In the last several years, strong resistance to the fumigant phosphine in the Lesser Grain Borer Rhyzopertha dominica has been identified in different geographical regions of Australia. The present study reveals that phosphine resistance levels of strongly resistant R. dominica strains recently isolated from New South Wales (SRNSW) and South Australia (SRSA), are, respectively, ~225-fold and ~100-fold greater than the baseline response of a sensitive reference strain. Molecular analysis indicates that high-level resistance in these strains was derived independently from that in a previously described strain from Queensland (SRQLD). Genetically, however, the three independent outbreaks of high-level resistance are quite similar. Resistance in both SRQLD and SRNSW is conferred by two genes, rph1 and rph2. Resistance in SRSA is also conferred by two genes, one of which is an allele of rph1. The second resistance factor in SRSA seems to be a weak resistance allele of rph2. High-level resistance in all three strains is neither mitochondrially encoded nor sex-linked, whereas it is incompletely recessive.
The present study demonstrates that the rph1 gene also contributes to resistance in strong resistant strain SR2QLD, isolated from central Queensland. The rph1 gene is therefore shared among all four resistance outbreaks in different geographical regions in Australia. As with SRSA, a weak resistance allele of rph2 also seems to contribute to resistance in SR2QLD. Combining the resistance genes from the four strongly resistant strains SRQLD, SR2QLD, SRNSW and SRSA, produced a less than 2 fold enhancement of the resistance phenotype. This result is consistent with the conclusion that the four strains simply harbour alternative alleles at the two major resistance loci, rph1 and rph2 and that any additional resistance genes are of minor effect. Thus, the number of genetic mechanisms by which insects become resistant to phosphine is severely constrained and may be restricted to as few as two genes.
Toxicological analysis reveals that the phosphine resistance allele at the rph1 locus confers cross resistance against the cytochrome c oxidase toxin, dimethyl disulphide (DMDS). Thus, the rph1 locus is responsible for a common mechanism of resistance to both DMDS and phosphine. A second resistance gene, rph2, which synergistically enhances the effectiveness of rph1 against phosphine, does not confer any additional cross resistance against DMDS. Furthermore, phosphine and DMDS exhibited a synergistic increase in toxicity that was equivalent in both resistant and sensitive strains of R. dominica. Thus, the synergism represents a unique mode of action against which existing resistance mechanisms are ineffective. The mitochondrial uncoupler compound, chlorfenapyr, produced an equivalent effect in both sensitive and resistant strains. This eliminates
the possibility that resistance results from a constitutive down regulation of oxidative phosphorylation, as this should result in hypersensitivity toward chlorfenapyr.
The phosphine resistant strains consistently show greater resistance to starvation than the sensitive strains. This cross resistance is likely contributed by the rph1 resistance factor as all of the resistant strains showed similar starvation resistance. This suggests that resistance against phosphine, starvation and the mitochondrial toxin, DMDS, results from a shared mechanism.