AMPLIFIED ESTERASES A2 AND B2. HAS RESISTANCE OCCURRED ONCE OR SEVERAL TIMES?

Abstract:
Elevation of esterase activity is the most common form of organophosphate resistance in the Culex pipiens complex. There are several electrophoretically distinguishable forms of esterase which can be elevated. Raymond et a1 (1991) have put forward the hypothesis that the most common form of elevation, involving esterases A, and B2, has occurred only once and then spread worldwide through migration. Their hypothesis is based on identical restriction fragment length patterns observed in the amplicon containing the & esterase from Culex strains worldwide. This does not, however, account for the different resistance patterns observed in the Culex strains which contain only this esterase mechanism. A study of five resistant strains, including two collected from the same cities as Raymond's study, and one insecticide susceptible strain, has now revealed that both the A2 and B2 esterases interact with a range of insecticides to a similar extent, hence both have a role in resistance. Within the different Culex populations the A2s can be separated on the basis of their physical characteristics and interactions with a range of organophosphates plus the carbamate propoxur. The B2 esterases are more similar between strains, but some of these are also kinetically distinct. The data suggest that the A2 and B2 esterases, as visualised on polyacrylamide or starch gels do not represent a single allelic form of either esterase, but that these both contain allelic variants with similar Rf values. This has already been documented for the B4/B5 esterases, and unpublished data suggests that the same is true for the BI esterase in Californian and Cuban populations of Culex quinquefasciatus. We propose that the hypothesis of a single amplification event of the B2 allele, and its subsequent spread through migration, is too simplistic. Data on the purified esterases from a range of strains suggests that either multiple allelic forms of A2 and B2 are present in a single strain or that different allelic forms occur in each strain. The elevated esterases have only been detected over the last thirty years, and their initial appearance in a number of populations is well documented. This time scale is insufficient for variation to occur on the scale demonstrated here, from the amplification of a single allelic form of both the A2 and B2.