Evidence for conservation of the vasopressin/oxytocin superfamily in Annelida.
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Annetocin is a structurally and functionally oxytocin-related peptide isolated from the earthworm Eisenia foetida. We present the characterization of the annetocin cDNA. Sequence analyses of the deduced precursor polypeptide revealed that the annetocin precursor is composed of three segments: a signal peptide, an annetocin sequence flanked by a Gly C-terminal amidation signal and a Lys-Arg dibasic processing site, and a neurophysin domain, similar to other oxytocin family precursors. The proannetocin showed 37.4-45.8% amino acid homology to other prohormones. In the neurophysin domain, 14 cysteines and amino acid residues essential for association of a neurophysin with a vasopressin/oxytocin superfamily peptide were conserved, suggesting that the Eisenia neurophysin can bind to annetocin. Furthermore, in situ hybridization experiments demonstrated that the annetocin gene is expressed exclusively in neurons of the central nervous system predicted to be involved in regulation of reproductive behavior. These findings confirm that annetocin is a member of the vasopressin/oxytocin superfamily. This is the first identification of the cDNA encoding the precursor of an invertebrate oxytocin-related peptide and also the first report of the identification of an annelid vasopressin/oxytocin-related precursor. (+info)
Cytoskeletal mechanisms of ooplasmic segregation in annelid eggs.
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Annelid embryos are comprised of yolk-deficient animal and yolk-filled vegetal blastomeres. This "unipolar" organization along the animal-vegetal axis (in terms of ooplasmic distribution) is generated via selective segregation of yolk-free, clear cytoplasm to the animal blastomeres. The pathway that leads to the unipolar organization is different between polychaetes and clitellates (i.e., oligochaetes and hirudinidans). In polychaetes, the clear cytoplasm domain, which is established through ooplasmic segregation at the animal side of the egg, is simply cut up by unequal equatorial cleavage. In clitellates, localization of clear cytoplasm to animal blastomeres is preceded by unification of the initially separated polar domains of clear cytoplasm, which result from bipolar ooplasmic segregation. In this article, I have reviewed recent studies on cytoskeletal mechanisms for ooplasmic localization during early annelid development. Annelid eggs accomplish ooplasmic rearrangements through various combinations of three cytoskeletal mechanisms, which are mediated by actin microfilaments, microtubules and mitotic asters, respectively. One of the unique features of annelid eggs isthat a homologous process is driven by distinct cytoskeletal elements. Annelid eggs may provide an intriguing system to investigate not only mechanical aspects of ooplasmic segregation but also evolutionary divergence of cytoskeletal mechanisms that operate in a homologous process. (+info)
Cell lineage analysis of pattern formation in the Tubifex embryo. I. Segmentation in the mesoderm.
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Annelids are strongly segmented animals that display a high degree of metamerism in their body plan. The embryonic origin of metameric segmentation was examined in an oligochaete annelid Tubifex using lineage tracers. Segmental organization arises sequentially in the anterior-to-posterior direction along the longitudinal axis of the mesodermal germ band, a coherent column of primary blast cells that are produced from the mesodermal teloblast. Shortly after its birth, each primary blast cell undergoes a spatiotemporally stereotyped sequence of cell divisions to generate three classes of cells (in terms of cell size), which together give rise to a distinct cell cluster. Each cluster is composed of descendants of a single primary blast cell; there is no intermingling of cells between adjacent clusters. Relatively small-sized cells in each cluster become localized at its periphery, and they form coelomic walls including an intersegmental septum to establish individuality of segments. A set of cell ablation experiments showed that these features of mesodermal segmentation are not affected by the absence of the overlying ectodermal germ band. These results suggest that each primary blast cell serves as a founder cell of each mesodermal segment and that the boundary between segments is determined autonomously. It is concluded that the metameric body plan of Tubifex arises from an initially simple organization (i.e., a linear series) of segmental founder cells. (+info)
Characterization of a new variant DNA (cytosine-5)-methyltransferase unable to methylate double stranded DNA isolated from the marine annelid worm Chaetopterus variopedatus.
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The enzyme S-adenosylmethionine-DNA (cytosine-5)-methyltransferase has been identified, first time for invertebrates, in embryos of the marine polychaete annelid worm Chaetopterus variopedatus. The molecule has been isolated from embryos at 15 h of development. It is a single peptide of about 200 kDa molecular weight, cross-reacting with antibodies against sea urchin DNA methyltransferase. The enzymatic properties of the molecule are similar to those of Dnmt1 methyltransferases isolated from other organisms, but with the peculiarity to be unable to make 'de novo' methylation on double stranded DNA. (+info)
MAP kinase, meiosis, and sperm centrosome suppression in Urechis caupo.
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Although MAP kinase is an important regulatory enzyme in many somatic cells, almost nothing is known about its functions during meiosis, except in frog and mouse oocytes. We investigated MAPK activation and function in oocytes of the marine worm Urechis caupo that are fertilized at meiotic prophase. Activity was first detected at 4-6 min after fertilization in immunoblots with anti-active MAPK, prior to germinal vesicle breakdown (GVBD). MAPK activation did not require new protein synthesis and was dependent on the increases in both intracellular pH and intracellular Ca(2+) that normally occur during activation. When MAPK activation was inhibited with PD98059 or U0126, GVBD still occurred, but meiosis was abnormal and there was a dramatic premature enlargement of sperm asters, which normally do not appear until second polar body formation. Failure of polar body formation and premature sperm aster enlargement also occurred when MAPK activation was inhibited by an entirely different treatment which involved lowering the pH of external seawater to interrupt the normal cytoplasmic pH increase. Thus, in Urechis, active MAPK appears to be required for (1) normal meiotic divisions and (2) suppressing the paternal centrosome until after the egg completes meiosis, a general phenomenon whose mechanism has been unknown. (+info)
Structure of the iron complex in methemerythrin.
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The coordination of the ligands about the iron atoms in methemerythrin from Themiste dyscritum has been deduced from a 2.8 A resolution electron density map. The complex can be described in terms of two trigonal antiprisms about the pair of iron atoms in each subunit, the antiprisms having one face in common. Ligands at eight of the nine coordination positions are protein side chains, the ninth presumably being water. Comparison of the electron density map for T. dyscritum methemerythrin with the sequence of Phascolopsis gouldii hemerythrin suggests six aromatic side chain ligands (five histidine and one tyrosine) and two nonaromatic side chain ligands. The latter provide atoms at two of the three vertices of the face shared by the two antiprisms, and these along with the presumed water at the third vertex form bridges between the iron atoms of each pair. (+info)
Mitochondrial genomes of Galathealinum, Helobdella, and Platynereis: sequence and gene arrangement comparisons indicate that Pogonophora is not a phylum and Annelida and Arthropoda are not sister taxa.
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We report a contiguous region of more than half (> 7,500 nt) of the mitochondrial genomes for Platynereis dumerii (Annelida: Polychaeta), Helobdella robusta (Annelida: Hirudinida), and Galathealinum brachiosum (Pogonophora: Perviata). The relative arrangements of all 22 genes identified for Helobdella and Galathealinum are identical to one another and to their arrangements in the mtDNA of the previously studied oligochaete annelid Lumbricus. In contrast, Platynereis differs from these taxa in the positions of several tRNA genes and in having two additional tRNA genes (trnC and trnM) and a large noncoding sequence in this region. Comparisons of relative gene arrangements and of the nucleotide and inferred amino acid sequences among these and other published taxa provide strong support for an annelid-mollusk clade that excludes arthropods, and for the inclusion of pogonophorans within Annelida, rather than giving them separate phylum status. Gene arrangement comparisons include the first use of a recently described method on previously unpublished data. Although a variety of alternative initiation codons are typically used by mitochondrial protein-encoding genes, ATG appears to be the initiator for all but one reported here. The large noncoding region (1,091 nt) identified in Platynereis has no significant sequence similarity to the noncoding region of Lumbricus, although each contains runs of TA dinucleotides and of homopolymers, which could potentially serve as signaling elements. There is strong bias for synonymous codon usage in Helobdella and especially in Galathealinum. In this latter taxon, 5 codons are completely unused, 13 are used three or fewer times, and G appears at third codon positions in only 26 of the 2,236 codons. Nucleotide composition bias appears to influence amino acid composition of the proteins. (+info)
Bilaterian origins: significance of new experimental observations.
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Several recent laboratory observations that bear on the origin of the Bilateria are reviewed and interpreted in light of our set-aside cell theory for bilaterian origins. We first discuss new data concerning the phylogeny of bilaterian phyla. Next, we use systematic, molecular, and paleontological lines of evidence to argue that the latest common ancestor of echinoderms plus hemichordates used a maximal indirect mode of development. Furthermore, the latest common ancestor of molluscs and annelids was also indirectly developing. Finally, we discuss new data on Hox gene expression patterns which suggest that both sea urchins and polychaete annelids use Hox genes in a very similar fashion. Neither utilizes the complete Hox complex in the development of the larva per se, while the Hox complex is expressed in the set-aside cells from which the adult body plan is formed. Our current views on the ancestry of the bilaterians are summarized in phylogenetic terms, incorporating the characters discussed in this paper. (+info)