Seasonal fluctuation in susceptibility to insecticides within natural populations of Drosophila melanogaster. Empirical observations of fitness costs of insecticide resistance.

Abstract

To investigate genetic variation and seasonal fluctuation in susceptibility to insecticides, natural populations of Drosophila melanogaster were collected from Katsunuma in mid summer and late fall for two consecutive years. After isofemale lines of each population collected in each season had been established in a laboratory, the susceptibility of each line to five insecticides, including permethrin, malathion, prothiophos, fenitrothion, and DDT, was examined. Lines of each population exhibited the broad ranges of variation in susceptibility to all chemicals. Comparison between populations in different seasons indicated that genetic variation in susceptibility to organophosphates fluctuated in consistency with the population size, in which the susceptibility increased in fall. In addition, highly significant correlations were observed among responses to organophosphates, and the correlations also fluctuated with seasons. On the other hand, genetic variation in susceptibility to permethrin and DDT was less fluctuated. These results suggest that not only a common resistance factor for organophosphate resistance but also different resistance factor(s) for each insecticide could be involved within a natural population, and that the fluctuation observed in the susceptibility to organophosphates could be associated with fitness costs of organophosphate resistance factor(s).

Seasonal fluctuation in susceptibility to insecticides within natural populations of Drosophila melanogaster. II. Features of genetic variation in susceptibility to organophosphate insecticides within natural populations of D. melanogaster

“…The mean susceptibility to the three chemicals tended to increase in the fall, when the population increased drastically on a large amount of squeezed grapes dumped outside (Watanabe and Oshima, 1970). This tendency was repeatedly observed in the Katsunuma population in 1997 and 1998 (Miyo et al, 2000). In a series of studies, we have analyzed the relationship between resistance to organophosphates and fitness, from the standpoint that the resistance factor for the three organophosphate insecticides had disadvantageous effects on fitness under no insecticide condition, especially under density-independent conditions Miyo et al, 2003;Miyo and Charlesworth, 2004).…”

“…In this Katsunuma population of D. melanogaster , we have observed seasonal fluctuations in susceptibility to three organophosphate insecticides (Miyo et al, 2000). The mean susceptibility to the three chemicals tended to increase in the fall, when the population increased drastically on a large amount of squeezed grapes dumped outside (Watanabe and Oshima, 1970).…”

“…In a series of studies, we established many isofemale lines collected from this natural population and investigated insecticide resistance, using these lines (Miyo et al, 2000(Miyo et al, , 2001. We demonstrated that one resistant line #1465, screened from the Katsunuma population of D. melanogaster , possessed two resistance factors for three organophosphate insecticides, one on the second chromosome and the other on the third chromosome .…”

“…To do this, we first reanalyzed mortality data sets used in the previous studies (Miyo et al, 2000(Miyo et al, , 2001, and susceptibility of isofemale lines from the same natural populations (10-286 lines) was analyzed by analysis of variance. Then, we further conducted a simulation study and investigated how genotype frequencies, mean resistance levels, genotypic variances in organophosphate resistance, and gene frequencies fluctuate under the density-independent condition during a relatively short period of time, using the approximation method for the change in gene frequency within a population which is not subject to a density-dependent limit by the number of individuals (density-independent condition), assuming random mating (Charlesworth, 1974(Charlesworth, , 1994.…”

“…Then, we further conducted a simulation study and investigated how genotype frequencies, mean resistance levels, genotypic variances in organophosphate resistance, and gene frequencies fluctuate under the density-independent condition during a relatively short period of time, using the approximation method for the change in gene frequency within a population which is not subject to a density-dependent limit by the number of individuals (density-independent condition), assuming random mating (Charlesworth, 1974(Charlesworth, , 1994. This is to test whether the observed seasonal fluctuations in susceptibility to the three organophosphates (Miyo et al, 2000) and in the observed genetic variances in susceptibility to the chemicals can be explained by disadvantageous effects of resistance genes on fitness under the density-independent condition.…”