Molecular analysis of cyclodiene resistance-associated mutations among populations of the sweetpotato whitefly bemisia tabaci

Two polymerase chain reaction (PCR)-based molecular diagnostics were used to investigate whether cyclodiene resistance is uniquely associated with the novel “B” biotype of the sweetpotato whitefly Bemisia tabaci (Gennadius) and thus establish whether resistance could have acted as a driving force in the recent and rapid spread of this biotype. Previous studies have shown that a single point mutation coding for an alanine to serine replacement in the Drosophila Rdl gene confers high levels of resistance to cyclodiene insecticides. Following identification of an analogous point mutation in the B. tabaci Rdl homologue, PCR amplification of specific alleles demonstrated that the corresponding alanine to serine replacement is not confined to the B biotype but is also present in indigenous whitefly populations found on crop plants. Single-stranded conformational polymorphism (SSCP) analysis of the same region of the Rdl gene was used to confirm whitefly genotype and examine the degree of nucleotide polymorphism among whitefly strains. A comparison of SSCP banding patterns revealed a remarkable lack of nucleotide variation among strains conforming to the B biotype, whereas several of the non-B strains exhibited different banding patterns. Sequence analysis of these strains revealed one or more nucleotide polymerphisms including a novel resistance-associated mutation in one collection from the Sudan. These results show that cyclodiene resistance is not uniquely associated with the B biotype. However, the lack of genetic variability in the Rdl gene among B strains is consistent with the recent origin and spread of this novel biotype.