An ascidian glycine-rich RNA binding protein is not induced by temperature stress but is expressed under a genetic program during embryogenesis.

We have cloned a putative ascidian glycine-rich RNA binding protein gene, CiGRP1. Its maternal transcript and protein are stored in the unfertilized egg. They are gradually decreased during the first few rounds of cleavage. The CiGRP1 zygotic transcript and protein start to accumulate at the gastrula stage. The CiGRP1 transcript is expressed in the brain precursor and mesenchyme precursor cells of the gastrula and the neurula stage, and the brain and mesenchyme cells of the tailbud stage embryo. The CiGRP1 protein is found in all nuclei and in the cytoplasm of brain and mesenchyme cells. Although many glycine-rich RNA binding protein homologs of plants and vertebrates are cold-inducible, CiGRP1 cannot be induced by cold shock or heat shock at the transcriptional and translational levels during embryogenesis. The temporal expression pattern and the tissue-restricted expression pattern of CiGRP1 suggest that it has important roles in the very early stage of development and in the brain and the mesenchyme tissue specification.

The transcript coding for an RNA-binding protein is localized in the anterior side of the ascidian 2-cell stage embryo.

We have cloned a putative ascidian RNA-binding protein gene, CiRGG1, which has three RNA-recognition motifs and two arginine-glycine-glycine domains. Its deduced amino acid sequence has a weak similarity to nucleolin, which is one of the known components of messenger ribonucleoproteins. Its maternal transcript is transiently accumulated into the anterior side of 2-cell stage embryos. Zygotic expression of CiRGG1 is strongest in the brain, mesenchyme and endoderm of the trunk region of the tailbud stage embryo. This mRNA is the first example of an anterior-localized molecule in the ascidian 2-cell stage embryo.

Interactions between cytoskeletal components during myoplasm rearrangement in ascidian eggs.

Ooplasmic segregation in ascidian eggs consists of two phases of cytoplasmic movement, the first phase is mediated by the microfilament system and the second is mediated by the microtubule system. Recently, two novel proteins, p58 and myoplasmin-C1, which are localized to the myoplasm, were suggested to have important roles in muscle differentiation. In order to analyze the molecular mechanisms underlying ooplasmic segregation, the interactions between actin, tubulin, p58 and myoplasmin-C1 were examined. During the first segregation, microtubule meshwork in the unfertilized egg disappeared. At the second segregation, a novel structure of the microtubules that extended from the sperm aster and localized in the cortical region of the myoplasm was found. Moreover, uniform distribution of the cortical actin filament was observed at the second segregation. During the course of myoplasm rearrangement, p58 and myoplasmin-C1 are colocalized and can form a molecular complex in vitro. This complex of p58 and myoplasmin-C1 is a good candidate for a cytoskeletal component of the myoplasm, and is likely to be involved in the correct distribution of cytoplasmic determinants.

An alternatively spliced gene encoding a Y-box protein showing maternal expression and tissue-specific zygotic expression in the ascidian embryo.

An ascidian Y-box protein gene was cloned, designated as CiYB, which consists of a highly conserved cold shock domain and an auxiliary tail domain with alternating modules of acidic and basic amino acids. CiYB is a single copy gene in the ascidian genome. During oogenesis and early development, CiYB produces three different transcripts (CiYB1, CiYB2 and CiYB3) by alternate splicing. CiYB1 and CiYB2 were expressed during oogenesis, suggesting that they are recruited into maternal ribonucleoprotein particles. According to gel mobility shift assay, the CiYB1 protein has the ability to bind RNA. The sequence preference of RNA binding is similar to that of the Xenopus Y-box protein (FRGY2), which is a major component of the maternal messenger ribonucleoprotein particles (mRNP) in the oocyte. These results suggest that the ascidian Y-box protein may have an important role for masking and translational regulation of maternal mRNA. Furthermore, CiYB1, CiYB2 and CiYB3 were expressed zygotically in a tissue restricted manner. CiYB1 was expressed specifically in muscle precursor blastomeres and tail muscle cells suggesting its important role in muscle differentiation.

Expression of pharyngeal gill-specific genes in the ascidian Halocynthia roretzi.

The most primitive chordates may have arisen with a shift to internal feeding through the use of the pharyngeal gill slits and endostyle for extracting suspended food from the water. Therefore, the pharyngeal gill and endostyle, in addition to notochord and nerve cord, are structures key to an understanding of the molecular developmental mechanisms underlying the origin and evolution of chordates. In this and a following study, isolation of cDNA clones for genes that are specifically expressed in the pharyngeal gill or endostyle in the ascidian Halocynthia roretzi was attempted. Differential screening of a pharyngeal gill cDNA library and an endostyle cDNA library with total pharyngeal-gill cDNA probes yielded cDNA clones for two pharyngeal gill-specific genes, HrPhG1 and HrPhG2. Northern blot analysis showed a 3.0-kb transcript of HrPhG1 and a 2.0-kb transcript of HrPhG2. Predicted amino acid sequences of the gene products suggested that both genes encode secretory proteins with no significant match to known proteins. In adults, both HrPhG1 and HrPhG2 genes were only expressed in the pharyngeal gill and not in other tissues including the endostyle, body-wall muscle, gonad, gut and digestive gland. HrPhG1 and HrPhG2 transcripts were undetectable in embryos and larvae, and were first detected in juveniles 3 days after initiation of metamorphosis. In situ hybridization revealed that the expression of HrPhG1 and HrPhG2 was restricted to differentiating pharyngeal-wall epithelium, with intense signals in the area surrounding the stigma or gill slit. These genes may serve as probes for further analyses of molecular mechanisms underlying the occurrence of pharyngeal gill and formation of gill slits during chordate evolution.

Expression of endostyle-specific genes in the ascidian Halocynthia roretzi.

The endostyle is a special organ in the pharynx of Urochordata, Cephalochordata and Cyclostomata. This organ may have arisen in their common ancestor with a shift to internal feeding for extracting suspended food from the water. In addition, the endostyle has functional homology to the vertebrate thyroid gland. The endostyle is therefore another key structure in the understanding of the origin and evolution of chordates. Following a previous report of the pharyngeal gill-specific genes, we report here the isolation and characterization of cDNA clones for endostyle-specific genes HrEnds1 and HrEnds2 of the ascidian Halocynthia roretzi. These cDNA clones were obtained by differential screening of an endostyle cDNA library and a pharyngeal gill cDNA library with total endostyle cDNA probes. Both transcripts were abundant in the library; each represented about 10% of the cDNA clones of the library. The HrEnds1 transcript was small in size, about 600 bp in length. Although the predicted amino acid sequence of the gene product showed no similarity to known proteins, mean hydropathy profiles suggested that HrENDS1 is a type Ib protein or secreted protein. The HrEnds2 transcript was about 2.5 kb in length. Although the HrEnds2 gene product showed no sequence similarity to known proteins, mean hydropathy profiles suggested that HrENDS2 is a secreted protein. The transcripts of both genes were not detected in embryos, larvae and early juveniles but were evident in 1-month-old young adult after several compositional zones were organized in the endostyle. In situ hybridization revealed that distribution of transcripts of both genes was restricted to zone 6, the protein-secreting glandular element of the endostyle. These genes may be useful for further analysis of molecular mechanisms involved in endostyle development.