An Ultrastructural Study of the Egg and Early Cleavage Stages of Lymnaea stagnalis, a Pulmonate Mollusc

Ultrastructural observations were made on the egg and earlycleavage stages (2-, 4-, 8-, and 24-cell stages). An attemptwas made to discern the nature of the "subcortical accumulations"(SCA) previously described at the light microscope level ofresolution. A variety of organelles can be found at the peripheryof the egg and embryos, but none of them are specific to thisregion. Although the morphological basis for the SCA could notbe attributed to a particular morphological entity it is apparentthat the SCA are not due to a specific large organelle (suchas yolk platelets, lipid droplets, mitochondria) or a particulardistribution or aggregation of such organelles. The SCA couldbe due to a cytochemical differentiation in the cytoplasmicmatrix but it seems more likely that they reflect a differentialaccumulation of small organelles, possibly ribosomes.     

Loss of Dispensable Endonuclease Activity in Relief of Polarity by <Emphasis Type="Italic">suA</Emphasis>

NONSENSE mutations in several microorganisms exert a pleio-tropic “polar” effect, reducing the level of expression of those genes in the same Operon which lie on the operator-distal side of the mutant gene'. Although ribosomes terminate translation and are quickly discharged from the messenger when they encounter a nonsense codon², they can efficiently re-attach to the messenger at the “start” codons of downstream genes, both in RNA phage^2–5 and in E. coli⁶. Thus, polarity must be caused by unavailability of the messenger template beyond the nonsense codon. In RNA phage, this unavailability has been attributed to a conformational masking of the messenger^2–5     

Stored Messenger Ribonucleoprotein Particles in Differentiated Sclerotia of Physarum polycephalum

Starvation induces vegetative microplasmodia of Physarum polycephalum to differentiate into translationally-dormant sclerotia. The existence and the biochemical nature of stored mRNA in sclerotia is examined in this report. The sclerotia contain about 50% of the poly(A)-containing RNA [poly(A)+RNA] complement of microplasmodia as determined by [³H]-poly(U) hybridization. The sclerotial poly(A)+RNA sequences are associated with proteins in a ribonucleoprotein complex [poly(A)+mRNP] which sediments more slowly than the polysomes. Sclerotial poly(A)+RNP sediments more rapidly than poly(A)+RNP derived from the polysomes of microplasmodia despite the occurrence of poly(A)+RNA molecules of a similar size in both particles suggesting the existence of differences in protein composition. Isolation of poly(A)+RNP by oligo (dT)-cellulose chromatography and the analysis of its associated proteins by polyacrylamide gel electrophoresis show that sclerotial poly(A)+RNP contains at least 14 major polypeptides, 11 of which are different in electrophoretic mobility from the polypeptides found in polysomal poly(A)+RNP. Three of the sclerotial poly(A)+RNP polypeptides are associated with the poly(A) sequence (18, 46, and 52 × 10³ mol. wt. components), while the remaining eight are presumably bound to non-poly(A) portions of the poly(A)+RNA. Although distinct from polysomal poly(A)+RNP, the sclerotial poly(A)+RNP is similar in sedimentation behavior and protein composition (with two exceptions) to the microplasmodial free cytoplasmic poly(A)+RNP. The results suggest that dormant sclerotia store mRNA sequences in association with a distinct set of proteins and that these proteins are similar to those associated with the free cytoplasmic poly(A)+RNP of vegetative plasmodia.     

Écologie des Théidées

The Recent thecideid brachiopod Lacazella mediterranea (Risso), studied in the Mediterranean and on the Great Bahama Bank, shows a depth range of 20–150 m (optimum 50–95 m), determined by the need of subdued light. A hard substratum is essential for the cementing of the shell. Brachiopods of the genera Crania. Argyrotheca , and Megathiris belong to the same ecological niche - more or less dependence on light and substratum results in a differentiation of the species in biocoenoses as well and taphocoenoses. Several Mesozoic species are studied in coral, bryozoan, poriferan, and rudist environments. Species like Thecidea papillata (Schlotheim) and Danella recurvirostris (Defrance) seem to have acquired a free life, independent of the substratum, and others, like Praelacazella baltoviensis Barczyk stood fairly intense light in shallow water. Diptych thecideids and megathyridids occurred in bryozoan and coral environments.     

Some Observations on the Structure of the Peristomial Membranelle of Spirostomum ambiguum

SYNOPSIS. Electron-microscopic observations of Spirostomum ambiguum have demonstrated additional details of superficial and deep tubular connections with peristomial and somatic kinetosomes. The superficial peristomial tubules appear to connect adjacent rows of kinetosomes. Anatomically, they course distally from the proximal kinetosomal plate. The deep tubules run proximally from the kinetosomal plate. Those in the somatic region appear to enter the endoplasm; those in the peristomial region leave the kinetosome as bundles of either 10 or 11 tubules which steadily converge to form 2 compact rows of 10 tubular bundles. These tubules connect to 2 of the 3 rows of 10 cilia each, the rows of 3 being separated by membranous folds protruding perpendicular to the peristomial groove. The rows of bundles converge further, enter the endoplasm and fan out again into tubular sheets, some of which appear to course in an antero-posterior direction. Another set of tubules arises from each of the kinetosomes in the 3rd row of 10 kinetosomes and courses proximally at a different angle from those arising from the 2 other kinetosome rows. Terminations have not been observed for the deep somatic or peristomial tubules. Their possible role in producing the forceful longitudinal contraction of Spirostomum is discussed.     

Taxonomic isolation and the accumulation of herbivorous insects: a comparison of introduced and native trees

Abstract.

• 1 Evidence from leaf-mining insects on Fagaceous hosts suggests that range expansions of insects onto introduced trees often involve species that feed on native hosts closely related to the introduced host.
• 2 An examination of the herbivorous entomofauna of British trees illustrates that the size of the entomofauna is partially determined by the taxonomic isolation of the host tree.

     

Pale biological implications of some Upper Ordovician juvenile asteroids (Echinodermata)

Blake, D. B. 1990 10 15: Pale biological implications of some Upper Ordovician juvenile asteroids (Echinodermata). Lethaia , Vol. 23, pp. 347–357. Oslo. ISSN 0024–1164.
Two patterns have been recognized in the early dorsal skeletal development of modern asteroids. The skeleton of well-preserved juveniles of the Late Ordovician species Promopalaeaster finei is similar in fundamental ossiclar arrangement to one of these patterns, suggesting continuity of developmental sequence. Similarities include presence of large terminal ossicles and double rows of marginal ossicles. Ancient and modern juveniles differ in that in P. finei , podial pores leading to the interior of the arms are lacking, thus providing an ontogenetic argument that such pores are phylogenetically derived within the class. An unpaired interbrachial marginal, or axillary, is present distal to the oral frame; positioning supports earlier suggestions that the axillary is the homologue of the odontophore ossicle of modern asteroids. Comparisons between P. finei and another Ordovician species, Macroporaster matutinus , suggest the need for greater mouth frame flexibility contributed to the evolution of the modern odontophore. Axillary development is linked to the common absence of actinal ossicles from Paleozoic species. ▭ Asteroidea, Echinodermata, functional morphology, ontogeny, phylogeny, Ordovician, Paleozoic .     

THE BETA-BINOMIAL MODEL: ACCOUNTING FOR INTER-TRIAL VARIATION IN REPLICATED DIFFERENCE AND PREFERENCE TESTS

Binomial tests are commonly used in sensory difference and preference testing under the assumptions that choices are independent and choice probabilities do not vary from trial to trial. This paper addresses violations of the latter assumption (often referred to as overdispersion) and accounts for variation in inter-trial choice probabilities following the Beta distribution. Such variation could arise as a result of differences in test substrate from trial to trial, differences in sensory acuity among subjects or the existence of latent preference segments. In fact, it is likely that overdispersion occurs ubiquitously in product testing. Using the Binomial model for data in which there is inter-trial variation may lead to seriously misleading conclusions from a sensory difference or preference test. A simulation study in this paper based on product testing experience showed that when using a Binomial model for overdispersed Binomial data, Type I error may be 0.44 for a Binomial test specification corresponding to a level of 0.05. Underestimation of Type I error using the Binomial model may seriously undermine legal claims of product superiority in situations where overdispersion occurs. The Beta-Binomial (BB) model, an extension of the Binomial distribution, was developed to fit overdispersed Binomial data. Procedures for estimating and testing the parameters as well as testing for goodness of fit are discussed. Procedures for determining sample size and for calculating estimate precision and test power based on the BB model are given. Numerical examples and simulation results are also given in the paper. The BB model should improve the validity of sensory difference and preference testing.     

HANDS-FREE, "POP-UP," ADHESIVE TAPE METHOD FOR MICROBIAL SAMPLING OF MEAT SURFACES

Microbial contamination of food is a concern to both food producers and consumers. For the food production industry surface sampling of foods is one of the simplest ways to monitor the microbial load. The objective of this experiment was to investigate the feasibility of using the less expensive and quicker "Pop-up" tape method instead of the conventional swab/rinse method for the microbial sampling of meat surfaces. An analyst can place the unit on the wrist and then use both hands to lay out all the necessary materials and take the sample with one hand. The "Pop-up" tape method was able to measure microbial loads up to 2.2 log CFU/cm² on meat surfaces. The conventional swab/rinse method was able to measure up to 8.3 log CFU/cm² on meat surfaces. The correlation coefficient (R) between the two methods was 0.91 (n=42). These data show that the "Pop-up" tape method is a viable alternative to other methods for estimating microbial surface contamination.     

THE POWER OF THE " A" -" NOT A" METHOD

The " A" - " Not A" method is a rating method with two categories. It is often treated as a discrimination method. Unlike forced choice procedures, the Thurstonian model for this method involves a choice criterion. In statistical tests, it is treated as a comparison of two proportions. In this paper, the power for hypothesis tests involving the monadic and replicated monadic " A" - " Not A" method is discussed. The power functions and the sample sizes needed for 80% power are given based on Thurstone's δ. Designs with equal and unequal allocations for A and A (Not A) samples are considered. The power of the method is also compared with that of four forced choice methods under the assumption that the perceptual variance is identical among methods. The comparison shows that, in general, the power for the five methods ranks from high to low: the 3-AFC, 2-AFC, " A" - " Not A", triangular and duo-trio. The comparison also shows that, based on the same number of panelists and/or the same sample size for the A and A⁻ samples for the methods, if the panelists are not too discrepant and the choice criterion in the " A" - " Not A" method is not too strict or too lax, the power of the " A" - " Not A" method is very close to that of the 2-AFC method.