Principles of Meiotic Chromosome Assembly

During meiotic prophase, chromosomes organise into a series of chromatin loops emanating from a proteinaceous axis. Yet, while much is known about meiotic chromosome morphology, the mechanism of assembly, and how assembly is regulated, remain open questions. Here we combine genome-wide chromosome conformation analysis (Hi-C) with Saccharomyces cerevisiae genetics and in silico polymer modelling to elucidate the mechanisms that shape meiotic chromosomes. Entering meiosis, grid-like Hi-C interaction patterns emerge that correspond to the localisation of - and depend upon - the meiotic cohesin subunit Rec8. Such patterns fit a model of loop extrusion, where a heterogeneous population of expanding loops develop along the chromosome, with growth limited by barriers. While grid-like patterns emerge independently of chromosome synapsis, synapsis itself generates additional compaction that matures differentially depending on telomeric distance and chromosome size. Our results elucidate fundamental principles of meiotic chromosome assembly and demonstrate the essential role of cohesin within this evolutionarily conserved process.