Malathion resistance in Tribolium strains and their hybrids: inheritance patterns and possible enzymatic mechanisms (Coleoptera, Tenebrionidae).

(1) The genetics of malathion resistance in two strains of the flour beetle, Tribolium castaneum, was investigated. In CTC-12, resistance is polygenic, while in Kano, it is due to a dominant allele at a single autosomal locus. Reciprocal hybrids with the susceptible control strains bb and pp showed an overdominant response in particular when Kano was the male parent in the original cross. (2) Three possible genetic mechanisms to explain these results are discussed. The model which best explains the genetic results, particularly the difference between the reciprocal crosses, assumes a modifier resistance allele on the Y chromosome. (3) The levels of activity of total esterases, carboxylesterases, mixed-function oxidases, epoxide hydrase, and glutathione transferase in the parent strains and their hybrids were measured quantitatively. Although total esterase activity may not be relevant for the breakdown of malathion, it was inhibited by the pesticide. The activity of the microsomal enzymes was high in CTC-12, low in bb, and intermediate in the hybrids, while carboxylesterases were very active in Kano as well as in the hybrids with bb and low in the latter. These patterns agree with the genetics of resistance in the two strains. A higher level of GSH transferase in the Kano x bb hybrids than in Kano seems to indicate a possible biochemical mechanism for their overdominant resistance.