RESUMEN
We studied an imported host-parasitoid community in Hawaii, asking to what extent the species covaried in a systematic fashion even though all species were exotic to Hawaii, and occurred in an artificial agroecosystem (a commercial guava, Psidium guajava L., orchard). Using knock-down pyrethrin sprays we were able to accurately quantify numbers of the host, [oriental fruit fly, Bactrocera dorsalis (Hendel)] and its four major parasitoid species [Biosteres arisanus (Sonan), Diachasmimorpha longicaudata (Ashmead), Psyttalia incisi (Silvestri), and Bi. vandenboschi (Fullaway)] at hourly intervals. We found that the parasitoids' activity and abundance was well correlated with the activity and abundance of their host, and that all four parasitoid species covaried in concert with one another. In fact, the magnitude of correlation between the different species in this system was greater than the correlation with temperature. This show clearly that an entirely exotic community, reassembled piecemeal as a result of biocontrol efforts, can end up with patterns of temporal covariation that are highly coincident. One other interesting result concerns the speed with which sprayed trees were recolonized by the fruit fly and its parasitoids. The time that it took each species to reach its mean density prior to removal by the first pyrethrin spray at 0600 hours varied. It took 2 h for female B. dorsalis to recolonize guava trees to pre-spray levels. It took 3 h for Bi. arisanus, 4 h for D. longicaudata, 7 h for Bi. vandenboschi and 14 h for P. incisi to reach pre-spray levels. The fact that Bi. arisanus recolonized vacant trees almost as rapidly as did the fruit fly pest suggest that there is little opportunity for the fruit fly to escape in space and time by "staying one step ahead of its enemies".
RESUMEN
Warfarin is a potent inhibitor of vitamin K 2,3-epoxide reduction to vitamin K in vitro and in vivo. Dithiothreitol, an in vitro reductant for the vitamin K 2,3-epoxide reductase, antagonizes inhibition of the reductase by warfarin via mechanisms that have not been determined [Zimmermann, A., & Matschiner, J. T. (1974) Biochem. Pharmacol. 23, 1033-1040]. Experiments with rat hepatic microsomes were undertaken to characterize the interactions that exist between vitamin K 2,3-epoxide, warfarin, and dithiothreitol. Increasing concentrations of dithiothreitol decreased inhibition of the reductase by warfarin. When dithiothreitol was present prior to exposure of the reductase to warfarin, there was less inhibition than when the same concentration of dithiothreitol was present after its exposure to warfarin. Moreover, maximum inhibition of the reductase by warfarin occurred at a much slower rate when dithiothreitol was present initially. Inhibition of the reductase by warfarin was greater when the substrate concentration was 100 microM vitamin K 2,3-epoxide than when it was 10 microM epoxide. On the basis of these data, we conclude that (i) dithiothreitol reduces either directly or indirectly a critical disulfide within the reductase that it reoxidized during reduction of the epoxide substrate, (ii) warfarin and vitamin K 2,3-epoxide are not competitive with respect to one another, and (iii) warfarin binding, which produces inhibition, occurs solely to the disulfide form of the reductase. Once it is bound, warfarin inhibits further reduction of the critical disulfide by dithiothreitol. Dithiothreitol therefore antagonizes warfarin by maintaining the reductase in the reduced state.
