What Can Be Learnt about Selection from Gene Frequency Distribution?

Polymorphism has been studied at the Esterase 6 locus in the Yellow Fever mosquito Aedes aegypti (L.) in laboratory stocks. At least 12 alleles are present, with up to four coexisting in a stock. The allele frequency distribution is quite sharply peaked at a mode of about 0.25. The experimental data are compared with the results of simulation based on two models, one in which the initial global distribution is taken to be the stationary distribution obtained from the neutral model assuming M = 4 mu Ne = 1 and the other in which the initial global distribution is generated from the experimental populations studied. The results suggest that the patterns observed are not likely to arise through random fluctuation of frequencies in neutral alleles, but that some kind of selection maintains polymorphism, either in the wild or in the laboratory, or both.     

Regulation of gallbladder cholesterol concentration in the hamster. Role of hepatic cholesterol level

The goal of this investigation was to determine how alterations in hepatic cholesterol metabolism influence the cholesterol content of gallbladder bile in hamsters. Although the rate of hepatic cholesterol synthesis was varied over 600-fold, there was no direct relationship between the rate of cholesterol synthesis and the cholesterol content of gallbladder bile. However, expansion of the hepatic cholesterol pool by 42-fold resulted in an 11-fold increase in gallbladder bile cholesterol. Examination of four subfractions of the hepatic cholesterol pool revealed that the cholesterol content of gallbladder bile was most consistently correlated with the free cholesterol level in both hepatic tissue and hepatic microsomes from all experimental groups. In most groups of animals in which gallbladder bile cholesterol was increased, plasma lipoprotein cholesterol levels were also increased. It was concluded that in hamsters, under these experimental conditions, changes in the cholesterol content of gallbladder bile were directly related to alterations in cholesterol content of the liver and most closely related to alterations in the free cholesterol content of that tissue.     

The subcellular location of isozymes of NADP-isocitrate dehydrogenase in tissues from pig, ox and rat

Antibodies against purified NADP-isocitrate dehydrogenase from pig liver cytosol and pig heart were raised in rabbits. The purified enzymes from these sources are different proteins, as demonstrated by differences in electrophoretic mobility and absence of crossreactivity by immunotitration and immunodiffusion. The NADP-isocitrate dehydrogenase in the soluble supernatant homogenate fraction from pig liver, kidney cortex, brain and erythrocyte hemolyzate was identical with the purified enzyme from pig liver cytosol, as determined by electrophoretic mobility and immunological techniques. The enzyme in extracts of mitochondria from pig heart, kidney, liver and brain was identical with the purified pig heart enzyme by the same criteria. However, the 'mitochondrial' isozyme was the major component also in the soluble supernatant fraction of pig heart homogenate. The 'cytosolic' isozyme accounted for only 1-2% of total NADP-isocitrate dehydrogenase in pig heart, as determined by separation of the isozymes with agarose gel electrophoresis and immunotitration. The mitochondrial isozyme was also the predominant NADP-isocitrate dehydrogenase in porcine skeletal muscle. The ratio of cytosolic/mitochondrial isozyme for porcine whole tissue extract, determined by immunotitration, was about 2 for liver and 1 for kidney cortex and brain. The distribution of isozymes in cell homogenate fractions from ox and rat tissues corresponded to that observed in organs of porcine origin. The mitochondrial and cytosolic isozymes from ox and rat tissues exhibited crossreactivity with the antibodies against the pig heart and pig liver cytosol enzyme, respectively, and the electrophoretic migration patterns were similar qualitatively to those found for the isozymes in porcine tissues. Nevertheless, there were species specific differences in the characteristics of each of the corresponding isozymes. NAD-isocitrate dehydrogenase was not inhibited by the antibodies, confirming that the protein is distinct from that of either isozyme of NADP-isocitrate dehydrogenase.     

The ecology of flower and bulbil production in Polygonum viviparum

Polygonum viviparum produces inflorescences which bear both sexual flowers and asexual bulbils in highly variable proportions. This paper investigates the proportion of flowers and bulbils per inflorescence at 12 sites in central Norway. It is shown that there is a 'trade—off between flowers and bulbils so that one cannot be increased without reducing the other'. On average, the balance is tilted in favour of bulbils, but there are significant differences in the proportion of bulbils per inflorescence among sites and among quadrats within 11 of the sites. Of these 11 sites there are 6 in which the proportion of bulbils per inflorescence is correlated with factors of the biotic environment. The factors include: (1) the amount of vascular plant cover, which is negatively correlated with the proportion of bulbils at 2 sites; (2) the cover of P. viviparum , which is positively correlated at 1 site; (3) the number of P. viviparum inflorescences, which is negatively correlated at 3 sites. The results suggest that both intra– and inter–specific factors of the biotic environment play a role in the control of bulbil and flower production in P. viviparum.     

Comparative rates of transfer of N-acetylneuraminic acid to acceptors bearing one or more Gal(beta 1-4)GlcNAc terminus by the Gal(beta 1-4)GlcNAc(NeuAc-Gal) (alpha 2-6)-sialyltransferase from embryonic chicken liver. Utilization of oligosaccharides as acceptors in sialyltransferase assays.

Using a number of branched and unbranched oligosaccharides, glycoproteins and artificial glycoproteins bearing Gal(beta 1-4)GlcNAc-R termini as acceptors (where R represents H, oligosaccharide, oligosaccharide-protein or fatty acid-protein), the comparative rates of transfer of NeuAc by the Gal(beta 1-4)GlcNAc(NeuAc-Gal) (alpha 2-6)-sialyltransferase of embryonic chicken liver were determined. Acceptor substrates were utilized at levels approximating physiological, near the Km value of the best acceptor, desialylated alpha 1 acid glycoprotein. The sialyltransferase has a marked preference for multi-branched acceptors. From the specificity data, it is concluded that the enzyme binds at least two Gal(beta 1-4)GlcNAc termini of an acceptor molecule, and that the relative orientation of the branches is an important factor determining the rate of catalysis by the enzyme. The use of oligosaccharides as acceptors to study sialyltransferase catalyses is emphasized. Results are discussed in the context of the mode of assembly of sialoside termini of known glycoprotein structures in vivo.     

Tryptophan and glucose metabolism in rat liver cells. The effects of DL-6-chlorotryptophan, 4-chloro-3-hydroxyanthranilate and pyrazinamide.

Liver cells pre-incubated with 1 mM-DL-6-chlorotryptophan are less sensitive to tryptophan-mediated inhibition of gluconeogenesis; this effect is apparent both at physiological (0.1 mM) and higher (0.5 mM) concentrations of tryptophan. 4-Chloro-3-hydroxyanthranilate (1-100 microM) has effects similar to those of DL-6-chlorotryptophan. The effects of both compounds are consistent with a decrease in quinolinate formation, a consequence of inhibition of 3-hydroxyanthranilate oxidase. Pyrazinamide (0.25-5.0 mM) significantly decreased flux through the glutarate pathway and potentiated tryptophan-mediated inhibition of gluconeogenesis; these changes were apparent at physiological concentrations of tryptophan. The effects of pyrazinamide are consistent with an increase in quinolinate formation resulting from inhibition of picolinate carboxylase.     

Effect of host density on searching behaviour of Nemeritis canescens (Hymenoptera: Ichneumonidae)

Observations were made over one hour of individual parasites, N. canescens, searching for their hosts, larvae of the moth Plodia interpunctella, at five different host densities. Records were made of the total number of hosts encountered at each density, the total number of eggs laid and the handling time per host.Holling's disc equation was found to fit the data well and gave estimates for rate of parasite search (a) and handling time (b) close to values actually observed. Despite the good fit, rate of search and handling time both declined as host density increased. The time spent handling already parasitised hosts, avoidance time was found to be about half the value of handling time.

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Resume Les observations ont été faites durant une heure sur des parasites isolés, N. canescens, prospectant des lots de chenilles de Plodia interpunctella — leurs hôtes — à cinq densités différentes. Pour chaque densité, ont été notés: le nombre total d'hôtes rencontrés, le nombre total d'ufs pondus et le temps d'examen par hôte.L'équation de Holling s'est bien adaptée aux données et a fourni des estimations pour le taux de prospection des parasites (a) et le temps d'examen (b) proches des valuers réellement observées. Malgré une bonne adaptation, le taux de prospection et le temps d'examen diminuent tous deux quand la densité de l'hôte augmente. Le temps dépensé à l'examen des hôtes déjà parasités, temps de rejet correspond à environ la moitié du temps d'examen.