| Two populations (Jackson Pike, laboratory-derived) of Chironomus riparius (Diptera:Chironomidae) were examined in terms of response to selection with DDT and malathion. Specific studies included: (1) examination of population specific fitness (growth, development, reproduction) and biochemical characteristics (mixed function oxidase (MFO), general esterase (GEST), acetylcholinesterase (AChE), and glutathione S-transferase (GSH) activity) before selection, (2) population specific biochemical and fitness response to selection with DDT and malathion, (3) population specific genetic (heritability of and genetic correlations between female fitness traits) and fitness response after selection with DDT and malathion, and (4) population specific change in genetic structure (change in genotype frequency at three enzyme in loci) and respiration rate after selection with DDT and malathion.;Initial population comparisons before selection with either compound indicate significantly different biochemical and fitness characteristics as well as susceptibility to insecticide exposure. For example, Jackson Pike (JP) larvae were significantly less susceptible to malathion, parathion, DDT, and propoxur than laboratory-derived (LAB) larvae. JP larvae were also characterized by significantly increased enzymatic activity (GSH), differential oxidative metabolism (MFO), reduced target site sensitivity (AChE) and reduced GEST activity.;Selection with DDT and malathion did not result in significantly changing JP sensitivity to either compound or any biochemical characteristic. DDT-selection, however, resulted in lighter JP females but no significant change in fitness trait heritability, fitness trait correlations, genotype frequencies at the glucose-6-phosphodehydrogenase (G6PDH), phosphoglucomutase (PGM), and phosphoglucoisomerase (PGI) loci or larval respiration was found. Malathion-selection of JP larvae resulted in reduced fecundity but no significant change in fitness trait heritability, genotype frequency at the G6PDH, PGI, PGM loci or larval oxygen consumption. Malathion selection of JP larvae did, however, result in a negative genetic correlation between female growth and development time, whereas a positive correlation was observed between growth and fecundity.;LAB larvae showed no significant increase in resistance to either compound after selection. DDT selection produced significantly increase GSH activity with adverse effects on female fitness, non-significant increase in development time heritability, no significant change in trait relationships (genetic correlations), genotypic frequencies at G6PDH, PGI, PGM enzyme loci or oxygen consumption. Malathion-selection of LAB larvae were characterized by a significantly different MFO spectrum and decreased AChE sensitivity to inhibition, reduced heritability of pupal weight and developmental period, no change in genetic trait relationships, respiration or genotype frequencies.;Results from all population comparisons suggest a population and insecticide specific response to DDT and malathion selection. For example, JP response suggests site-specific selection due to xenobiotics exposure, indirectly producing a population resistant to DDT and malathion. Conversely, LAB response suggests (1) limited variability for response due to selection pressures present under laboratory culture conditions, or (2) this population may be in the initial stages of developing resistance to DDT and malathion. |
