In house flies sex is determined by M on the Y chromosome.  "Autosomal" or "atypical" strains have been described where the M factor is located on one or more of the five autosomes (I-V) or the X.  In Florida (FL), 100% of the males possess the M factor on chromosome III based on collections made in 1973 and 2002 leading to the idea that there is some selective advantage for autosomal males in FL due to its stability for the past 30 yr.  Collections made in 2002 from North Carolina (NC) had 20% III^M and 2.35% with both the Y^M and III^M mechanisms in the same individual.  Fewer III^M males were located in New York (4.35%).  The Maine population was entirely XY^M.  However, it has not been determined how rapidly these mechanisms change over time.

To study the evolution of sex determination we compared changes in the linkage of M in flies from a NC dairy in 2002, 2006 and in the original population that had been maintained in the lab from 2002-2006.   The field collections in both 2002 and 2006 consisted of 78% XY^M, 20% III^M, and <3% with both III^M and XY^M.  However, after 4 years in the laboratory, frequencies had changed to 15% X^MY^M and 85% XY^M.  These results indicate that the linkage of M can change rapidly or be maintained, depending on the environmental conditions.