Sex chromosome evolution in the house fly: Genomic distribution and structure of polymorphic male determining loci

An essential characteristic of life is reproduction. Many organisms reproduce sexually through male and female individuals. The two sexes often carry different sex chromosomes that determine sexual fates of individuals, such as in humans, where females have two X- chromosomes and males have one X and one Y. Evolutionary theory proposes that sex chromosomes emerge from a pair of regular chromosomes (known as autosomes) by obtaining sex-determining genes via, for instance, mutations or gene translocation. However, this hypothesis remains largely untested due to lack of suitable systems. The house fly (Musca domestica) may be an irritating insect, but it is very interesting for studying sex chromosome evolution, because it harbours populations with different sex-determining chromosomes. A typical XY sex chromosome system, like in humans, can be found in some populations, where the Y chromosome carries a male determining locus (M). Atypically, in some populations, any of the other five autosomes or even the X chromosome can act as “Y chromosome” because they also can carry the M locus. The aim of my thesis is to better understand how various M loci are formed and how they are maintained in nature. I localize M loci on different chromosomes, report variation in genomic locality and frequency of M loci in neigbhouring populations and compare the DNA sequences of various M loci. I reconstruct how various M loci may have originated and translocated in the housefly genome. My findings add to our knowledge about how sex-determination systems can change and how sex chromosomes can evolve.