Comparaison des potentialités biologiques de trois coccinelles prédatrices de la cochenille farineuse du maniocPhenacoccus manihoti (Hom. Pseudococcidae)

Résumé H. raynevali est la principale espèce de coccinelle importée de l'Amérique latine et acclimatée par l'I.I.T.A. pour lutter contre la cochenille du maniocP. manihoti. E. flaviventris etD. hennesseyi sont 2 autres coccinelles prédatrices présentes au sein de la biocoenose de la cochenille du manioc. Après avoir étudié leur biologie au laboratoire, on a pu comparer leurs potentialités respectives. A partir de ces résultats, on a tenté de déterminer le r?le que chaque espèce prédatrice pourrait éventuellement jouer dans les conditions naturelles et de définir les périodes les plus propices à la manifestation d'une meilleure efficacité.      

Oxalic acid production and its role in pathogenesis of Sclerotinia sclerotiorum

Abstract Sunflower plants were inoculated with a virulent isolate of Sclerotinia sclerotiorum and with the same isolate nutritionally conditioned to produce small amounts of oxalic acid. The preconditioned isolate behaved as hypovirulent. Tomato plants were inoculated with four S. sclerotiorum isolates of increasing virulence. A close correlation among disease severity, accumulation of oxalic acid, decrease in pH and inhibition of polyphenoloxidase in both infected host tissues was demonstrated. Oxalic acid production as an important factor of virulence in S. sclerotiorum is emphasized and its effect on the phenolic metabolism of the host via inhibition of polyphenoloxidase is suggested.     

The nature of the neutral Na+−Cl−-coupled entry at the apical membrane of rabbit gallbladder epithelium: I. Na+/H+, Cl−/HCO 3 − double exchange and Na+−Cl− symport

Summary Cl^– influx at the luminal border of the epithelium of rabbit gallbladder was measured by 45-sec exposures to³⁶Cl^– and³H-sucrose (as extracellular marker). Its paracellular component was evaluated by the use of 25mm SCN^– which immediately and completely inhibits Cl^– entry into the cell. Cellular influx was equal to 16.7eq cm^–2 hr^–1 and decreased to 8.5eq cm^–2 hr^–1 upon removal of HCO ₃ ^– from the bathing media and by bubbling 100% O₂ for 45 min. When HCO ₃ ^– was present, cellular influx was again about halved by the action of 10^–4 m acetazolamide, 10^–5 to 10^–4 m furosemide, 10^–5 to 10^–4 m 4-acetamido-4-isothiocyanostilbene-2,2-disulfonate (SITS), 10^–3 m amiloride. The effects of furosemide and SITS were tested at different concentrations of the inhibitor and with different exposure times: they were maximal at the concentrations reported above and nonadditive. In turn, the effects of amiloride and SITS were not additive. Acetazolamide reached its maximal action after an exposure of about 2 min. When exogenous HCO ₃ ^– was absent, the residual cellular influx was insensitive to acetazolamide, furosemide and SITS. When exogenous HCO ₃ ^– was present in the salines, Na⁺ removal from the mucosal side caused a slow decline of cellular Cl^– influx; conversely, it immediately abolished cellular Cl^– influx in the absence of HCO ₃ ^– . In conclusion, about 50% of cellular influx is sensitive to HCO ₃ ^– , inhibitable by SCN^–, acetazolamide, furosemide, SITS and amiloride and furthermore slowly dependent on Na⁺. The residual cellular influx is insensitive to bicarbonate, inhibitable by SCN^–, resistant to acetazolamide, furosemide, SITS and amiloride, and immediately dependent on Na⁺. Thus, about 50% of apical membrane NaCl influx appears to result from a Na⁺/H⁺ and Cl^–/HCO ₃ ^– exchange, whereas the residual influx seems to be due to Na⁺–Cl^– contranport on a single carrier. Whether both components are simultaneously present or the latter represents a cellular homeostatic counterreaction to the inhibition of the former is not clear.     

Th17 Effector Cells Support B Cell Responses Outside of Germinal Centres

Th17 cells are pro-inflammatory CD4⁺T cells, which are important in immune responses against fungal pathogens and extracellular bacteria and have also been implicated in various autoimmune syndromes. However, their role in supporting B cell responses in these scenarios remains unclear, representing a significant lapse in our understanding of the role Th17 play in vaccine responses and the regulation of autoimmunity. We employed T cell and B cell receptor transgenic mice specific for model antigens, and adoptive transfer approaches that allowed the tracking of cognate B and T cells in situ and ex vivo using immunological methods. We have found that T cells activated under Th17 polarising conditions have a greater capacity to provide cognate B cell help compared with Th1 polarised populations, supporting higher expansion of antigen specific B cells and enhanced antibody titres. This advantage is associated with the increased persistence of Th17 polarised cells in areas of the lymph nodes where they can provide help (i.e. the B cell follicles). Also the Th17 cells are characterised by their higher expression of ICOS, a costimulatory molecule important for B cell help. Surprisingly, contrary to published reports, Th17 cells were not detected inside germinal centres, although they were found in close proximity to cognate B cells in the follicle early in the genesis of the humoral immune response. These data indicate that, Th17 cells have a more significant role earlier in the initiation/development of the germinal centre response and/or germinal centre-independent events, consistent with their early effector status.     

Biochemical and genetic analysis of the four DNA ligases of mycobacteria

Mycobacterium tuberculosis encodes an NAD(+)-dependent DNA ligase (LigA) plus three distinct ATP-dependent ligase homologs (LigB, LigC, and LigD). Here we purify and characterize the multiple DNA ligase enzymes of mycobacteria and probe genetically whether the ATP-dependent ligases are required for growth of M. tuberculosis. We find significant differences in the reactivity of mycobacterial ligases with a nicked DNA substrate, whereby LigA and LigB display vigorous nick sealing activity in the presence of NAD(+) and ATP, respectively, whereas LigC and LigD, which have ATP-specific adenylyltransferase activity, display weak nick joining activity and generate high levels of the DNA-adenylate intermediate. All four of the mycobacterial ligases are monomeric enzymes. LigA has a low K(m) for NAD(+) (1 microm) and is sensitive to a recently described pyridochromanone inhibitor of NAD(+)-dependent ligases. LigA is able to sustain growth of Saccharomyces cerevisiae in lieu of the essential yeast ligase Cdc9, but LigB, LigC, and LigD are not. LigB is distinguished by its relatively high K(m) for ATP (0.34 mm) and its dependence on a distinctive N-terminal domain for nick joining. None of the three ATP-dependent ligases are essential for mycobacterial growth. M. tuberculosis ligDDelta cells are defective in nonhomologous DNA end joining.     

Desiccation resistance along an aridity gradient in the cactophilic fly Drosophila buzzatii: sex-specific responses to stress

Stress resistance characters are valuable tools for the study of acclimation potential, adaptive strategies and biogeographic patterns in species exposed to environmental variability. Water stress is a challenge to terrestrial arthropods because of their small size and relatively high area: volume ratio. Fruit flies have been investigated to record adaptive morphological and physiological traits, as well as to test their responses to stressful factors. In this study, we investigate the ability to cope with water stress, by examining variation in desiccation resistance in a species that lives mainly in desert lands. Specifically, we explored the genetic and ecological basis of desiccation resistance in populations of Drosophila buzzatii from Northern Argentina. We used a common garden experiment with desiccation treatments on a number of isofemale lines from four populations along an aridity gradient. Our results revealed significant among-population differentiation and substantial amounts of genetic variation for desiccation resistance. We also detected significant genotype-by-environment and genotype-by-sex interactions indicative that desiccation resistance responses of the lines assayed were environment- and sex-specific. In addition, we observed clinal variation in female desiccation resistance along gradients of altitude, temperature and humidity; that desiccation resistance is a sexually dimorphic trait, and that sexual dimorphism increased along the aridity and altitudinal gradients. Based on current evidence, we propose that the observed sex-specific responses may reflect different life history traits, and survival and reproductive strategies in different ecological scenarios.