Preliminary Study of the Genetics of House Fly (Musca domestica) Resistance to Malathion1

Abstract

The genetics of malathion resistance in a strain of house flies ( Musca domestica L.) was studied by selecting two strains, one susceptible and one resistant to malathion, from a slightly resistant field strain. The method of selection was by collecting eggs from individual females and increasing the population to sufficient numbers to determine their response. After three generations of selection a uniform malathion-resistant strain was obtained. A susceptible strain was obtained in one generation although selection was continued another generation. The parent strain was 1.7 times as resistant as the susceptible strain, while the resistant strain was 62 times as resistant. All insects used to determine response to malathion were laboratory-reared females which were tested 4 days after peak emergence with solutions made by diluting 95% technical malathion in glass-distilled acetone. Flies were held in recovery jars containing food, at a constant temperature until 24-hour mortality counts were taken. Strain differences other than malathion resistance were present. The resistant strain required a longer period from egg to adult than the susceptible strain. The males in the susceptible strain emerged significantly earlier than the females. The resistant strain was observed to have a lower fecundity than the parent and susceptible strains. The sex ratio in the selected strains did not differ from a 1:1 ratio. The F1 hybrids of reciprocal crosses between the susceptible and resistant strain segregated into 50% susceptible and 50% resistant individuals. The least tenuous explanation for this segregation is that malathion resistance is inherited by two allelic groups exhibiting incomplete duplicate dominance epistasis. The segregation of the F1 hybrids cannot be explained on the basis of a single gene or independent assortment of multiple genes, as the resistant strain is homogeneous malathion-resistant. The F2 hybrids did not differ significantly from the F1 hybrids. Maternal effects were present in the F1 hybrids but had disappeared in the F2 generation. That the F2 progeny of reciprocal crosses did not differ indicates that no sex-linkage is involved in the inheritance of malathion resistance.