Geographic Distribution of Phosphine Resistance and Frequency of Resistance Genes in Two Species of Grain Beetles, and , in North America.

Resistance to the fumigant phosphine (PH) was studied for 28 populations of from eight states of the USA and four provinces of Canada, as well as for 34 populations of from twelve states of the USA and four provinces of Canada, using both a discriminating dose bioassay and molecular marker analysis. We used a molecular marker analysis for a point mutation in the gene that encodes dihydrolipoamide dehydrogenase and facilitates the "strong resistance" phenotype in both species. Our results showed that PH resistance was correlated with higher frequencies of the strong resistance R allele in both species (R = 0.59 in and R = 0.79 in ). We also found that recessive R allele frequency did not correlate well with the geographic distribution of the resistant populations of these two species (R = 0.21 in and R = 0.15 in ). Therefore, populations of both species with higher R allele frequencies had higher resistance levels to PH. Our results showed that the geographic distribution of PH resistance in both species varied and was not related geographically, but this supports the idea that the adaptive evolution of PH resistance in these species is caused by selection pressure for their resistance genes.