The Ordovician-Silurian boundary stratotype: consequences of its approval by the IUGS

Lespérance, Pierre J., Barnes, Christopher R., Berry, William B. N., Boucot, Arthur J. & Mu En-zhi 1987 07 15: The Ordovician-Silurian boundary stratotype: consequences of its approval by the IUGS.
The Ordovician-Silurian stratotype at Dob's Linn, Scotland is the second systemic boundary approved by the International Union of Geological Sciences (IUGS). A review of the internationally accepted criteria required of a stratotype shows that few of these are possessed by the Dob's Linn section. The strongest attribute of the section is the presence of zonal graptolites, although the boundary is recognized primarily on a single biological event (base of acuminatus Zone) for which the evolutionary relationships of the taxa are not established. In approving this boundary proposal in 1985, which received only simple majority support within the Ordovician-Silurian Boundary Working Group (OSBWG) and which fails to meet most of the accepted prerequisites for a stratotype, the International Commission on Stratigraphy (ICS) and the IUGS have established an unfortunate precedent. It follows that future systemic boundaries need not meet the accepted standards. It raises serious questions on the assessment and voting procedures of the International Commission on Stratigraphy and on the credence accorded the recommendations developed by the International Subcommission on Stratigraphic Classification of IUGS.     

Estimates of gene flow, genetic substructure and population heterogeneity in bracken (Pteridium aquilinum)

Bracken [ Pteridium aquilinum (L.) Kuhn] is a cosmopolitan species and is a noxious weed in many areas. Because of its abundance, particularly in Britain, bracken affords an ideal system for investigating various aspects of population genetics and evolution. High mobility of dispersal units (spores) suggests that rates of gene flow among distant populations should be high. Gene flow is a major evolutionary force that influences the genetic structure of populations. To examine the effects of gene flow on population heterogeneity and population substructuring in bracken, starch gel electrophoresis of enzymes was used to provide the necessary genetic database. Allele frequency data at 21 loci were obtained for seven British populations, one Majorcan and one from the eastern United States. A model was employed to estimate the amount of gene flow ( Nm ) at several levels. Gene flow among British populations was extremely high ( Nm = 36.51), one of the highest estimates reported for plants. Among eight European populations gene flow was lower (but still considered high) at Nm = 2.47. Trans-Atlantic gene flow was low ( Nm = 0.0926).
F -statistics were used to assess population heterogeneity and substructuring. The data indicate that, compared with other species, there is very little genetic differentiation among British populations of bracken. Indeed, it appears that the whole island is behaving as a single randommating population. This result is consistent with high levels of gene flow. Only one population (on the Isle of Arran) showed statistically significant genetic substructuring. Habitat heterogeneity on the island and age structure are hypothesized as possible causes of this result.
The data reported here support previous studies demonstrating that bracken is genetically polymorphic and is an outcrossing species.     

Functional Aspects of the Salt Glands of the Plumbaginaceae

X-ray microanalysis and diffuse reflectance infrared Fouriertransform spectrometry were used to determine the presence andratios of elements in salt secretions from salt glands of greenhouseand experimentally-manipulated leaves of five species of thePlumbaginaceae Sodium, magnesium, silica, sulphur, phosphorus,chloride, potassium, calcium and carbonate were detected insecretions of greenhouse-grown plants. The salt glands of excisedleaves challenged by solutions of KI, KCl, NaCl, Na₂SO₄, MgCl₂and MgSO₄ secreted principally the ions of the challenging solutions. Key words: Ion transport, secretion     

The Behaviour of Peroxidase and Polyphenol Oxidase during the Growth and Senescence of Tobacco Leaves

The activities of polyphenol oxidase and peroxidase per unitarea of attached tobacco leaves increased as the leaves expanded,and reached a stable level in the mature leaves. After the onsetof senescence the enzyme activities fell rapidly, but at a laterstage they showed a small rise. Enzyme activities in tissuestaken from non-senescent leaves increased during incubationat 20 °C. These increases were sensitive to inhibitors ofprotein synthesis. Enzyme activities in tissues taken from leavesin early senescence increased during incubation at 35 °Cbut not at 20 °C. These increases were very largely insensitiveto inhibitors of protein synthesis and were not apparently relatedto de novo protein synthesis. There were no increases in enzymeactivities in tissues taken from leaves in late senescence andincubated at 35 °C or 20 °C.     

A survey of angiosperm species to detect hypodermal Gasparian bands. I. Roots with a uniseriate hypodermis and epidermis

PERUMALLA, C. J., PETERSON, C. A. & ENSTONE, D. E., 1990. A survey of angiosperm species to detect hypodermal Casparian bands. I. Roots with a uniseriate hypodermis and epidermis. Roots of 181 species from 53 families were surveyed to determine the frequency of Casparian bands in hypodermal layers. For six species, inconclusive data were obtained. The roots of the remaining 175 species were divided into three categories on the basis of this survey. In the first, a hypodermis is absent (12 species): no wall modifications were observed in the outer cortex and this region was permeable to the apoplastic dye Cellufluor. In the second, a hypodermis is present, but a hypodermal Casparian band is absent (seven species). In roots of six species, no wall modifications were detected in the hypodermis; the one remaining species had lignified phi thickenings which were permeable to Cellufluor. In the third, both a hypodermis and a hypodermal Casparian band are present (156 species). These Casparian bands consisted of suberin deposits throughout the width of the anticlinal walls of the hypodermis. The tangential walls of the hypodermis were also suberized, indicating that suberin lamellae were probably also present. Hypodermal Casparian bands were found in roots of hydrophytic, mesophytic and xerophytic species and in members of primitive as well as advanced families. The widespread occurrence of these bands (in 89% of the species surveyed) suggests that they were present in the type ancestral to the flowering plants and that this feature has been retained by many species in this group. The epidermal cell walls of the majority of species examined were suberized but were permeable to Cellufluor.     

Description of a New Species of Microsporidia from Muscidifurax raptor (Hymenoptera: Pteromalidae), a Pupal Parasitoid of Muscoid Flies

ABSTRACT. A microsporidian parasite, Nosema muscidifuracis n. sp., has been found in Muscidifurax raptor , a parasitoid of muscoid flies. Stages of the parasite developed in direct contact with the host cell cytoplasm and were detected in midgut epithelium, Malpighian tubules, ovaries (including oocytes) and fat body of larvae and adults. Spores were also detected within eggs deposited on the host. Light and electron microscopy revealed a developmental cycle with diplokaryotic stages dividing by binary fission and disporous sporulation sequences producing diplokaryotic spores of three morphological classes, differing significantly only in length of the polar filament. Two of the classes were found in larvae, pupae and adults. One of these, with about five turns in the coiled polar filament, is presumed to be responsible for transmission from cell to cell within the host (autoinfection) and the other, with about 10 turns, responsible for transmission from host to host. A third class, with about 15 turns in the polar filament, was found in eggs of M. raptor . It is, presumably, either involved in initiation and spread of the infection at eclosion or is responsible for horizontal transmission to a new host individual when eggs are cannibalized.