Investigating meiotic chromosome structure during meiotic prophase in C. elegans

Abstract

Meiosis is the specialised cell division program that allows the formation of haploid gametes from diploid germ cells, playing an essential role in the life cycle of sexuallyreproducing organisms. Meiosis involves dramatic changes in chromosome structure during the long prophase that precedes the first meiotic division. At the onset of meiosis axial elements (AE) containing cohesin, the complex that provides sister chromatid cohesion (SCC), are established along each chromosome. Following AE assembly, homologous chromosomes pair with one another and crossover (CO) recombination events are formed between them. COs, together with SCC, provide the basis of chiasmata: temporary physical attachments between homologous chromosomes that ensure their correct orientation on the first meiotic spindle. Therefore, COs play an essential role during meiosis and their number and position appear to be highly regulated. However, the functional interplay between chromosome structure and CO formation and distribution remains poorly understood. In this project, I have combined the experimental advantages of the C. elegans germ line with super resolution microscopy (SRM) techniques to study the structural changes that chromosomes undergo during meiotic prophase. I develop methods to image three-dimensionally intact meiotic nuclei using structural illumination microscopy (SIM) and Imaris image software analysis, making it possible to measure structural features of individual meiotic chromosomes, including CO sites. When combined with the extensive genetic resources available in C. elegans, this method provides a powerful tool to investigate the functional regulation of meiotic chromosome structure by different protein complexes such as cohesin. I also describe that a mutation in MEI-2, a component of the microtubule severing complex katanin, results in altered CO distribution, likely by affecting early steps of meiotic recombination. This finding reveals an unexpected role for katanin during meiotic prophase and suggests that the microtubule network plays an important role in the regulation of CO distribution.