Geographic variation in the chromosomal location of the male sex determining factor (M) was studied in four house fly, Musca domestica L., populations from the eastern United States. We found a strong clinal trend (29° 41′ latitude in Florida to 44° 2′ in Maine) in which the percentage of standard XYM males increased with increasing latitude. In Florida, 100% of the males possessed the M factor on the third autosome (IIIM). North Carolina had 20% IIIM males and 2.35% with both YM and IIIM. Fewer IIIM males were found in New York (4.35%). Populations from Maine contained 100% XYM males. In two of three standard laboratory-susceptible strains, all males carried M on an autosome (“autosomal males” or AM): CS (IIIM) and SRS (VM). Insecticide bioassays of four field-collected strains led us to conclude that resistance is not correlated with sex determination over a broad range of insecticides. For example, high levels of resistance to permethrin (86–99% survival at a diagnostic concentration) were found in all four field-collected strains. The five other insecticides evaluated showed varying levels of resistance among field strains. We conclude that a cline is present in house fly populations from the eastern United States with 100% IIIM males in the south and entirely YM males in the north and that insecticide resistance is not a key factor influencing the evolution or linkage of M.
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Vol. 98 • No. 1