Spatially and temporally regulated expression of the LIM class homeobox gene Hrlim suggests multiple distinct functions in development of the ascidian, Halocynthia roretzi

Localization of mitochondrial large ribosomal RNA in the myoplasm of the early ascidian embryo

Mitochondrial large ribosomal RNA (mtlrRNA) is transferred out of mitochondria and associates with germinal granules in Drosophila and Xenopus embryos. It has been revealed that mtlrRNA outside of mitochondria is required for formation of the germ-line progenitor, or pole cells in Drosophila. In the present study, the distribution of mtlrRNA was examined in embryos of the ascidian, Halocynthia roretzi, during cleavage stages by whole-mount in situ hybridization. Until the 4-cell stage, the distribution of mtlrRNA coincided with that of mitochondria. which are localized to the cortical cytoplasm in the posterior region of the embryos. Both mitochondria and mtlrRNA were preferentially partitioned into muscle-lineage blastomeres during cleavage stages. After the 8-cell stage, a discrepancy in intracellular localization of mitochondria and mtlrRNA became evident. Mitochondria translocated into central yolkless cytoplasm, while mtlrRNA remained in the posterior cortex in the posterior muscle-lineage b astomeres. The significance of the cortical localization of mtlrRNA in muscle precursor cells in ascidian embryos is obscure. However, the results suggest that mtlrRNA is also transferred out of mitochondria in early ascidian embryos and may play some roles in developmental processes.

Cloning and embryonic expression of Hrsna, a snail family gene of the ascidian Halocynthia roretzi: implication in the origins of mechanisms for mesoderm specification and body axis formation in chordates

snail family genes encode a transcription factor with specific zinc finger motifs. In vertebrates, they are expressed in the entire mesoderm in early embryogenesis and later in the paraxial mesoderm and the tail-bud, suggesting roles in specification and morphogenesis of the paraxial mesoderm. In the present study, a snail family gene Hrsna from a member of the chordates, an ascidian (Halocynthia roretzi), was cloned to obtain an insight into the origin of the mechanisms of mesoderm specification and body axis formation as observed in vertebrates. Expression of Hrsna during ascidian embryogenesis was found to be quite similar to that of vertebrate snail genes. First, before gastrulation, Hrsna was initially expressed in most precursors of mesodermal tissues including the notochord where As-T, the ascidian homolog of brachyury, was expressed. Hrsna expression persisted in the paraxial mesoderm, the mesenchyme and muscle, but not in the notochord precursors. Also, just as vertebrate snail family genes are expressed in the border of the neural plate that develops into dorsal neural tube and neural crest cells, so Hrsna expression was detected in the precursors of lateral and dorsal regions of the neural tube. However, Hrsna expression was not detected in the tip of the tail, unlike in vertebrate counterparts. In the light of the present findings, similarity and dissimilarity of mechanisms governing mesoderm specification and body axis formation between ascidians and vertebrates are discussed.

Retinoic acid affects patterning along the anterior-posterior axis of the ascidian embryo

Because retinoic acid (RA) is known to affect anterior posterior patterning in vertebrate embryos, it was questioned whether it shows similar effects in a more primitive chordate, the ascidian Halocynthia roretzi. Ascidian embryos treated with RA exhibited truncated phenotypes in a dose-dependent manner similar to the anterior truncations seen in vertebrate embryos. The most severely affected larvae possessed a round trunk without the papillae characteristic of the anterior terminal epidermis. Retinoic acid also altered the expression of HrHox-1 and Hroth in a dose-dependent manner. Expression of HrHox-1 increased, whereas expression of Hroth decreased with increasing levels of RA. In treated embryos, HrHox-1 was first expressed pan-ectodermally, then degraded in all but specific regions of the embryo. By contrast, initiation of Hroth expression was not affected, but epidermal expression was lost while expression in the neural tube narrowed toward the anterior in tail-bud embryos. These alterations in the expression of homeobox genes appear to correlate closely to the morphological defects elicited by RA treatment, suggesting broad conservation of developmental patterning mechanisms within the Phylum Chordata.

Tripartite organization of the ancestral chordate brain and the antiquity of placodes: insights from ascidian Pax-2/5/8, Hox and Otx genes

Ascidians and vertebrates belong to the Phylum Chordata and both have dorsal tubular central nervous systems. The structure of the ascidian neural tube is extremely simple, containing less than 400 cells, among which less than 100 cells are neurons. Recent studies suggest that, despite its simple organization, the mechanisms patterning the ascidian neural tube are similar to those of the more complex vertebrate brain. Identification of homologous regions between vertebrate and ascidian nervous systems, however, remains to be resolved. Here we report the expression of HrPax-258 gene: an ascidian homologue of vertebrate Pax-2, Pax-5 and Pax-8 genes. Molecular phylogenetic analyses indicate that HrPax-258 is descendant from a single precursor gene that gave rise to the three vertebrate genes. The expression pattern of HrPax-258 suggests that this subfamily of Pax genes has conserved roles in regional specification of the brain. Comparison with expression of ascidian Otx (Hroth) and a Hox gene (HrHox1) by double-staining in situ hybridizations indicate that the ascidian brain region can be subdivided into three regions; the anterior region marked by Hroth probably homologous to the vertebrate forebrain and midbrain, the middle region marked by HrPax-258 probably homologous to the vertebrate anterior hindbrain (and maybe also midbrain) and the posterior region marked by Hox genes which is homologous to the vertebrate hindbrain and spinal cord. Later expression of HrPax-258 in atrial primordia implies that basal chordates such as ascidians have already acquired a sensory organ that develops from epidermal thickenings (placodes) and expresses HrPax-258; we suggest it is homologous to the vertebrate ear. Therefore, placodes are not likely to be a newly acquired feature in vertebrates, but may have already been possessed by the earliest chordates.

Expression of a twist-related gene, Bbtwist, during the development of a lancelet species and its relation to cephalochordate anterior structures

Mesoderm formation plays a crucial role in the establishment of the chordate body plan. In this regard, lancelet embryos develop structures such as the anteriorly extended notochord and the lateral divertecula in their anterior body. To elucidate the developmental basis of these structures, we examined the expression pattern of a lancelet twist-related gene, Bbtwist, from the late gastrula to larval stages. In late-gastrula embryos, the transcripts of Bbtwist were detected in the presumptive first pair of somites and the middorsal wall of the primitive gut. The expression of Bbtwist was then upregulated in the lateral wall of somites and the notochord. At the late-neurula stage, it was also expressed in the anterior wall of the primitive gut, as well as in the evaginating lateral diverticula. No signal was detected in the left lateral diverticulum when it was separated from the gut, while in the right one, the gene was expressed later during the formation of the head coelom in knife-shaped larvae, and in the anterior part of the notochord in the same larvae. In 36-h larvae, only faint expression was detected in the differentiating notochordal and paraxial mesoderm in the caudal region. These expression patterns suggest that Bbtwist is involved in early differentiation of mesodermal subsets as seen in Drosophila and vertebrates. The expression in the anterior notochord may be related to its anterior expansion. The expression in the anterior wall of the primitive gut and its derivative, the lateral diverticula, suggests that lancelets share the capability to produce a mesodermal population from the tip of the primitive gut with nonchordate deuterostome embryos.

DeLIMiting development

LIM-homeodomain transcription factors (LIM-HD) regulate expression of genes that pattern the body and generate cell specificity during development in invertebrates and vertebrates. It is especially interesting that most are expressed in and participate in the development of the nervous system. LIM-HD proteins are themselves regulated by both intramolecular and intermolecular interactions mediated by the LIM domains. LIM domains positively regulate LIM-HD activity by promoting protein-protein interactions that allow cooperative binding to regulatory regions of tissue-specific promoters. They also negatively regulate LIM-HD activity, possibly by preventing HD association with DNA. Interaction of LIM domains with other proteins relieves this interference, permitting DNA binding and providing a mechanism for refining LIM-HD activity. The recurrence of LIM-HD proteins in fundamental developmental processes emphasizes the importance of their function and regulation and provides an opportunity to identify mechanisms and molecules underlying patterning and cell specification.

Distinct neuronal lineages of the ascidian embryo revealed by expression of a sodium channel gene

The ascidian larva contains tubular neural tissue, one of the prominent anatomical features of the chordates. The cell-cleavage pattern and cell maps of the nervous system have been described in the ascidian larva in great detail. Cell types in the neural tube, however, have not yet been defined due to the lack of a suitable molecular marker. In the present work, we identified neuronal cells in the caudal neural tube of the Halocynthia embryo by utilizing a voltage-gated Na+ channel gene, TuNa I, as a molecular marker. Microinjection of a lineage tracer revealed that TuNa I-positive neurons in the brain and in the trunk epidermis are derived from the a-line of the eight-cell embryo, which includes cell fates to epidermal and neural tissue. On the other hand, TuNa I-positive cells in the more caudal part of the neural tissue were not stained by microinjection into the a-line. These neurons are derived from the A-line, which contains fates of notochord and muscle, but not of epidermis. Electron microscopic observation confirmed that A-line-derived neurons consist of motor neurons innervating the dorsal and ventral muscle cells. Isolated A-line blastomeres have active membrane excitability distinct from those of the a-line-derived neuronal cells after culture under cleavage arrest, suggesting that the A-line gives rise to a neuronal cell distinct from that of the a-lineage. TuNa I expression in the a-line requires signals from another cell lineage, whereas that in the A-line occurs without tight cell contact. Thus, there are at least two distinct neuronal lineages with distinct cellular behaviors in the ascidian larva: the a-line gives rise to numerous neuronal cells, including sensory cells, controlled by a mechanism similar to vertebrate neural induction, whereas A-line cells give rise to motor neurons and ependymal cells in the caudal neural tube that develop in close association with the notochord or muscle lineage, but not with the epidermal lineage.

Neural tube is partially dorsalized by overexpression of HrPax-37: the ascidian homologue of Pax-3 and Pax-7

The origin and elaboration of the central nervous system played an important role in chordate and vertebrate history. All chordates possess a dorsal tubular central nervous system, but elaboration of dorsoventral and segmental pattern is far more pronounced in cephalochordates and vertebrates than in the more basal urochordates. Analysis of the urochordates, therefore, should allow deduction of the neural organization and neuronal patterning mechanisms that predated overt dorsoventral and segmental complexity. Here we report functional studies of the ascidian Pax gene (HrPax-37). The spatiotemporal expression pattern of HrPax-37 has suggested involvement in two distinct developmental processes: specification of dorsal cell fates of ectoderm during neurulation, and regional differentiation of the neural tube in later stages. Here we show that HrPax-37 is descendent from the precursor of the Pax-3 and Pax-7 genes implicated in specification of dorsal fate in the vertebrate neural tube. We also demonstrate that injection of HrPax-37 RNA into fertilized eggs causes ectopic expression of the dorsal neural marker tyrosinase gene in neurulae, confirming a regulatory role in dorsal patterning of the neural tube comparable to its vertebrate homologues. These results suggest that dorsal specification in the neural tube by Pax-3/7 subfamily genes was established in the ancestors of extant chordates during emergence of the dorsal tubular nervous system.

Conservation of Pax-6 in a lower chordate, the ascidian Phallusia mammillata

The Pax-6 genes of vertebrates and invertebrates encode transcription factors with both a paired domain and a homeodomain. They are expressed in the developing eye and in the central nervous system. Loss-of-function mutations in mammals and in flies result in a reduction or absence of eyes and targeted expression of the Drosophila and the mouse Pax-6 genes induces ectopic eye structures in Drosophila. These findings lead to the proposal that the morphogenesis of the different types of eyes is controlled by a Pax-6-dependent genetic pathway and that the various eye types are of monophyletic origin. We have isolated a Pax-6 homologous gene from the ascidian Phallusia mammillata, because ascidians occupy an important position in early chordate evolution. Furthermore, the Phallusia larva has a simple photosensitive ocellus. Phallusia Pax-6 shares extensive sequence identity and conserved genomic organization with the known Pax-6 genes of vertebrates and invertebrates. Expression of Phallusia Pax-6 is first detected at late gastrula stages in distinct regions of the developing neural plate. At the tailbud stage, it is expressed in the spinal cord and the brain vesicle, where the sensory organs (ocellus and otolith) form, suggesting an important function in their development. Ectopic expression of the ascidian Pax-6 gene in Drosophila leads to the induction of supernumerary eyes indicating a highly conserved gene regulatory function for Pax-6 genes.

Hroth an orthodenticle-related homeobox gene of the ascidian, Halocynthia roretzi: its expression and putative roles in the axis formation during embryogenesis

To obtain insight into the axis-forming mechanism in ascidian embryogenesis, Hroth, an ascidian counterpart of orthodenticle/otx, was isolated from Halocynthia roretzi and its expression in embryogenesis was examined by whole mount in situ hybridization. It was revealed that Hroth is expressed in both involuting mesoendoderm and anterior ectoderm during gastrulation while later expression is restricted to the sensory vesicle and anterior epidermis. Expression pattern of Hroth around gastrulation was compared with that of Hrlim, the ascidian LIM class homeobox gene that is known to be expressed during gastrulation. In the light of the present findings on the expression of Hroth, properties of the axis-forming mechanism in ascidian embryogenesis are discussed.

An ascidian homologue of vertebrate BMPs-5-8 is expressed in the midline of the anterior neuroectoderm and in the midline of the ventral epidermis of the embryo

The ascidian tadpole larva is thought to be the prototype for the ancestral chordate. Although ascidians show a highly determinate mode of development, recent studies suggest significant roles of cell-cell interaction during embryogenesis. To elucidate the signaling molecules responsible for the cellular interaction, we investigated an ascidian homologue of the transforming growth factor beta (TGF-beta) superfamily. HrBMPa is an ascidian member of the 60A subclass of the BMP subfamily. Molecular phylogenetic analysis suggested that HrBMPa branched prior to further divergence of vertebrate BMPs-5-8. The zygotic expression of HrBMPa was initiated around gastrulation. HrBMPa transcripts were first evident in precursor cells of the spinal cord, notochord, epidermis and nervous system, although signals in the first two regions quickly disappeared. In neurulae and early tailbud embryos, transcripts were evident in the adhesive organ, midline of the anterior dorsal neuroectoderm and midline of both ventral and dorsal ectoderm, suggesting that HrBMPa plays a major role in neuroectodermal cell differentiation during embryogenesis. This HrBMPa expression profile resembled that of Xenopus BMP-7, implying a primordial function of BMP-7 among vertebrate BMPs-5-8.

Homeobox genes exhibit evolutionary conserved regionalization in the central nervous system of an ascidian larva

Animals in each subgroup of the phylum Chordata exhibit a similar process by which they form a tubular central nervous system (CNS). However, little is known about spatial relationship among the CNSs of chordates; vertebrates, cephalochordates and urochordates (tunicates). Ascidians constitute a major animal group in the subphylum Urochordata. In the present study, we examined the expression patterns of labial and orthodenticle related genes of the ascidian, Halocynthia roretzi, in the developing larval CNS. These homeobox genes exhibited region-specific expression patterns that are strikingly similar to those of murine Hoxb-1 and Otx2. The regionalization as characterized by the expression of these genes supports the division of the ascidian larval CNS suggested by the previous morphological studies. Furthermore, conservation of the expression pattern of the homeobox genes suggests that such regionalization occurred in the CNS of a putative common ancestor of chordates.