The Effect of Hadacidin on Bud Development and its Implications for Apical Dominance

Hadacidin, which is known to inhibit the enzyme adenylo-succinate synthetase, was applied locally to lateral buds on decapitated Pisum sativum L. plants. In controls the buds grew out normally, but the treated buds were almost completely inhibited. After a delay, the buds overcame the inhibition, and their subsequent outgrowth could be further hastened by the local application of a cytokinin. The inhibition, unlike that due to IAA, was not transported to a second bud, and it could be largely reversed by kinetin or dimethylallylaminopurine. Adenine itself produces little or no reversal, however. There was some reversal by aspartic acid, which is known to reverse, at least partially, the hadacidin inhibition of the isolated enzyme. It is deduced that lateral bud development is dependent on the synthesis of a cytokinin, which may take place by a biochemical route similar, but probably not identical, to that for adenine. It follows that this synthesis takes place locally in the bud itself.     

Changes in Enzyme Activities and Lipid Content of Echinoderm Egg Membranes, at Maturation and Fertilization

Membrane enriched fractions of eggs show changes in relativelipid and acylglyceride fatty acid content at maturation andfertilization, when many critical membrane changes occur. Manyof the changes in lipids seen at fertilization in sea urchinsare reversed from those seen at maturation in starfish. Maybea special permissive lipid content appears at maturation anddisappears at fertilization. Mycostatin alters the lipid contentof unfertilized eggs, prevents fertilization, blastula cellaggregation and normal development, with decreasing susceptibilitywith age, suggesting altered cell surfaces during development.Membrane enriched fractions also show changes in enzyme specificactivity at fertilization. Attempts were made to localize theseenzymes in various surface components (membrane or associatedcoats or adherent cortex) and several fractions were separated.Egg surfaces are complex and therefore difficult to study. DTTor pronase treatment, for example, changes the activity of G6PDin the surface of whole eggs, with further changes occurringin such eggs following fertilization. Increases in G6PD activityin ghosts and supernate of fertilized eggs appear due to isoenzymechanges instead of one enzyme changing compartment. Isozymepatterns of G6PD extracted from pellets differ from those ofsupernates. Patterns also differ following fertilization orpartial activation by ammonia or calcium ionophore. Some isozymesseem to be breakdown products of larger forms, and can be producedby treatment with papain, DTT, or urea. Isozyme studies supporta dynamic model of the egg surface. Surface changes may occurenzymatically, by insertion of vesicles, or by release of materialfrom vesicles.     

Cold tolerance in diapausing and non-diapausing stages of the flesh fly, Sarcophaga crassipalpis

ABSTRACT. Supercooling points (SCP) and low temperature tolerance were determined for larval, pupal and adult stages of Sarcophaga crassipalpis Macquart (Diptera: Sarcophagidae). No stage tolerates tissue-freezing. Ontogenetic changes in SCP profiles are similar for comparable developmental stages of diapause and non-diapause groups. Feeding larvae have SCPs near -7°C which decrease to -11°C in the postfeeding wandering phase of the final larval instar. The lowest SCPs are recorded for pupae at -23°C. The capacity to survive at -17°C varies with age of the diapausing pupae: 10-day-old pupae are less cold tolerant than pupae that have been in diapause for 45–80 days. Although the SCP of non-diapausing pupae is as low as in diapausing pupae, non-diapausing pupae are extremely sensitive to low temperature exposure and do not survive to adult eclosion when exposed to -17°C for as little as 20 min. The use of hexane to break pupal diapause has no effect on SCPs or low temperature tolerance.     

Declining acceptability of a food plant for the polyphagous grasshopper Schistocerca americana: the role of food aversion learning

ABSTRACT Successive contacts with a food that was initially acceptable to the acridid Schistocerca americana (Drury) (Orthoptera: Acrididae) were monitored. It was found that acceptability, as measured by meal length, declined with experience until the food, spinach, was completely rejected. Reduced acceptability of spinach was maintained even after feeding for three meals on a fully acceptable and suitable food, a period of 2–3 h. Experiments with spinach-filled capsules placed in the gut suggested that the decline in acceptability was not due to direct feedback from the gut or haemolymph on the sensory or feeding control systems. Food aversion learning is indicated as the mechanism underlying the decline in acceptability.