RESISTANCE IN A LABORATORY POPULATION OF CULEX-QUINQUEFASCIATUS (DIPTERA, CULICIDAE) TO BACILLUS-SPHAERICUS BINARY TOXIN IS DUE TO A CHANGE IN THE RECEPTOR ON MIDGUT BRUSH-BORDER MEMBRANES

Direct binding experiments with isolated brush border membrane fractions (BBMF) from larvae of a susceptible laboratory strain of Culex quinquefasciatus Say, indicated the presence of a single class of Bacillus sphaericus binary toxin receptors. The dissociation constant (K-d) was approximately 11 nM and the maximum binding capacity (B-max) approximately 8 pmol/mg BBMF protein. Similar binding experiments with a field population of C. quinquefasciatus that had been selected in the laboratory to more than 100000-fold resistance to B. sphaericus binary toxin failed to reveal the presence of any specific binding. Thus this resistant strain had lost the functional receptor for B. sphaericus toxin. The binding characteristics of BBMF from the F-1 larval progeny (susceptible femalesXresistant males) were very close to those of the parental susceptible strain, consistent with the resistance being recessive.