Gap junction structures: Analysis of the x-ray diffraction data

Models for the spatial distribution of protein, lipid and water in gap junction structures have been constructed from the results of the analysis of X-ray diffraction data described here and the electron microscope and chemical data presented in the preceding paper (Caspar, D. L. D., D. A. Goodenough, L. Makowski, and W.C. Phillips. 1977. 74:605-628). The continuous intensity distribution on the meridian of the X-ray diffraction pattern was measured, and corrected for the effects of the partially ordered stacking and partial orientation of the junctions in the X-ray specimens. The electron density distribution in the direction perpendicular to the plane of the junction was calculated from the meridional intensity data. Determination of the interference function for the stacking of the junctions improved the accuracy of the electron density profile. The pair-correlation function, which provides information about the packing of junctions in the specimen, was calculated from the interference function. The intensities of the hexagonal lattice reflections on the equator of the X-ray pattern were used in coordination with the electron microscope data to calculate to the two-dimensional electron density projection onto the plane of the membrane. Differences in the structure of the connexons as seen in the meridional profile and equatorial projections were shown to be correlated to changes in lattice constant. The parts of the junction structure which are variable have been distinguished from the invariant parts by comparison of the X-ray data from different specimens. The combination of these results with electron microscope and chemical data provides low resolution three- dimensional representations of the structures of gap junctions.     

Colonial origin for Emetazoa: major morphological transitions and the origin of bilaterian complexity

A new hypothesis for the evolution of Bilateria is presented. It is based on a reinterpretation of the morphological characters shared by protostomes and deuterostomes, which, when taken together with developmental processes shared by the two lineages, lead to the inescapable conclusion that the last common ancestor of Bilateria was complex. It possessed a head, a segmented trunk, and a tail. The segmented trunk was further divided into two sections. A dorsal brain innervated one or more sensory cells, which included photoreceptors. "Appendages" or outgrowths were present. The bilaterian ancestor also possessed serially repeated "segments" that were expressed ontogenetically as blocks of mesoderm or somites with adjoining fields of ectoderm or neuroectoderm. It displayed serially repeated gonads (gonocoels), each with a gonoduct and gonopore to the exterior, and serially repeated "coeloms" with connections to both the gut and the exterior (gill slits and pores). Podocytes, some of which were serially repeated in the trunk, formed sites of ultrafiltration. In addition, the bilaterian ancestor had unsegmented coeloms and a contractile blood vessel or "heart" formed by coelomic myoepithelial cells. These cells and their underlying basement membrane confine the hemocoelic fluid, or blood, in the connective tissue compartment. A possible scenario to account for this particular suite of characters is one in which a colony of organisms with a cnidarian grade of organization became individuated into a new entity with a bilaterian grade of organization. The transformation postulated encompassed three major transitions in the evolution of animals. These transitions included the origins of Metazoa, Eumetazoa, and Bilateria and involved the successive development of poriferan, cnidarian, and bilaterian grades of organization. Two models are presented for the sponge-to-cnidarian transition. In both models the loss of a flow-through pattern of water circulation in poriferans and the establishment of a single opening and epithelia sensu stricto in cnidarians are considered crucial events. In the model offered for the cnidarian-to-bilaterian transition, the last common ancestor of Eumetazoa is considered to have had a colonial, cnidarian-grade of organization. The ancestral cnidarian body plan would have been similar to that exhibited by pennatulacean anthozoans. It is postulated that a colonial organization could have provided a preadaptive framework for the evolution of the complex and modularized body plan of the triploblastic ancestor of Bilateria. Thus, one can explore the possibility that problematica such as ctenophores, the Ediacaran biota, archaeocyaths, and Yunnanozoon reflect the fact that complexity originated early and involved the evolution of a macroscopic compartmented ancestor. Bilaterian complexity can be understood in terms of Beklemishev "cycles" of duplication and colony individuation. Two such cycles appear to have transpired in the early evolution of Metazoa. The first gave rise to a multicellular organism with a sponge grade of organization and the second to the modularized ancestor of Bilateria. The latter episode may have been favored by the ecological conditions in the late Proterozoic. Whatever its cause, the individuation of a cnidarian-grade colony furnishes a possible explanation for the rapid diversification of bilaterians in the late Vendian and Cambrian. The creation of a complex yet versatile prototype, which could be rapidly modified by selection into a profusion of body plans, is postulated to have affected the timing, mode, and extent of the "Cambrian explosion." During the radiations, selective loss or simplification may have been as creative a force as innovation. Finally, colony individuation may have been a unique historical event that imprinted the development of bilaterians as the zootype and phylotypic stage. (ABSTRACT TRUNCATED)     

The role of nitric oxide in cancer

Nitric oxide (NO) is a pleiotropic regulator, critical to numerous biological processes, including va-sodilatation, neurotransmission and macrophage-mediated immunity. The family of nitric oxide synthases (NOS) comprises inducible NOS (iNOS), endothelial NOS (eNOS), and neuronal NOS (nNOS). Interestingly, various studies have shown that all three isoforms can be involved in promoting or inhibiting the etiology of cancer. NOS activity has been detected in tumour cells of various histogenetic origins and has been associated with tumour grade, proliferation rate and expression of important signaling components associated with cancer development such as the oestrogen receptor. It appears that high levels of NOS expression (for example, generated by activated macrophages) may be cytostatic or cytotoxic for tumor cells, whereas low level activity can have the opposite effect and promote tumour growth. Paradoxically therefore, NO (and related reactive nitrogen species) may have both genotoxic and angiogenic pro     

Paired sequence difference in ribosomal RNAs: evolutionary and phylogenetic implications

Ribosomal RNAs have secondary structures that are maintained by internal Watson-Crick pairing. Through analysis of chordate, arthropod, and plant 5S ribosomal RNA sequences, we show that Darwinian selection operates on these nucleotide sequences to maintain functionally important secondary structure. Insect phylogenies based on nucleotide positions involved in pairing and the production of secondary structure are incongruent with those constructed on the basis of positions that are not. Furthermore, phylogeny reconstruction using these nonpairing bases is concordant with other, morphological data.      

Mitochondrial gene order is not conserved in arthropods: prostriate and metastriate tick mitochondrial genomes

The entire mitochondrial genome was sequenced in a prostriate tick, Ixodes hexagonus, and a metastriate tick, Rhipicephalus sanguineus. Both genomes encode 22 tRNAs, 13 proteins, and two ribosomal RNAs. Prostriate ticks are basal members of Ixodidae and have the same gene order as Limulus polyphemus. In contrast, in R. sanguineus, a block of genes encoding NADH dehydrogenase subunit 1 (ND1), tRNA(Leu)(UUR), tRNA(Leu)(CUN), 16S rDNA, tRNA(Val), 12S rDNA, the control region, and the tRNA(Ile) and tRNA(Gln) have translocated to a position between the tRNA(Glu) and tRNA(Phe) genes. The tRNA(Cys) gene has translocated between the control region and the tRNA(Met) gene, and the tRNA(Leu)(CUN) gene has translocated between the tRNA(Ser)(UCN) gene and the control region. Furthermore, the control region is duplicated, and both copies undergo concerted evolution. Primers that flank these rearrangements confirm that this gene order is conserved in all metastriate ticks examined. Correspondence analysis of amino acid and codon use in the two ticks and in nine other arthropod mitochondrial genomes indicate a strong bias in R. sanguineus towards amino acids encoded by AT-rich codons.      

A FINE STRUCTURAL INVESTIGATION OF SURFACE SPECIALIZATIONS AND THE CORTICAL REACTION IN EGGS OF THE CNIDARIAN BUNODOSOMA CAVERNATA

Developing oocytes of the cnidarian Bunodosoma cavernata are located within the mesoglea of the mesenteries of the gastrovascular cavity. The cortex of the more mature vitellogenic oocytes contains numerous, electron-dense, membrane-bound, cortical granules. The surface of these oocytes possesses prominent radially projecting structures termed cytospines. Each cytospine has a core of microfilaments, 50–70 Å in diameter, that extends basally as a rootlet through the cortical layer. During spawning, ova lacking any extraneous investments are released from the enclosing gastrodermis. As a consequence of fertilization or events associated with the earliest stages of development the ova undergo a massive cortical reaction. This reaction, which occurs during or just after release of the ova, involves extensive reorganization of the cortical layer. The cortical granule membranes and egg surface membrane fuse and vesiculate resulting in the massive discharge of granule contents. This event is accompanied by the loss of vesicular remnants of cortical ooplasm and the disruption of cytospine organization. Light and electron microscope comparisons of unreacted and reacted eggs show that the reaction results in a significant decrease in egg diameter with the oolemma of the reacted egg reorganizing in a position centripetal to its original location.     

Studies on the tardigrades. I. Fine structure of the anterior foregut of Milnesium tardigradum Doyère

The fine structure of the anterior foregut of the tardigrade Milnesium tardigradum is presented. The oral region consists of a terminal mouth opening surrounded by six plate-like lips lying within a circlet of six prominent papillae. The buccal cavity is enclosed within a thick cuticular tube which possesses appendage structures, the stylet sheaths, stylet supports and paired protrusible stylets. Two large salivary glands envelop the buccal structures and contain voluminous amounts of secretory product. The arrangement, possible functions and phyletic significance of these structures are discussed.