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Selfish genetic elements can distort the sex ratios of their hosts by increasing their own transmission to the next generation in a non-mendelian fashion. These elements can be either nuclear genes on a sex chromosome or cytoplasmically inherited microbes, and achieve an increased transmission by manipulating gametogenesis or host reproduction. Often these selfish elements benefit from a female biased population (for example heritable microbes are passed on maternally in the egg cytoplasm), while non-selfish, autosomal genes are selected to produce a balanced sex ratio. These differing reproductive strategies cause a genetic conflict that results in an “evolutionary arms race” that can promote the evolutionary change of sex determination systems. In this thesis, I investigate an extreme sex ratio distortion in a species of booklouse, Liposcelis sp. This species contains two distinct female types, one of which carries a maternally transmitted selfish genetic element that results in exclusively female offspring being produced. Recently, a candidate for the sex ratio distortion was identified as a horizontally transferred bacterial gene, that we have called Odile, and that is present in the genome of the (distorter) female carrying the distorting element. The gene originates from the endosymbiotic bacterium Wolbachia that is well known for its ability to distort the sex ratio of its hosts. I investigated this horizontal gene transfer event and attempt to characterize Odile. I provide evidence that this Wolbachia gene has been integrated into the genome of the distorter females and is not a bacterial contaminant. I found that the Odile gene has been duplicated and may have been horizontally transferred from Wolbachia independently to at least three other insect genomes. Additionally, I found that Odile is transcribed at low levels in a life-stage specific manner that is suggestive of a role in development. Additionally, I looked into male mate choice in this species as one aspect of the persistence of the distorting element. I found that male Liposcelis sp. do not discriminate between the two female types and do not spend more time mating with one female type over the other. These results contribute to ongoing research into the extreme sex ratio distortion found in this species and the candidate gene that may be the cause. Selfish genetic elements are an important driver of sex determination evolution, and Liposcelis sp. provides a unique and exciting system to investigate the implications of selfish elements in a genome further. / Graduate / 2019-12-17