In the house fly, Musca domestica L., sex is determined by a dominant factor, M, located on the Y chromosome.  However, there are "autosomal male" (A^M) strains in which the M factor is located on one or more of the five autosomes (I-V) or even on X.  The appearance of autosomal males is not random. Latitudinal clines exist in Europe and the United States.  It is unknown whether the frequency of A^M versus Y^M males is stable in populations over time.  Populations that contain males with multiple M factors (III^M and Y^M for example) or males homozygous for an A^M factor contain an F factor to produce females. 

The linkage of sex determination under two different environmental conditions was studied in the house fly.  We compared the linkage of M in North Carolina populations after a four year period in the laboratory or the field.  In 2002 male house flies were 77.65% Y^M, 20% III^M, and 2.35% with both III^M and Y^M.  After 4 years in the laboratory, the frequency had changed to include homozygous males and had eliminated autosomal males.  The frequencies in the field were similar to that of 4 years ago with the addition of the X^MY^M genotype.  The presence of homozygous M males offered the opportunity to evaluate the frequency of the female determining F factor.  F was found to be present in both the lab and field populations, but frequencies varied.