Date of Completion

Spring 5-1-2023

Thesis Advisor(s)

Stacey L. Hanlon

Honors Major

Molecular and Cell Biology

Disciplines

Genetics and Genomics

Abstract

B chromosomes are supernumerary, heterochromatic genetic elements that are found in hundreds of different plant and animal species. Recently, B chromosomes were discovered in a stock of Drosophila melanogaster and are carried at a high copy number of 10-12 B chromosomes per cell. B chromosomes are not known to carry any active genes, but when placed in a wild-type genetic background, they cause a significant increase in the frequency of chromosome 4 missegregation during meiosis. This project aimed to understand the relationship between a female’s B chromosome copy number and how often she passes on too many (or too few) copies of chromosome 4 to her progeny. To do this, females with B chromosomes were crossed to males that do not carry B chromosomes but do carry a specially marked copy of chromosome 4. This cross allows us to phenotypically identify instances where the parental female passed on a normal (one copy) or abnormal (zero or two copies) number of chromosome 4 to her progeny. To assess the B chromosome copy number in the parental female, I obtained karyotypes from metaphase chromosome spreads from her ovaries. I then plotted the frequency of chromosome 4 missegregation as a function of the number of B chromosomes the female carried. Our preliminary results indicate that there is not a distinct relationship between B chromosome copy number and nondisjunction frequency, and that any number of B chromosomes is enough to cause significant missegregation of chromosome 4 during female meiosis. Future studies will focus on the mechanism of how the B chromosomes promote aberrant chromosome 4 segregation, which will have major impacts on our understanding of how the presence of extra chromosomes during meiosis can lead to birth defects and infertility.

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