RUI: Behavior of Trivalents in Metaphase I and Anaphase I

Bucknell University, Lewisburg PA

Investigators

Abstract

Every time a cell divides, it must distribute the correct number of chromosomes to its daughter cells. Chromosomes must be positioned correctly to ensure correct chromosome distribution during cell division. Failure to correctly position chromosomes in cell division leads to aneuploidy (extra or missing chromosomes) and can be catastrophic for the cell and, ultimately, the organism. Chromosomes must attach to the spindle, typically with one attachment site facing each spindle pole. This bipolar attachment guarantees that partner chromosomes will separate from one another in anaphase. In metaphase, after forming a bipolar attachment, chromosomes of nearly all organisms align in the center of the spindle forming the metaphase plate. The alignment of chromosomes in the center of the spindle in metaphase may be simply a consequence of the "tug of war" that the chromosome is experiencing as a result of a bipolar attachment, with both sides of the chromosome under equal tension. Alternatively, the positioning of chromosomes at the center of the spindle could be essential for guaranteeing correct distribution in anaphase. The fact that chromosomes align at the center of the spindle in nearly all cell types (the behavior is nearly universal), and that mutants that fail to align chromosomes produce aneuploid cells, suggests that that metaphase alignment at the center of the spindle is important for directing appropriate chromosome separation and movement in anaphase. The aim of this project is to learn more about how chromosomes align at the center of the spindle, and how the placement and the timing of this alignment can impact distribution of chromosomes during anaphase. The Broader Impact activities will address the problem that there are very few tools available to biology instructors for teaching cell division. A series of narrated videos of different types of cell division will be created for use in the classroom, thus beginning a collection of materials for future projects. These videos will be distributed nationally to any instructor who desire to use them in their courses. In meiosis I, most chromosomes connect with one homologous or partially homologous partner to form a bivalent. Bivalents align on the metaphase plate and then homologues initiate segregation at anaphase I onset. However, in males from some taxa (e.g., praying mantids and the cellar spider Pholcus phalangioides), the X1, X2, and Y chromosomes form a trivalent during meiosis I. The X1, X2, and Y chromosomes each have a single kinetochore, with X1 and X2 associating with one pole in metaphase I, while the Y chromosome associates with the other pole. Though the trivalent has two kinetochores facing one pole and one facing the other, it aligns with all autosomes on the metaphase I plate. In addition, metaphase I appears to be so important that anaphase I is delayed for the trivalent, with the autosomes separating approximately 20 minutes before the X1, X2, and Y chromosomes. This project aims to determine how forces are balanced to align the trivalent on the metaphase plate, and then how the chromosomes that make up the trivalent segregate correctly into daughter cells, using electron microscopy, laser microsurgery, immunofluorescence staining, and small molecule inhibitors.