The male sex determining factor (M) in the house fly is linked to the Y chromosome in the ancestral condition, but can also be linked to another chromosome (I-V or X). However, descriptions of the linkage and frequency of M factors in different populations throughout the world are vastly incomplete. We collected house flies from a dairy in Ipswich, Australia, and determined that M was linked to chromosomes II, III, IV, and Y. Most males (69.8%) were homozygous for M on autosome II and/or III, and 92.3% of the males had multiple M factors. In all, there were 13 different male genotypes found. The high frequency of M, the presence of M on four different linkage groups, and the large number of male genotypes found in this population make it unique relative to other populations of house flies that have been examined.

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