Inhibition of Leukaemia Virus Replication by Polyadenylic Acid

A PREVIOUS communication from this laboratory demonstrated that the DNA polymerase of the Rauscher leukaemia virus is strongly inhibited in vitro by unprimed, single stranded polyribonucleotides¹ as a result of competition between the polymers and the active template for the same enzyme binding site. This inhibition was apparently specific, since partially purified preparations of DNA polymerase from Escherichia coli and BALB/c mouse embryos were not inhibited in the same conditions. We attempted to determine therefore whether single stranded polyribonucleotides would have any effect on the activities of oncogenic RNA viruses in cultured cells.     

An Electron Microscopic Study of Double Fertilization in Allohexaploid Wheat Triticum aestivum L.

Double fertilization has been examined by electron microscopyin allohexaploid wheat, Triticum aestivum L. cv. Mardler. Serialsections through fertilized ovules revealed that following dischargeof the pollen tube contents into the degenerate synergid thelatter extended to form a continuum between the egg and centralcells. The two naked sperm nuclei were seen at the far end ofthis extended synergid. These observations suggest that thedegenerate synergid may play a role in transporting the spermnuclei to the site where they can be accepted by the egg andcentral cell. In comparison with double fertilization in Plumbagozeylanica L., we also suggest that the degenerate synergid preventsmale cytoplasms from being transmitted to the egg and centralcell. The present study also confirms that in the fertilizedcentral cell the maternal and paternal genomes remain physicallyseparate at least until the first nuclear division of the coenocyticphase of endosperm development. Double fertilization, degenerate synergid, Triticum aestivum     

The Cardiac Lethal Mutant of Ambystoma mexicanum: A Re-examination

SYNOPSIS. Hearts of embryonic axolotls (Ambystoma mexicanum)homozygous for gene c do not beat in situ. Under appropriateculture conditions they rapidly commence beating, albeit lessvigorously than similarly explanted hearts of comparably stagednormal siblings. As part of this symposium, we have shown acine record demonstrating heartbeat in mutant hearts and comparingit to normal heartbeat. Myocardia of normal embryos exhibita characteristic pattern of birefringent, striated myofibrils.Mutants of the same stage contain hearts with birefringent fibrilsorganized in a pattern similar to that found in normal myocardia.A striking difference is that obvious striations are lackingin fibrils of the mutant. Electrophoresis of normal and mutanthearts in SDS-polyacrylamide gels shows that the major myofibrillarproteins are present. We conclude that induction of c/c heartshas been relatively normal and suggest that, due to the rapidityof recovery, the observed phenomena are perhaps due to somethingas simple as a defect in ionic conditions.     

Population Size,Clones and Morphogenesis in Naegleria gruberi*

SYNOPSIS. The size of the population of Naegleria gruberi at the time the amoebae are offered the opportunity to become flagellated is not a critical factor in the morphogenesis of this organism. Small populations (1-15 cells) readily become flagellated. Small populations (1-5 cells) washed several times also become flagellated. Clonal populations (25) have been established. All clones yield flagellates under the usual conditions. It is suggested that the physiological state of the amoebae is a factor in determining the number of cells that will undergo morphogenesis at any given time.     

Warm-Bodied Fish

SYNOPSIS. Two groups of fishes, the tuna and the lamnid sharks,have evolved ounter-currentheat-exchange mechanisms for conservingmetabolic heat and raising their body temperatures. Warm musclecan produce more power, and considering the other adaptationsfor fast swimming in these fish, it seems likely that the selectiveadvantages of greater speed made possible by the warm musclewere important in the evolution of this system. Some tunas suchas the yellowfin and skip jack are at a fixed temperature differenceabove the water, but bluefin tuna can thermoregulate. Telemetryexperiments show that the bluefin tuna can maintain a constantdeep body temperature during marked changes in the temperatureof its environment.     

Effects of Zinc on Meristem Size and Proximity of Root Hairs and Xylem Elements to the Root Tip in a Zinc-tolerant and a Non-tolerant Cultivar of Festuca rubra L.

A 4 d exposure to zinc (0.1 and 02 µg Zn cm^–3) reducedthe length of the root apical meristem in a Zn-sensitive cultivar(S59) of Festuca rubra L. to a much greater extent than in aZn-tolerant cultivar (Merlin). In S59, Zn treatment also inducedroot hair and xylem formation much closer to the root cap boundarythan in control roots, whereas Merlin was only marginally affectedby Zn treatment. The data are discussed in relation to previouslyestablished effects of Zn on the cell cycle and other cellularcharacters of the two cultivars. zinc, meristem size, root hair, xylem, Festuca rubra     

Phylogeny of Palaearctic Syrphidae (Diptera): evidence from larval stages

We estimated the phylogeny of Palaearctic Syrphidae using 187 larval morphological characters obtained from about 65% of the fauna (85 supraspecific taxa represented by 118 species) and based the analysis at the generic, level. The root of the syrphid tree was established from an outgroup consisting of other Aschizan families: the Platypezidae, Phoridae and Pipunculidae, with the tree rooted on the Lonchopteridae. The Syrphidae was the most derived Aschizan family. The Pipunculidae was the sister group to the Syrphidae. Eumerus was basal within Syrphidae. A trend exists towards increasing complexity of integumental folds and grooves across the Aschiza. In movement, the integument collapses along the line of these grooves. Grooves are evidence of muscles forming functional groups. Elaboration of independent groups of muscles appears to underlie much of the evolution of larval form within Aschiza. The basal feeding modes of syrphid larvae are mycophagy and phytophagy. Above these feeding modes, all remaining syiphids fall into one of two lineages comprising entomophages and saprophages, each of which has a single origin within the apparently polyphyletic Volucella. Major morphological innovation is associated with shifts between feeding modes; within feeding modes, change is gradual and tends towards increasing complexity. Change is mostly in structures associated with the mouthparts, thorax, anal segment and locomotory organs. Generic diagnoses with biological and taxonomic notes and a key to genera using larval characters are provided.