Variation in the amount and activity of esterase 6 in a natural population of Drosophila melanogaster

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Abstract
Forty-two homozygous lines each isoallelic for the Esterase 6 (Est-6) locus were extracted from a natural population of Drosophila melanogaster. Homogenates of 4–5 day old virgin adults of each sex from several replicate cultures of each line were assayed for EST6 activity. Depending on the line, males had from three to nine times more EST6 activity than females. Both sexes showed highly significant differences in EST6 activity among lines, with 32 and 27 fold differences between highest and lowest lines for males and females respectively. However, the variation in EST6 activity among lines was only weakly correlated across the two sexes. Female EST6 activity did not differ significantly across the six electrophoretic variants of EST6 found among the 42 lines. On the other hand, a significant proportion of the variation among lines in male EST6 activity could be explained by differences among the six electromorphs. However, most of these differences were due to the relatively high activities of males from two relatively rare electromorphs and there were no significant differences in male activity among the four more common EST6 electromorphs. Radial immunodiffusion assays with polyclonal anti-EST6 antibody established that differences among lines in male EST6 activity were largely due to differences in the number of EST6 protein molecules, with negligible differences in their specific activities. It is concluded that the natural population segregates for genetic variance with large effects on the amount of EST6 protein; that there is little overlap in the variance expressed in the two sexes; and that most of the variance is different from the polymorphisms for electrophoretically detectable variants of EST6.