Divergent evolution of male-determining loci on proto-Y chromosomes of the housefly

Houseflies provide a good experimental model to study the initial evolutionary stages of a primary sex-determining locus because they possess different recently evolved proto-Y chromosomes that contain male-determining loci (M) with the same male-determining gene, Mdmd. We investigate M-loci genomically and cytogenetically revealing distinct molecular architectures among M-loci. M on chromosome V (M^V) has two intact Mdmd copies in a palindrome. M on chromosome III (M^III) has tandem duplications containing 88 Mdmd copies (only one intact) and various repeats, including repeats that are XY-prevalent. M on chromosome II (M^II) and the Y (M^Y) share M^III-like architecture, but with fewer repeats. M^Y additionally shares M^V-specific sequence arrangements. Based on these data and karyograms using two probes, one derives from M^III and one Mdmd-specific, we infer evolutionary histories of polymorphic M-loci, which have arisen from unique translocations of Mdmd, embedded in larger DNA fragments, and diverged independently into regions of varying complexity.