Cell age-related differences in the interaction of a potential-sensitive fluorescent dye with nuclear envelopes of Acetabularia mediterranea

Abstract. To study whether an electrical potential difference exists across the nuclear envelope or inner nuclear membrane of plant cells, the authors have used an optical probe of membrane potential, the cationic fluorescent dye, DiOC₆(3) (MW = 572.5). This dye was microinjected into the nucleoplasm of isolated Acetabularia nuclei (which are still surrounded by a thin layer of cytoplasm) and its subnuclear localization visualized by fluorescence microscopy. Striking differences, which seemed to be correlated with the developmental stage of the isolated nucleus, were observed. In nuclei isolated from cells at the stage of early cap stage formation, the dye was restricted to the nuclear envelope. In nuclei isolated from cells with intermediate or fully developed caps, there was increased nucleoplasmic staining, and the staining of the envelope was frequently diminished or abolished. In all nuclei, the dye remained within the nucleus after injection. Cytoplasmic staining was only observed when nuclei isolated from cells at the stage of early cap formation were incubated in a hyper- or hypo-tonic medium. Various ionophores, injected before the dye into the nucleoplasm, had no effect on the subsequent nuclear localization of DiOC₆(3), although they did rapidly induce nucleolar condensation in nuclei isolated from cells at the stage of early cap formation. The results suggested that the electrical properties of Acetabularia nuclear envelopes or inner nuclear membranes change during cell maturation. Furthermore, the retention of the dye in the nucleoplasm under isotonic conditions indicated that the nuclear pores were not open channels for molecules of this size.     

Visualization of mitochondria and nuclei in living plant cells by the use of a potential-sensitive fluorescent dye

Abstract Cationic potential-sensitive dyes have previously been used to selectively stain mitochondria in living animal cells (Johnson, Walsh & Chen, 1980; Johnson et al., 1981). The present work demonstrates that the cyanine dye 3,3′-dihexyloxacarbocyanine iodide (DiOC₆(3)) can also be used as a mitochondrial stain in living plant cells. The stained mitochondria were easily visualized by fluorescence microscopy. The accumulation of DiOC₆(3) in mitochondria seemed to be potential-dependent since it was prevented by protonophores, valinomycin and inhibitors of electron transport. It was often observed that DiOC₆(3) also stained the nuclear membrane of some cells. This fluorescence, limited to the perinuclear region, was possibly due to a potential across one or both nuclear membranes, although it was not completely dissipated by any of the ionophores or inhibitors tested. Our observations demonstrate the usefulness of using DiOC₆(3) for studying relative membrane potentials of plant mitochondria and, perhaps, other organelles and membrane systems in living plant cells.     

The mixosaurid ichthyosaur Contectopalatus from the Middle Triassic of the German Basin

The skull of the mixosaurid species Contectopalatus atavus (Quenstedt, 1851-52) is the most bizarre of any known ichthyosaur. It possesses a very high sagittal crest formed by the nasal, frontal and parietal bones which grows higher during ontogeny. This skull structure - found to a lesser extent in the other mixosaurid genera Mixosaurus and Phalarodon - is a synapomorphy of the family Mixosauridae. It is here interpreted as correlated with a unique arrangement of the jaw adductor musculature among tetrapods, with the internal jaw adductors extending over most of the skull roof up to the external narial opening. This reconstruction would increase the biting force considerably and the hypothesis is supported by peculiarities of the dentition and jaws of Contectopalatus. Contectopalatus probably reached a length of about 5 meters. It is therefore the largest known mixosaurid and one of the largest Triassic ichthyosaurs. The general text-book picture of mixosaurs as small, rather unspecialized, primitive ichthyosaurs is incorrect. Mixosaurs were a highly specialized, uniquely adapted and very diverse ichthyosaur family, some members of which rank among the marine top predators of their time.     

A BASAL MIXOSAURID ICHTHYOSAUR FROM THE MIDDLE TRIASSIC OF CHINA

Abstract:  New specimens of a mixosaurid ichthyosaur are described from the People's Republic of China. They consist of an almost complete skull, forefin and shoulder girdle of a single individual, and a second partial skull from the Anisian Guanling Formation of Guizhou Province that belongs to the same taxon. A phylogenetic analysis shows this species to be the sister-taxon of all other mixosaurids. It is distinguished from Mixosaurus , Contecotpalatus and Phalarodon by features of its dentition and cranial morphology. There is only one maxillary tooth row. The posterior maxillary teeth are blunt and rounded, and much wider than the premaxillary teeth, but they are not strongly elongated anteroposteriorly. The teeth are subthecodontously implanted. The postorbital skull segment is long. The jugal-quadratojugal notch is deep and these two elements remain separated externally. The postorbital separates supratemporal and postfrontal. There are no postaxial ossifications of the forefin. Autapomorphies include a long ventral process of the postorbital lamina of the postorbital as well as characteristic morphologies of the coracoid and humerus. The material is therefore clearly distinguished from other known mixosaurids. It can be referred, on the basis of the features of the humerus and coracoid, to Mixosaurus maotaiensis Young, 1965. As it is generically distinct from Mixosaurus , a new generic name, Barracudasaurus gen. nov. is proposed. The phylogenetic relevance of the external separation of jugal and quadratojugal in some Triassic ichthyosaurs is discussed and it is concluded that it does not demonstrate the initial presence of a lower temporal fenestra in ichthyosaurs. The new material suggests that mixosaurids could have also originated in the Eastern rather than the Western Tethys, as previously assumed.     

AN ALMOST COMPLETE JUVENILE SPECIMEN OF THE CHELONIID TURTLE CTENOCHELYS STENOPORUS (HAY, 1905) FROM THE UPPER CRETACEOUS NIOBRARA FORMATION OF KANSAS,USA

Abstract: A new, unusually well‐preserved juvenile specimen of Ctenochelys stenoporus from the Niobrara Formation is described. The skull has come apart at its sutures and all bones of the braincase and ear region are preserved three‐dimensionally. This allows a detailed reconstruction of the important brain structures of a basal juvenile cheloniid turtle. It is compared with adult Ctenochelys specimens, and the major ontogenetic changes in the skull and postcranial skeleton are described. Furthermore, the specimen is compared with other fossil and extant cheloniids with well‐known braincases and the differences between basal and advanced cheloniids turtles are specified.     

Endogenous pararetroviruses of allotetraploid Nicotiana tabacum and its diploid progenitors, N. sylvestris and N. tomentosiformis

Endogenous pararetroviruses (EPRVs) represent a new class of dispersed repetitive DNA in plants. The genomes of many Nicotiana species and other solanaceous plants are rich in EPRVs. Distinct EPRV families are present in N. sylvestris ( Ns ) and in N. tomentosiformis ( Nto ), the two diploid progenitors of allotetraploid N. tabacum . Nicotiana EPRVs represent an interesting type of repetitive sequence to analyse in polyploids because of their potential impact on plant fitness and the epigenetic architecture of plant genomes. The Ns EPRV family appears identical in N. sylvestris and N. tabacum , indicating little change has occurred in either species since polyploid formation. By contrast, the Nto EPRV family is larger in N. tomentosiformis than in N. tabacum , suggesting either preferential elimination from the polyploid genome or specific accumulation in the diploid genome following polyploidization. The lability of Nto EPRVs might be enhanced by a frequent association with gypsy retrotransposons. Although some EPRVs are probably benign, others are potentially pathogenic or, conversely, determinants of virus resistance. Normally quiescent EPRVs can be reactivated and cause symptoms of infection in hybrids of species that differ in their EPRV content. EPRVs that furnish immunity to the free virus exemplify the selective value of so-called 'junk' DNA. Variation in the abundance and distribution of EPRVs among related species can be useful in taxonomic and evolutionary studies.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 627–638.