyoDs revealed their role in the activation of myomixer expression in yellowfin seabream (Acanthopagrus latus) (Hottuyn, 1782)

Myoblast determination protein (MyoD), a muscle-specific basic helix-loop-helix (bHLH) transcription factor, plays a pivotal role in regulating skeletal muscle growth and development. However, the regulation mechanism of MyoD has not been determined in marine fishes. In the present study, we isolated the MyoD1 (AlMyoD1) and MyoD2 (AlMyoD2) genomic sequences and analyzed the expression patterns in different tissues of yellowfin seabream (Acanthopagrus latus). The open reading frame (ORF) sequences of AlMyoD1 and AlMyoD2 encoded 297 and 271 amino acids possessing three common characteristic domains, respectively, containing a myogenic basic domain, a bHLH domain, and a ser-rich region (helix III). Phylogenetic and genome structure analyses exhibited classic phylogeny and highly conserved exon/intron architecture. Furthermore, the AlMyoD1 and AlMyoD2 transcription levels were higher in white muscle than in the other tissues. In order to further study AlMyoD function in muscle, promoter sequence analysis found that several E-box binding sites were present. Additionally, binding sites of Almyomixer involved in mammal myoblast fusion, which expression was also the highest in white muscle, were found in the promoter of AlMyoD. Pomoter activity assays further confirmed that both AlMyoD1 and AlMyoD2 can dramatically activate Almyomixer expression, and the AlMyoD1 M2 and AlMyoD2 M5 E-box binding sites were functionally important for Almyomixer transcription based on mutation analysis and electrophoretic mobile shift assays (EMSA). In summary, two MyoDs play a core role in Almyomixer regulation and may promote myofibre formation during muscle development and growth by regulating Almyomixer expression.

Regulation and evolution of muscle development in tunicates

For more than a century, studies on tunicate muscle formation have revealed many principles of cell fate specification, gene regulation, morphogenesis, and evolution. Here, we review the key studies that have probed the development of all the various muscle cell types in a wide variety of tunicate species. We seize this occasion to explore the implications and questions raised by these findings in the broader context of muscle evolution in chordates.

The Ciona myogenic regulatory factor functions as a typical MRF but possesses a novel N-terminus that is essential for activity

Electroporation-based assays were used to test whether the myogenic regulatory factor (MRF) of Ciona intestinalis (CiMRF) interferes with endogenous developmental programs, and to evaluate the importance of its unusual N-terminus for muscle development. We found that CiMRF suppresses both notochord and endoderm development when it is expressed in these tissues by a mechanism that may involve activation of muscle-specific microRNAs. Because these results add to a large body of evidence demonstrating the exceptionally high degree of functional conservation among MRFs, we were surprised to discover that non-ascidian MRFs were not myogenic in Ciona unless they formed part of a chimeric protein containing the CiMRF N-terminus. Equally surprising, we found that despite their widely differing primary sequences, the N-termini of MRFs of other ascidian species could form chimeric MRFs that were also myogenic in Ciona. This domain did not rescue the activity of a Brachyury protein whose transcriptional activation domain had been deleted, and so does not appear to constitute such a domain. Our results indicate that ascidians have previously unrecognized and potentially novel requirements for MRF-directed myogenesis. Moreover, they provide the first example of a domain that is essential to the core function of an important family of gene regulatory proteins, one that, to date, has been found in only a single branch of the family.

Deuterostome Genomics: Lineage-Specific Protein Expansions That Enabled Chordate Muscle Evolution

Fish-like larvae were foundational to the chordate body plan, given the basal placement of free-living lancelets. That body plan probably made it possible for chordate ancestors to swim by beating a tail formed of notochord and bilateral paraxial muscles. In order to investigate the molecular genetic basis of the origin and evolution of paraxial muscle, we deduced the evolutionary histories of 16 contractile protein genes from paraxial muscle, based on genomic data from all five deuterostome lineages, using a newly developed orthology identification pipeline and a species tree. As a result, we found that more than twice as many orthologs of paraxial muscle genes are present in chordates, as in nonchordate deuterostomes (ambulacrarians). Orthologs of paraxial-type actin and troponin C genes are absent in ambulacrarians and most paraxial muscle protein isoforms diversified via gene duplications that occurred in each chordate lineage. Analyses of genes with known expression sites indicated that some isoforms were reutilized in specific muscles of nonvertebrate chordates via gene duplications. As orthologs of most paraxial muscle genes were present in ambulacrarians, in addition to expression patterns of related genes and functions of the two protein isoforms, regulatory mechanisms of muscle genes should also be considered in future studies of the origin of paraxial muscle.

The regulatory pathway from genes directly activated by maternal factors to muscle structural genes in ascidian embryos

Striated muscle cells in the tail of ascidian tadpole larvae differentiate cell-autonomously. Although several key regulatory factors have been identified, the genetic regulatory pathway is not fully understood; comprehensive understanding of the regulatory pathway is essential for accurate modeling in order to deduce principles for gene regulatory network dynamics, and for comparative analysis on how ascidians have evolved the cell-autonomous gene regulatory mechanism. Here, we reveal regulatory interactions among three key regulatory factors, Zic-r.b, Tbx6-r.b and Mrf, and elucidate the mechanism by which these factors activate muscle structural genes. We reveal a cross-regulatory circuit among these regulatory factors, which maintains the expression of Tbx6-r.b and Mrf during gastrulation. Although these two factors combinatorially activate muscle structural genes in late-stage embryos, muscle structural genes are activated mainly by Tbx6-r.b before gastrulation. Time points when expression of muscle structural genes become first detectable are strongly correlated with the degree of Tbx6-r.b occupancy. Thus, the genetic pathway, starting with Tbx6-r.b and Zic-r.b, which are activated by maternal factors, and ending with expression of muscle structural genes, has been revealed.

[Phylogenetic analysis and expression patterns of tropomyosin in amphioxus]

In amphioxus, we found a mesoderm related gene, tropomyosin, which encodes a protein comprising 284 amino acid residues, sharing high identities with other known Tropomyosin proteins both in vertebrates and invertebrates. Phylogenetically, amphioxus Tropomyosin fell outside the invertebrate clade and was at the base of the vertebrate protein family clade, indicating that it may represent an independent branch. From the early neurula to the larva stage, whole-mount in situ hybridization and histological sections found transcripts of amphioxus tropomyosin gene. Weak tropomyosin expression was first detected in the wall of the archenteron at about 10 hours-post-fertilization neurula stage, while intense expression was revealed in the differentiating presumptive notochord and the muscle. Transcripts of tropomyosin were then expressed in the formed notochord and somites. Gene expression seemed to continue in these developing organs throughout the neurular stages and remained till 72-hours, during the early larval stages. In situ study still showed tropomyosin was also expressed in the neural tube, hepatic diverticulum, notochord and the spaces between myotomes in adult amphioxus. Our results indicated that tropomyosin may play an important role in both embryonic development and adult life.

Muscle development in Ciona intestinalis requires the b-HLH myogenic regulatory factor gene Ci-MRF

The activity of myogenic regulatory factor (MRF) genes is essential for vertebrate muscle development, whereas invertebrate muscle development is largely independent of MRF function. This difference indicates that myogenesis is controlled by distinct regulatory mechanisms in these two groups of animals. Here we used overexpression and gene knockdown to investigate the role in embryonic myogenesis of the single MRF gene of the invertebrate chordate Ciona intestinalis (Ci-MRF). Injection of Ci-MRF mRNA into eggs resulted in increased embryonic muscle-specific gene activity and revealed the myogenic activity of Ci-MRF by inducing the expression of four muscle marker genes, Acetylcholinesterase, Actin, Troponin I, and Myosin Light Chain in non-muscle lineages. Conversely, inhibiting Ci-MRF activity with antisense morpholinos down-regulated the expression of these genes. Consistent with the effects of morpholinos on muscle gene activity, larvae resulting from morpholino injection were paralyzed and their "muscle" cells lacked myofibrils. We conclude that Ci-MRF is required for larval tail muscle development and thus that an MRF-dependent myogenic regulatory network probably existed in the ancestor of tunicates and vertebrates. This possibility raises the question of whether the earliest myogenic regulatory networks were MRF-dependent or MRF-independent.

A Zn-finger/FH2-domain containing protein, FOZI-1, acts redundantly with CeMyoD to specify striated body wall muscle fates in theCaenorhabditis eleganspostembryonic mesoderm

Striated muscle development in vertebrates requires the redundant functions of multiple members of the MyoD family. Invertebrates such as Drosophila and Caenorhabditis elegans contain only one MyoD homolog in each organism. Earlier observations suggest that factors outside of the MyoD family might function redundantly with MyoD in striated muscle fate specification in these organisms. However, the identity of these factors has remained elusive. Here, we describe the identification and characterization of FOZI-1, a putative transcription factor that functions redundantly with CeMyoD(HLH-1) in striated body wall muscle (BWM) fate specification in the C. elegans postembryonic mesoderm. fozi-1 encodes a novel nuclear-localized protein with motifs characteristic of both transcription factors and actin-binding proteins. We show that FOZI-1 shares the same expression pattern as CeMyoD in the postembryonic mesodermal lineage, the M lineage, and that fozi-1-null mutants exhibit similar M lineage-null defects to those found in animals lacking CeMyoD in the M lineage (e.g. loss of a fraction of M lineage-derived BWMs). Interestingly, fozi-1-null mutants with a reduced level of CeMyoD lack most, if not all, M lineage-derived BWMs. Our results indicate that FOZI-1 and the Hox factor MAB-5 function redundantly with CeMyoD in the specification of the striated BWM fate in the C. elegans postembryonic mesoderm, implicating a remarkable level of complexity for the production of a simple striated musculature in C. elegans.

Macho-1 functions as transcriptional activator for muscle formation in embryos of the ascidian Halocynthia roretzi

Various kinds of maternal factor that play crucial roles in embryogenesis are present and localized in the ascidian egg cytoplasm. Localized maternal mRNA of the macho-1 gene is a muscle determinant in the embryo of the ascidian Halocynthia roretzi. The macho-1 protein has a zinc-finger domain and accumulates in nuclei, being expected to function as a DNA-binding transcription factor. In the present study, we show that macho-1 is, indeed, a DNA-binding transcriptional activator, and directly or indirectly regulates the expression of six downstream genes. Macho-1 was required and sufficient for expression of the muscle actin, myosin, calcium transporter, myogenic factor, Tbx6, and snail genes, whose expression is initiated in muscle blastomeres at the cleavage stages in normal embryos. Furthermore, when macho-1 conjugated with a transcription-repression domain of Drosophila engrailed (En(R)) was expressed in embryos, it repressed expression of these downstream genes. In contrast, expression of macho-1 fused with a transcription-activation domain of VP16 caused ectopic expression of these muscle genes in non-muscle blastomeres. PCR-assisted binding-site selection and gel-retardation assay showed that macho-1 protein binds to the consensus target sequence (TGGGTGGTC) for GLI/Zic-family proteins, and that three guanine residues with underlines are crucial for the specificity. The 5' promoter region of the muscle actin gene supported expression of the reporter gene only in muscle cells at late stage. By contrast, when the target sequence was added to the promoter, it well reproduced early expression of muscle actin at the cleavage stage, indicating that macho-1 can recognize the target sequence in vivo. In conclusion, the maternal muscle determinant macho-1 functions as a transcription factor that positively regulates gene expression for muscle formation in ascidian embryos.

Decoding cis-regulatory systems in ascidians

Ascidians, or sea squirts, are lower chordates, and share basic gene repertoires and many characteristics, both developmental and physiological, with vertebrates. Therefore, decoding cis-regulatory systems in ascidians will contribute toward elucidating the genetic regulatory systems underlying the developmental and physiological processes of vertebrates. cis-Regulatory DNAs can also be used for tissue-specific genetic manipulation, a powerful tool for studying ascidian development and physiology. Because the ascidian genome is compact compared with vertebrate genomes, both intergenic regions and introns are relatively small in ascidians. Short upstream intergenic regions contain a complete set of cis-regulatory elements for spatially regulated expression of a majority of ascidian genes. These features of the ascidian genome are a great advantage in identifying cis-regulatory sequences and in analyzing their functions. Function of cis-regulatory DNAs has been analyzed for a number of tissue-specific and developmentally regulated genes of ascidians by introducing promoter-reporter fusion constructs into ascidian embryos. The availability of the whole genome sequences of the two Ciona species, Ciona intestinalis and Ciona savignyi, facilitates comparative genomics approaches to identify cis-regulatory DNAs. Recent studies demonstrate that computational methods can help identify cis-regulatory elements in the ascidian genome. This review presents a comprehensive list of ascidian genes whose cis-regulatory regions have been subjected to functional analysis, and highlights the recent advances in bioinformatics and comparative genomics approaches to cis-regulatory systems in ascidians.

Self/nonself recognition in ascidian fertilization: vitelline coat protein HrVC70 is a candidate allorecognition molecule

Ascidians are hermaphrodites releasing sperm and eggs simultaneously, but many species are self-sterile because of a self/nonself-recognition system in spermegg interaction. Here, we show that a 70-kDa vitelline coat protein, HrVC70, consisting of 12 epidermal growth factor-like repeats, plays a key role in self/nonself recognition during ascidian fertilization. We discovered that the amount of HrVC70 of the self-sterile mature oocytes is markedly higher than that of the self-fertile immature oocytes and that the selfsterile mature oocytes become self-fertile by acid treatment, which is able to release the HrVC70 from isolated vitelline coats. In addition, fertilization is strongly inhibited by the pretreatment of sperm with HrVC70 from a different individual, but not from the same individual, and the number of nonself sperm bound to HrVC70-agarose was significantly higher than that of self-sperm. A sequence analysis of HrVC70 disclosed that several amino acid residues in a restricted region are substituted at an individual level, with no identical sequences among the 10 individuals tested. Furthermore, genomic DNA analysis revealed that the epidermal growth factor-like domains correspond to the exons, and each intron is highly conserved among even- and odd-numbered introns, suggesting that multiple gene duplications or amplification of this region might have taken place during evolution. It was also found that diversity in cDNA sequences is derived from genomic DNA polymorphism probably elicited by crossing over and specific nucleotide substitutions. These results indicate that HrVC70 is a candidate allogeneic recognition molecule in the gamete interaction of the ascidian Halocynthia roretzi.

A genomewide survey of developmentally relevant genes in Ciona intestinalis. IX. Genes for muscle structural proteins

Ascidians are simple chordates that are related to, and may resemble, vertebrate ancestors. Comparison of ascidian and vertebrate genomes is expected to provide insight into the molecular genetic basis of chordate/vertebrate evolution. We annotated muscle structural (contractile protein) genes in the completely determined genome sequence of the ascidian Ciona intestinalis, and examined gene expression patterns through extensive EST analysis. Ascidian muscle protein isoform families are generally of similar, or lesser, complexity in comparison with the corresponding vertebrate isoform families, and are based on gene duplication histories and alternative splicing mechanisms that are largely or entirely distinct from those responsible for generating the vertebrate isoforms. Although each of the three ascidian muscle types - larval tail muscle, adult body-wall muscle and heart - expresses a distinct profile of contractile protein isoforms, none of these isoforms are strictly orthologous to the smooth-muscle-specific, fast or slow skeletal muscle-specific, or heart-specific isoforms of vertebrates. Many isoform families showed larval-versus-adult differential expression and in several cases numerous very similar genes were expressed specifically in larval muscle. This may reflect different functional requirements of the locomotor larval muscle as opposed to the non-locomotor muscles of the sessile adult, and/or the biosynthetic demands of extremely rapid larval development.

Differential mesodermal expression of two amphioxus MyoD family members (AmphiMRF1 and AmphiMRF2)

To explore the evolution of myogenic regulatory factors in chordates, we isolated two MyoD family genes (AmphiMRF1 and AmphiMRF2) from amphioxus. AmphiMRF1 is first expressed at the late gastrula in the paraxial mesoderm. As the first somites form, expression is restricted to their myotomal region. In the early larva, expression is strongest in the most anterior and most posterior somites. AmphiMRF2 transcription begins at mid/late gastrula in the paraxial mesoderm, but never spreads into its most anterior region. Through much of the neurula stage, AmphiMRF2 expression is strong in the myotomal region of all somites except the most anterior pair; by late neurula expression is downregulated except in the most posterior somites forming just rostral to the tail bud. These two MRF genes of amphioxus have partly overlapping patterns of mesodermal expression and evidently duplicated independent of the diversification of the vertebrate MRF family.

Specification of developmental fates in ascidian embryos: molecular approach to maternal determinants and signaling molecules

Tadpole larvae of ascidians represent the basic body plan of chordates with a relatively small number and few types of cells. Because of their simplicity, ascidians have been intensively studied. More than a century of research on ascidian embryogenesis has uncovered many cellular and molecular mechanisms responsible for cell fate specification in the early embryo. This review describes recent advances in our understanding of the molecular mechanisms of fate specification mainly uncovered in model ascidian species--Halocynthia roretzi, Ciona intestinalis, and Ciona savignyi. One category of developmentally important molecules represents maternal localized mRNAs that are involved in cell-autonomous processes. In the second category, signaling molecules and downstream transcription factors are involved in inductive cell interactions. Together with genome-wide information, there is a renewed interest in studying ascidian embryos as a fascinating model system for understanding how single-celled eggs develop a highly organized chordate body plan.

Patterning the marginal zone of early ascidian embryos: localized maternal mRNA and inductive interactions

Early animal embryos are patterned by localized egg cytoplasmic factors and cell interactions. In invertebrate chordate ascidians, larval tail muscle originates from the posterior marginal zone of the early embryo. It has recently been demonstrated that maternal macho-1 mRNA encoding transcription factor acts as a localized muscle determinant. Other mesodermal tissues such as notochord and mesenchyme are also derived from the vegetal marginal zone. In contrast, formation of these tissues requires induction from endoderm precursors at the 32-cell stage. FGF-Ras-MAPK signaling is involved in the induction of both tissues. The responsiveness for induction to notochord or mesenchyme depends on the inheritance of localized egg cytoplasmic factors. Previous studies also point to critical roles of directed signaling in polarization of induced cells and in subsequent asymmetric divisions resulting in the formation of two daughter cells with distinct fates. One cell adopts an induced fate, while the other assumes a default fate. A simple model of mesoderm patterning in ascidian embryos is proposed in comparison with that of vertebrates.

Muscle development and lineage-specific expression of CiMDF, the MyoD-family gene of Ciona intestinalis

The expression pattern of CiMDF, the MyoD-family gene of Ciona intestinalis, was analyzed in unmanipulated and microsurgically derived partial embryos. CiMDF encodes two transcripts during development (coding for distinct proteins), the smaller of which, CiMDFa, was detected in maternal RNA. Zygotic activity of CiMDF initiated in cleaving embryos of 32-64 cells. Both CiMDFa and CiMDFb transcripts were detected at this time; however, CiMDFa accumulated more rapidly before declining in abundance such that, by the early tail-formation stage, CiMDFb was more prevalent. Microsurgical isolations of various lineage blastomeres from the eight-cell stage were used to analyze CiMDF expression in the two embryonic lineages that give rise to larval tail muscle-autonomously specified primary cells and conditionally specified secondary cells. CiMDFa and CiMDFb transcripts were detected in both lineages, suggesting that neither functioned in a lineage-specific manner. The data also demonstrated that CiMDF expression was autonomous in the primary lineage (i.e., cells derived from the B4.1 blastomeres) and correlated with histospecific differentiation of muscle. In the secondary lineage (i.e., cells derived from the A4.1 and b4.2 blastomeres), CiMDF expression was conditional and, as in the primary lineage, correlated with muscle differentiation. These experiments reveal similar patterns of CiMDF activity in the primary and secondary muscle lineages and imply a requirement for the expression of this gene in both lineages during larval tail muscle development.

Ascidian embryos as a model system to analyze expression and function of developmental genes

Ascidians have served as an appropriate experimental system in developmental biology for more than a century. The fertilized egg develops quickly into a tadpole larva, which consists of a small number of organs including epidermis, central nervous system with two sensory organs, endoderm and mesenchyme in the trunk, and notochord and muscle in the tail. This configuration of the ascidian tadpole is thought to represent the most simplified and primitive chordate body plan. Their embryogenesis is simple, and lineage of embryonic cells is well documented. The ascidian genome contains a basic set of genes with less redundancy compared to the vertebrate genome. Cloning and characterization of developmental genes indicate that each gene is expressed under discrete spatio-temporal pattern within their lineage. In addition, the use of various molecular techniques in the ascidian embryo system highlights its advantages as a future experimental system to explore the molecular mechanisms underlying the expression and function of developmental genes as well as genetic circuitry responsible for the establishment of the basic chordate body plan. This review is aimed to highlight the recent advances in ascidian embryology.

Loss-of-function mutations reveal that the Drosophila nautilus gene is not essential for embryonic myogenesis or viability

nautilus (nau), the single Drosophila member of the bHLH-containing myogenic regulatory family of genes, is expressed in a subset of muscle precursors and differentiated fibers. It is capable of inducing muscle-specific transcription as well as myogenic transformation, and plays a role in the differentiation of a subset of muscle precursors into mature muscle fibers. In previous studies, the nau zygotic loss-of-function phenotype was determined using genetic deficiencies in which the gene is deleted. We note that this genetic loss-of-function phenotype differs from the loss-of-function phenotype determined using RNA interference (L. Misquitta and B. M. Paterson, 1999, Proc. Natl. Acad. Sci. USA 96, 1451-1456). The present study re-examines this loss-of-function phenotype using EMS-induced mutations that specifically alter the nau gene, and extends the genetic analysis to include the loss of both maternal and zygotic nau function. In brief, embryos lacking nau both maternally and zygotically are missing a distinct subset of muscle fibers, consistent with its apparent expression in a subset of muscle fibers. The muscle loss is tolerated, however, such that the loss of nau both maternally and zygotically does not result in lethality at any stage of development.

Determinants of cell and positional fate in ascidian embryos

Ascidians have played a major role in studies to understand the function of cytoplasmic determinants in animal development. Special qualities, including eggs with colored cytoplasmic regions, an invariant cleavage pattern and cell lineage, embryos with low cell numbers, larvae with typical chordate features and only six different tissues, rapid development, and a small genome, combine to make these animals a unique system for studying cytoplasmic determinants. There is evidence for determinants that specify the cleavage pattern; the differentiation of epidermal, endodermal, and muscle cells; and cell movements associated with gastrulation. The muscle determinants appear to be modified in concert with tail and muscle regression in species that have evolved an anural, or tailless, larva. Several lines of evidence suggest that determinants may be localized maternal mRNAs, which encode transcription factors or signal transduction components responsible for initiating differential gene activity. Different approaches and strategies are being used to isolate and characterize the function of these localized maternal mRNAs.

RNA interference demonstrates a role for nautilus in the myogenic conversion of Schneider cells by daughterless

Schneider SL2 cells activate the myogenic program in response to the ectopic expression of daughterless alone, as indicated by exit from the cell cycle, syncytia formation, and the presence of muscle myosin fibrils. Myogenic conversion can be potentiated by the coexpression of DMEF2 and nautilus with daughterless. In RT-PCR assays Schneider cells express two mesodermal markers, nautilus and DMEF2 mRNAs, as well as very low levels of daughterless mRNA but no twist. Full-length RT-PCR products for nautilus and DMEF2 encode immunoprecipitable proteins. We used RNA-i to demonstrate that both endogenous nautilus expression and DMEF2 expression are required for the myogenic conversion of Schneider cells by daughterless. Coexpression of twist blocks conversion by daughterless but twist dsRNA has no effect. Our results indicate that Schneider cells are of mesodermal origin and that myogenic conversion with ectopic expression of daughterless occurs by raising the levels of daughterless protein sufficiently to allow the formation of nautilus/daughterless heterodimers. The effectiveness of RNA-i is dependent upon protein half-life. Genes encoding proteins with relatively short half-lives (10 h), such as nautilus or HSF, are efficiently silenced, whereas more stable proteins, such as cytoplasmic actin or beta-galactosidase, are less amenable to the application of RNA-i. These results support the conclusion that nautilus is a myogenic factor in Drosophila tissue culture cells with a functional role similar to that of vertebrate MyoD. This is discussed with regard to the in vivo functions of nautilus.

Characterization of a novel member of the FGFR family, HrFGFR, in Halocynthia roretzi

The cDNA for a novel member of the FGFR family, named HrFGFR, was isolated from a Halocynthia roretzi cDNA library prepared at the mid-tailbud stage. This cDNA was 3507b long, and the deduced amino acid sequence contained a motif characteristic of the vertebrate FGFRs. The existence of a single copy of the FGFR homologue gene in H. roretzi was suggested by restriction site analysis of multiple clones. HrFGFR mRNA was expressed strongly in the posterior region in the epidermis from the middle neurula stage. By contrast, Xenopus FGFR homologues are expressed in the anterior region and are known to induce anterior neural formation. A transition of the region expressing FGFR might have induced the more complicated brain or head formation characteristic of vertebrates.

Genetics of muscle determination and development

Skeletal muscles in vertebrates develop from somites as the result of patterning and cell type specification events. Here, we review the current knowledge of genes and signals implicated in these processes. We discuss in particular the role of the myogenic determination genes as deduced from targeted gene disruptions in mice and how their expression may be controlled. We also refer to other transcription factors which collaborate with the myogenic regulators in positive or negative ways to control myogenesis. Moreover, we review experiments that demonstrate the influence of tissues surrounding the somites on the process of muscle formation and provide model views on the underlying mechanisms. Finally, we present recent evidence on genes that play a role in regeneration of muscle in adult organisms.

Developmental gene activities in ascidian embryos

The fertilized egg of ascidians develops quickly into a tadpole-type larva consisting of several distinct types of tissues including epidermis, central nervous system, endoderm, mesenchyme, notochord, and muscle. This architecture of the ascidian larva represents the most simplified chordate body plan. Taking advantage of simple, well-defined cell lineages, the expression of developmental genes is analyzed at single-cell level. Advances in the methodology promote the ascidian embryo as a useful system for studying transcriptional control involved in the specification of embryonic cells and pattern formation of the embryo.

Evolutionary conservation of MyoD function and differential utilization of E proteins

The formation of striated muscle in both vertebrates and invertebrates involves the activity of the MyoD family of basic-helix-loop-helix (bHLH) transcription factors. The high degree of evolutionary conservation of MyoD-related proteins, both in the sequence of their bHLH domains and in their general developmental expression patterns, suggests that these factors are also conserved at the level of function. We have addressed this directly using MyoD and E protein factors from vertebrates, Drosophila, and Caenorhabditis elegans. Various MyoD and E factor combinations were tested for their ability to interact in vitro and to function in vivo in the myogenic conversion of 10T12 mouse fibroblasts. We found that the ability of different homo- and heterodimers to bind DNA in vitro was an accurate measure of biological activity in vivo. A second assessment of conserved function comes from the ability of these factors to rescue a C. elegans hlh-1 (CeMyoD) null mutation. We found that both Drosophila and chicken MyoD-related factors were able to rescue a C. elegans CeMyoD loss-of-function mutation. These results demonstrate a remarkable degree of functional conservation of these myogenic factors despite differences in E-protein interactions.

A role for nautilus in the differentiation of muscle precursors

In the Drosophila embryo, nautilus is expressed in a subset of muscle precursors and differentiated fibers and is capable of inducing muscle-specific transcription, as well as myogenic transformation. In this study, we examine the consequences of nautilus loss-of-function on the development of the somatic musculature. Genetic and molecular characterization of two overlapping deficiencies, Df(3R)nau-9 and Df(3R)nau-11a4, revealed that both of these deficiencies remove the nautilus gene without affecting a common lethal complementation group. Individuals transheterozygous for these deficiencies survive to adulthood, indicating that nautilus is not an essential gene. These embryos are, however, missing a subset of muscle fibers, providing evidence that (1) some muscle loss can be tolerated throughout larval development and (2) nautilus does play a role in muscle development. Examination of muscle precursors in these embryos revealed that nautilus is not required for the formation of muscle precursors, but rather plays a role in their differentiation into mature muscle fibers. Thus, we suggest that nautilus functions in a subset of muscle precursors to implement their specific differentiation programs.

Ascidian embryogenesis and the origins of the chordate body plan

For more than a century, ascidians have been a widely used system for classic embryological studies. Ascidians possess simple, well-defined cell-lineages, compact genomes, rapid development and world-wide distribution. Transgenic DNA can be introduced into developing embryos using simple electroporation methods. The ascidian larva represents the most simplified chordate body plan and provides a useful model for studying the molecular pathways underlying the morphogenesis and differentiation of the notochord and neural tube.

The generation and interpretation of positional information within the vertebrate myotome

How somitic cells become restricted to the muscle fate has been investigated on a number of levels. Classical embryological manipulations have attempted to define the source of inductive signals that control the formation of the myotome. Recently, these studies have converged with others dissecting the role of secreted proteins in embryonic patterning to demonstrate a role for specific peptides in inducing individual cell types of the myotome. Collectively, these investigations have implicated the products of the Wnt, Hedgehog (Hh) and Bone morphogenetic protein (Bmp) gene families as key myogenic regulators; simultaneously controlling both the initiation of myogenesis and the fate of individual myoblasts.

Tissue-specific alternative splicing of ascidian troponin I isoforms. Redesign of a protein isoform-generating mechanism during chordate evolution

In vertebrates, troponin I (TnI) exists as shorter and longer isoforms encoded by distinct genes expressed in skeletal and cardiac muscle, respectively. We report that the protochordate ascidian Ciona intestinalis expresses a homologous set of shorter and longer TnI isoforms in body wall muscle and heart, respectively. The heart-specific segment of the ascidian longer TnI isoform shares several sequence features with vertebrate cardiac TnI but lacks the protein kinase A phosphorylation sites implicated in sympatho-adrenal control of cardiac function. In contrast with vertebrates, the ascidian longer and shorter TnI isoforms are produced from a single gene by tissue-specific alternative RNA splicing; remarkably, the molecular mechanism of TnI isoform generation has been entirely reworked during ascidian/vertebrate evolution. Because alternative splicing is the more probable chordate ancestral condition, the long/cardiac versus short/somatic muscle pattern of TnI isoforms likely existed before the occurrence of the gene duplication events that created the vertebrate TnI gene family. Thus, gene duplication was apparently not the primary engine of isoform diversity in this aspect of TnI gene family evolution; rather, it simply provided an alternative (transcriptional) means of maintaining a previously established system of isoform diversity and tissue specificity based on alternative RNA splicing.

Drosophila myogenesis and insights into the role of nautilus

Several aspects of muscle development appear to be conserved between Drosophila and vertebrate organisms. Among these is the conservation of genes that are critical to the myogenic process, including transcription factors such as nautilus. From a simplistic point of view, Drosophila therefore seems to be a useful organism for the identification of molecules that are essential for myogenesis in both Drosophila and in other species. nautilus, the focal point of this review, appears to be involved in the specification and/or differentiation of a specific subset of muscle founder cells. As with several of its vertebrate and invertebrate counterparts, it is capable of inducing a myogenic program of differentiation reminiscent of that of somatic muscle precursors when expressed in other cell types. We therefore favor the model that nautilus implements the specific differentiation program of these founder cells, rather than their specification. Further analyses are necessary to establish the validity of this working hypothesis. Studies have revealed a critical role for Pax-3 in specifying a particular subset of myogenic cells, the progenitors of the limb muscles. These myogenic cells migrate from the somite into the periphery of the organism, where they differentiate. These myoblasts do not express MyoD or myf5 until they have arrived at their destination and begin the morphologic process of myogenesis (Bober et al., 1994; Goulding et al., 1994; Williams and Ordahl, 1994). They then begin to express these genes, possibly to put the myogenic plan into action. Thus, as with nautilus, MyoD and myf5 may be necessary for the manifestation of a muscle-specific commitment that has already occurred. By comparison with vertebrates, it was anticipated that the single Drosophila gene would serve the purpose of all four vertebrate genes. However, its restricted pattern of expression and apparent loss-of-function phenotype are inconsistent with this expectation. It remains to be determined whether nautilus functions in a manner similar to just one of the vertebrate genes. Since the myf5- and MyoD-expressing myoblasts are proliferative, the loss of one cell type appears to be compensated by proliferation of the remaining cell type. This apparent plasticity may obscure differences in mutant phenotype resulting from the loss of particular cells that express each of these genes. In Drosophila, by comparison, nautilus-expressing cells committed to the myogenic program undergo few, if any, additional cell divisions, and thus no other cells are available to compensate for the loss of nautilus. Therefore, the apparent differences between the Drosophila nautilus gene and its vertebrate counterparts may reflect, at least in part, differences in the developmental systems rather than differences in the function of the genes themselves.

Cell fate specification by localized cytoplasmic determinants and cell interactions in ascidian embryos

Tadpole larvae of ascidians show the basic body plan of chordates. An ascidian larva consists of only a few types of cells and has a relatively small number of cells. Cell lineages are invariant among individuals and have been described in detail. These advantages facilitate the analysis of how the fate of each blastomere becomes specified during development. Over a century of research on ascidian embryogenesis has uncovered many interesting features concerning cellular mechanisms responsible for the fate specification. During embryogenesis, the developmental fate of a blastomere is specified by one of three different mechanisms: localized maternal cytoplasmic determinants, inductive interactions, or lateral inhibition in an equivalence cell group.

Duplication of an amphioxus myogenic bHLH gene is independent of vertebrate myogenic bHLH gene duplication

Gene duplication is thought to be a major genetic change that may have permitted the evolution of vertebrates from invertebrates. The myogenic genes encode basic helix-loop-helix (bHLH) transcriptional factors essential for the formation of skeletal muscle. The invertebrate genome contains only a single myogenic bHLH gene, whereas the vertebrate genome contains four (MyoD, Myf-5, myogenin and MRF4). Since the tunicate genome contains a single myogenic bHLH gene, its duplication might have occurred some time during chordate evolution. To determine whether the duplication of the myogenic bHLH gene occurred prior to, or after the divergence of vertebrates from the cephalochordate lineage, we amplified target fragments from the amphioxus, Branchiostoma floridae, by means of PCR. Sequence analysis and genomic Southern analysis revealed that the amphioxus genome contains two myogenic bHLH genes (BMD1 and BMD2). A comparison of the amino acid sequences in the bHLH domain between BMD1, BMD2 and four vertebrate myogenic bHLH gene products, however, showed that neither BMD1 nor BMD2 resembled any of the four genes. These results suggested that the duplication of amphioxus myogenic bHLH gene occurred independently of that leading to the four myogenic bHLH genes in vertebrates.

Chasing tails in ascidians: developmental insights into the origin and evolution of chordates

The ascidian tadpole larva is regarded as a prototype of the ancestral chordate. Here we consider recent studies on the development of the tadpole larva that provide new insights into chordate origins and evolution. The notochord of ascidian larvae and vertebrates appear to be homologous structures based on their induction by endoderm and expression of the Brachyury (T) gene. The muscle cells of ascidian larvae also appear homologous to those of vertebrates based on their expression of bHLH myogenic and muscle-type actin genes, although they are specified by cytoplasmic determinants localized in the egg as well as embryonic induction. Studies of the tailless larvae of anural ascidians have resulted in the identification of Manx, a gene that may control tail development and evolution. These and other results support the ascidian tadpole prototype for the ancestral chordate.

MyoD and myogenesis in C. elegans

One of the goals in developmental biology is the identification of key regulatory genes that govern the transition of embryonic cells from a pluripotent potential to a specific, committed cell fate. During vertebrate skeletal myogenesis, this transition is regulated by the MyoD family of genes. C. elegans has muscle analogous to vertebrate skeletal muscle and has a gene (hlh-1) related to the MyoD family. The molecular and genetic characterization of hlh-1 shows that it is very similar to the vertebrate MyoD family in many respects, including its expression pattern and DNA binding activity. The hlh-1 product is required for proper myogenesis, but it is not required for myogenic commitment during embryogenesis in the nematode. The role of this MyoD-related gene in nematode myogenesis is discussed and compared to those of the vertebrate MyoD family.