More constraint on ParaHox than Hox gene families in early metazoan evolution.

Hox and ParaHox (H/P) genes belong to evolutionary-sister clusters that arose through duplication of a ProtoHOX cluster early in animal evolution. In contrast to bilaterians, cnidarians express, beside PG1, PG2 and Gsx orthologs, numerous Hox-related genes with unclear origin. We characterized from marine hydrozoans three novel Hox-related genes expressed at medusa and polyp stages, which include a Pdx/Xlox ParaHox ortholog induced 1 day later than Gsx during embryonic development. To reconstruct H/P genes' early evolution, we performed multiple systematic comparative phylogenetic analyses, which identified derived sequences that blur the phylogenetic picture, recorded dramatically different evolutionary rates between ParaHox and Hox in cnidarians and showed the unexpected grouping of [Gsx-Pdx/Xlox-PG2-PG3] families in a single metagroup distinct from PG1. We propose a novel more parsimonious evolutionary scenario whereby H/P genes originated from a [Gsx-Pdx/Xlox-PG2-PG3]-related ProtoHox gene, the "posterior" and "anterior" H/P genes appearing secondarily. The ProtoHOX cluster would have contained the three Gsx/PG2, Pdx/PG3, Cdx/PG9 paralogs and produced through tandem duplication the primordial HOX and ParaHOX clusters in the Cnidaria-Bilateria ancestor. The stronger constraint on cnidarian ParaHox genes suggests that the primary function of pre-bilaterian H/P genes was to drive cellular evolutionary novelties such as neurogenesis rather than axis specification.

BMP2/4 and BMP5-8 in jellyfish development and transdifferentiation.

Bone morphogenetic proteins (BMPs) have key roles in gastrulation, mesoderm induction and axial patterning. The multitude of bilaterian BMPs employed in these morphogenetic processes contrasts starkly with the scarcity of BMPs in Cnidaria, the most basal eumetazoan phylum. In coral, sea anemone and hydra species, BMPs have been found to be associated with larval and polyp axial patterning. In the hydrozoan jellyfish Podocoryne (Hydractinia) carnea the BMP2/4 and BMP5-8 genes are expressed unilaterally in the larva, corroborating a possible role in larval axial development. With the focal area of BMP expression in the anterior region, however, the jellyfish larva may have a developmental reversal of spatial polarity compared to the anthozoan larva. In medusa development, BMP genes are expressed in divergent expression territories within the presumptive radial canals and in various parts of the endoderm, indicative of an involvement in mesoderm patterning and gastrovascular system formation reminiscent of bilaterian BMP functions. In addition, the BMP2/4 and BMP5-8 genes may play roles in wound response and dedifferentiation or S-phase re-entry, respectively, as the former is expressed in striated muscle cells immediately after excision from the bell and the latter in the initial phase of muscle cell transdifferentiation.

The homeobox gene Msx in development and transdifferentiation of jellyfish striated muscle.

Bilaterian Msx homeobox genes are generally expressed in areas of cell proliferation and in association with multipotent progenitor cells. Likewise, jellyfish Msx is expressed in progenitor cells of the developing entocodon, a cell layer giving rise to the striated and smooth muscles of the medusa. However, in contrast to the bilaterian homologs, Msx gene expression is maintained at high levels in the differentiated striated muscle of the medusa in vivo and in vitro. This tissue exhibits reprogramming competence. Upon induction, the Msx gene is immediately switched off in the isolated striated muscle undergoing transdifferentiation, to be upregulated again in the emerging smooth muscle cells which, in a stem cell like manner, undergo quantal cell divisions producing two cell types, a proliferating smooth muscle cell and a differentiating nerve cell. This study indicates that the Msx protein may be a key component of the reprogramming machinery responsible for the extraordinary transdifferentation and regeneration potential of striated muscle in the hydrozoan jellyfish.

The germ line and somatic stem cell gene Cniwi in the jellyfish Podocoryne carnea.

In most animal phyla from insects to mammals, there is a clear division of somatic and germ line cells. This is however not the case in plants and some animal phyla including tunicates, flatworms and the basal phylum Cnidaria, where germ stem cells arise de novo from somatic cells. Piwi-like genes represent essential stem cell genes in diverse multicellular organisms. The cnidarian Piwihomolog Cniwiwas cloned from Podocoryne carnea, a hydrozoan with a full life cycle. CniwiRNA is present in all developmental stages with highest levels in the egg and the medusa. In the adult medusa, Cniwi expression is prominent in the gonads where it likely functions as a germ stem cell gene. The gene is also expressed, albeit at low levels, in differentiated somatic cells like the striated muscle of the medusa. Isolated striated muscle cells can be induced to transdifferentiate into smooth muscle cells which proliferate and differentiate into nerve cells. Cniwi expression is upregulated transiently after induction of transdifferentiation and again when the emerging smooth muscle cells proliferate and differentiate. The continuous low-level expression of an inducible stem cell gene in differentiated somatic cells may underlie the ability to form medusa buds from polyp cells and explain the extraordinary transdifferentation and regeneration potential of Podocoryne carnea.

Developmental and evolutionary aspects of the basic helix-loop-helix transcription factors Atonal-like 1 and Achaete-scute homolog 2 in the jellyfish.

The close functional link of nerve and muscle cells in neuromuscular units has led to the hypothesis of a common evolutionary origin of both cell types. Jellyfish are well suited to evaluate this theory since they represent the most basal extant organisms featuring both striated muscle and a nervous system. Here we describe the structure and expression of two novel genes for basic helix-loop-helix (bHLH) transcription factors, the Achaete-scute B family member Ash2 and the Atonal-like gene Atl1, in the hydrozoan jellyfish Podocoryne carnea. Ash2 is expressed exclusively in larval and adult endoderm cells and may be involved in differentiation of secretory cells. Atl1 expression is more widespread and includes the developing striated muscle as well as mechanosensory and nerve cell precursors in the medusa tentacles. Moreover, Atl1 expression is upregulated in proliferating nerve cell precursors arising from adult striated muscle cells by transdifferentiation in vitro. Likewise, the neuronal marker gene NP coding for the RFamide neuropeptide is expressed not only in mature nerve cells but also transiently in the developing muscle. The molecular evidence is concurrent to the hypothesis that muscle and nerve cells are closely linked in evolution and derive from a common myoepithelial precursor.

nanos expression at the embryonic posterior pole and the medusa phase in the hydrozoan Podocoryne carnea.

Summary The distinction between soma and germline is an important process in the development of animals with sexual reproduction. It is regulated by a number of germline-specific genes, most of which appear conserved in evolution and therefore can be used to study the formation of the germline in diverged animal groups. Here we report the isolation of two orthologs of one such gene, nanos (nos), in the cnidarian Podocoryne carnea, a species with representative zoological features among the hydrozoans. By studying nos gene expression throughout the Podocoryne biphasic life cycle, we find that the germline differentiates exclusively during medusa development, whereas the polyp does not contribute to the process. An early widespread nos expression in developing medusae progressively refines into a mainly germline-specific pattern at terminal stages of medusa formation. Thus, the distinction between germline and soma is a late event in hydrozoan development. Also, we show that the formation of the medusa is a de novo process that relies on active local cell proliferation and differentiation of novel cell and tissue types not present in the polyp, including nos-expressing cells. Finally, we find nos expression at the posterior pole of Podocoryne developing embryos, not related to germline formation. This second aspect of nos expression is also found in Drosophila, where nos functions as a posterior determinant essential for the formation of the fly abdomen. This raises the possibility that nos embryonic expression could play a role in establishing axial polarity in cnidarians.

Homologs of vascular endothelial growth factor and receptor, VEGF and VEGFR, in the jellyfish Podocoryne carnea.

Vascular endothelial growth factors (VEGF) are the major inducers of vasculogenesis and angiogenesis in vertebrates. Their effects are mediated by receptor tyrosine kinases of the VEGF receptor (VEGFR) family located on endothelial cells and include stimulation of cell survival, proliferation, migration, and tube formation as well as regulation of vascular permeability. Here, we report the presence of VEGF and VEGFR homologous genes in a basal invertebrate of the phylum Cnidaria. The marine jellyfish Podocoryne carnea features a gastrovascular system consisting of the feeding organ, or manubrium, the radial and ring canals, and the tentacle bulbs. Expression analysis indicates that both genes are involved in tentacle and gastrovascular canal formation, indicating an early recruitment of the VEGF signalling pathway for morphogenetic processes leading to tube formation in metazoans. The evolutionary origin of the VEGF signalling pathway resides in the common ancestor of the Cnidaria and Bilateria.

Basic leucine zipper transcription factors C/EBP and MafL in the hydrozoan jellyfish Podocoryne carnea.

Members of the CCAAT/enhancer binding protein (C/EBP) and the Maf protein subfamilies have been characterized in a variety of bilaterian organisms. This is the first report of C/EBP and MafL genes in a basal organism, the hydrozoan jellyfish Podocoryne carnea. Transcripts of both genes are present in all life cycle stages: egg, embryo, larva, polyp, and medusa. During early development, both factors appear to regulate metamorphosis of the larva to the primary polyp. Both genes are also expressed in the striated muscle of the developing and adult medusa. During in vitro transdifferentiation of striated muscle cells to smooth muscle and nerve cells, C/EBP is continuously expressed, whereas MafL expression is turned off during transdifferentiation and reactivated when nerve cells differentiate. Thus, both factors may be involved in muscle and nerve cell differentiation. In the mature medusa both genes are also implicated in gametogenesis. Developmental and evolutionary aspects of the gene structures and expression patterns are discussed.

The Sine oculis/Six class family of homeobox genes in jellyfish with and without eyes: development and eye regeneration.

The development of visual organs is regulated in Bilateria by a network of genes where members of the Six and Pax gene families play a central role. To investigate the molecular aspects of eye evolution, we analyzed the structure and expression patterns of cognate members of the Six family genes in jellyfish (Cnidaria, Hydrozoa), representatives of a basal, non-bilaterian phylum where complex lens eyes with spherical lens, an epidermal cornea, and a retina appear for the first time in evolution. In the jellyfish Cladonema radiatum, a species with well-developed lens eyes in the tentacle bulbs, Six1/2-Cr and Six3/6-Cr, are expressed in the eye cup. Six4/5-Cr is mainly expressed in the manubrium, the feeding, and sex organ. All three Six genes are expressed in different subsets of epidermal nerve cells, possibly of the RFamide type which are part of a net connecting the different eyes with each other and the effector organs. Furthermore, expression is found in other tissues, notably in the striated muscle. During eye regeneration, expression of Six1/2-Cr and Six3/6-Cr is upregulated, but not of Six4/5-Cr. In Podocoryne carnea, a jellyfish without eyes, Six1/2-Pc and Six3/6-Pc are also expressed in the tentacle bulbs, Six1/2-Pc additionally in the manubrium and striated muscle, and Six3/6-Pc in the mechanosensory nematocytes of the tentacle. The conserved gene structure and expression patterns of all Cladonema Six genes suggest broad conservation of upstream regulatory mechanisms in eye development.

Conservation of Brachyury, Mef2, and Snail in the myogenic lineage of jellyfish: a connection to the mesoderm of bilateria.

One major difference between simple metazoans such as cnidarians and all the bilaterian animals is thought to involve the invention of mesoderm. The terms diploblasts and triploblasts are therefore, often used to group prebilaterian and bilaterian animals, respectively. However, jellyfish contain well developed striated and smooth muscle tissues that derive from the entocodon, a mesoderm-like tissue formed during medusa development. We investigated the hypothesis, that the entocodon could be homologous to the third germ layer of bilaterians by analyzing the structures and expression patterns of the homologues of Brachyury, Mef2, and Snail in the jellyfish Podocoryne carnea. These are regulatory genes from the T-box, MADS-box and zinc finger families known to play important roles in bilaterian mesoderm patterning and muscle differentiation. The sequence and expression data demonstrate that the genes are structurally and functionally conserved and even more similar to humans or other deuterostomes than to protostome model organisms such as Drosophila or Caenorhabditis elegans. Based on these data we conclude that the common ancestor of the cnidarians and bilaterians not only shared genes that play a role in regulating myogenesis but already used them to develop and differentiate muscle systems similar to those of triploblasts.

Evolutionary aspects of developmentally regulated helix-loop-helix transcription factors in striated muscle of jellyfish.

The function of basic helix-loop-helix (bHLH) proteins in cell differentiation was shown to be conserved from Drosophila to vertebrates, exemplified by the function of MyoD in striated muscle differentiation. In phylogeny striated muscle tissue appears first in jellyfish and the question of its evolutionary position is controversially discussed. For this reason we have studied the developmental role of myogenic bHLH genes in medusa development. Based on their dimerization ability, four genes of the bHLH family of transcription factors were isolated from the hydrozoan jellyfish Podocoryne carnea. While the proteins Id and Ash group with cognate family members from bilaterians, Net-like and JellyD1 could not be unequivocally classified. Id is expressed during the medusa budding process and in the adult medusa, Ash and Net-like are expressed in all life cycle stages from egg to adult medusa and JellyD1 is expressed in the blastula and gastrula stages, the planula larva, and in late medusa bud stages. The dimerization specificity, the expression pattern, and the conservation of two residues specific for a MyoD bHLH domain suggest that JellyD1 is related to an ancestral MyoD gene. Id, Net-like, and JellyD1 are either expressed in the entocodon or its derived tissues, the striated and smooth muscle of the bell. These findings strengthen the hypothesis that the entocodon is a mesoderm-like structure and that the common ancestor of Cnidaria and Bilateria was more complex in cell-type architecture and body organization than commonly thought.