College of Science and Health Theses and Dissertations

Date of Award

Winter 3-19-2018

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Biology

First Advisor

William Gilliland, PhD

Second Advisor

Margaret Silliker, PhD

Third Advisor

Jason Bystriansky, PhD

Abstract

Physical connections established by homologous recombination are normally sufficient to establish proper co-orientation of chromosomes during prometaphase of female meiosis I. Nonexchange chromosomes can still segregate because they are connected by heterochromatic threads, which are thought to connect homologous chromosomes and ensure co-orientation in the absence of a chiasma. In Drosophila, the nonexchange chromosomes (such as the Muller F element, also called the “dot chromosome,” which never undergoes recombination) move out on the spindle during prometaphase I, and can be found positioned between the spindle poles and the exchange chromosomes at the metaphase plate. By metaphase I arrest, these chromosomes congress to a single mass. A previous study (Gilliland et al. 2015b) found a visible difference in the prometaphase dot-dot chromosome separation of two different Drosophila species (Drosophila melanogaster and Drosophila simulans). The mean dot-dot distance in D. melanogaster females (11.3 µm) was nearly twice as large as in D. simulans (6.1 µm). This difference in dot chromosome distances between these two species could be a result of their heterochromatin content; D. melanogaster has a larger amount of heterochromatin and has a longer average dot-dot chromosome distance, while D. simulans has less heterochromatin and a smaller dot-dot distance. A speculative further interpretation is that if the heterochromatic repeats on a chromosome form the threads connecting these nonexchange homologs, then having a greater amount of those repeats may increase thread length and enable those homologs to move farther apart from each other before the tether pulls tight enough to prevent further movement. A second difference between these species is that while D. melanogaster has many common polymorphic chromosome inversions, D. simulans is monomorphic with no common inversions (Lemeunier and Aulard 1992). As inversions block crossing over, increasing the abundance of inversions should make meioses with nonexchange chromosomes more common. Because other nonexchange chromosomes in D. melanogaster are positioned between the dots near the spindle poles and the exchange chromosomes at the metaphase plate during prometaphase, having the dots further out could provide more space for additional nonexchange chromosomes to move out on the spindle. If this additional space is beneficial, then the greater amount of space on the spindle provided by the longer dot-dot distance in D. melanogaster might help this species tolerate common inversions, leading to selection for increased dot-dot distances. We aimed to understand how these heterochromatic threads change chromosome positioning during Drosophila meiosis, or whether they might also have other evolutionary effects such as affecting the abundance of inversions across Drosophila species. We sampled 14 Drosophila species with and without common inversions and measured their average dot-dot distances during meiotic prometaphase to see if their distances correlate with either the abundance of inversions, the amount of heterochromatin or both. We did not find a strong correlation with either factor in these species, which suggests that neither inversions nor the amount of heterochromatin in the genome determine dot-dot distances. However, while doing this work, we noticed substantial variation in the size of the dot chromosomes among these species, and that the proportion of oocytes with chromosomes out on the spindle appeared strongly correlated with dot chromosome size. This suggests the variation in the time spent doing the prometaphase chromosome movements is proportional to the size of the dot chromosome in these species.v

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