The organization and pairing of chromosomes in meiosis

This dissertation seeks to relate the pairing of homologous chromosomes in meiosis to large-scale organizational processes occurring at the chromosome and nuclear level. Two grasses (maize and wheat) were used for cytological studies of meiotic prophase, and computer simulations were used to extend what is known about the kinetics of bouquet formation.Maize male meiosis was discovered to possess a centromere-based organization, reminiscent of the Rabl configuration, that comes before the clustering of telomeres and is lost upon the onset of telomere clustering. The centromeric ends of recently created telocentric chromosomes displayed both centromere polarization and telomere clustering, indicating that telomeres do not directly depend on centromeres for entry into the bouquet. Ring chromosomes were also found to enter the bouquet, indicating that the presence of telomere sequence was sufficient, and a physical end was not required. Meiotic chromosome organization in maize appears to be determined by the autonomous action of specialized chromosome regions, therefore, and not by geometrical properties of the chromosomes themselves.In hexaploid wheat, alterations of geometry (terminal deletions) in a pair of introgressed rye chromosomes, which disrupt meiotic crossing over, were demonstrated to exert their effect by causing a failure of homologous encounter and synapsis. Association of heterochromatin, generally thought to be an aid in pairing, was found to hinder pairing in cases where one arm lacks subtelomeric heterochromatin and its homolog contains it. Wheat therefore appears to rely more than maize on large-scale chromosome organization.The requirements for bouquet formation were further investigated by computer simulations, using a time-course study in rye for comparison. It was found that random motion of telomeres could not suffice to form a bouquet in the observed time, and a polarizing influence was required in addition. Further simulations indicate that this polarizing force could conceivably come from chromosome-chromosome association. The conclusions of the combined studies are: the bouquet is a specialized behavior of meiosis, requiring active processes, that is refractive to chromosome rearrangements and has the potential to affect homologous encounters and that geometrical properties of chromosomes can influence their ability to pair homologously.