Meiotic Drive Via Sex Ratio Chromosome In Drosophila Psuedoobscura
Open Access
- Author:
- Westlake, Jonathan George
- Area of Honors:
- Biology
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Dr. Stephen Wade Schaeffer, Thesis Supervisor
Dr. James Harold Marden, Thesis Honors Advisor - Keywords:
- Genetics
drosophila
biology
evolution
natural selection
chromosome - Abstract:
- Under normal circumstances in an XY sex determination system, an XY male mated to a female should produce 50% male and 50% female offspring. Selfish genetic elements can defy Mendel’s law of segregation through a biased transmission of the X chromosome over the Y leading to a biased sex ratio. Drosophila pseudoobscura has such a selfish genetic element on its X chromosome referred to as the Sex ratio chromosome. Males with a Sex Ratio X chromosome produce 95% daughters rather than the expected 50%. Cytogenetic analysis of the Sex ratio chromosome found three nonoverlapping inversion differences between the Standard and Sex Ratio X chromosomes, basal, medial, and terminal inversions. Inversion mutations occur when a chromosome breaks in two locations and the intervening segment rejoins in reverse order. The genetic consequence of the inversion is to suppress genetic exchange between maternal and paternal chromosomes during the formation of eggs and sperm. For nonoverlapping inversions, it may be possible for genetic exchange to shuffle the three different inversions during gamete formation. Here, we test whether genetic exchange shuffles the three nonoverlapping inversions in a three-point mapping cross. We used a genetic cross of an ST/SR female crossed to an ST/Y male. Male offspring from this cross were crossed to a female strain homozygous for a multiply marked Standard X chromosome and tested for which of the three inverted segments that they carried and the sex ratio of their offspring. We found no evidence for genetic exchange among any of the nonoverlapping inversions despite a significant genetic distance between the medial and terminal inversion. Sex ratio males confirmed the strong biased transmission of the X chromosome versus the Y chromosome. We conclude that the three nonoverlapping inversions suppress genetic exchange even in regions where genetic exchange could occur.