Meiotic crossing-over: from spatial distribution to in situ chromosomal architecture
To haploidise their genome, sexually reproducing organisms employ a specialised cell division program – meiosis – which consists of one round of DNA replication followed by two consecutive rounds of chromosome segregation: meiosis I and II. While meiosis II resembles mitosis, the ability of cells to segregate homologous chromosomes entails several specialised events. In most organisms, physical linkage and subsequent disjunction of maternal and paternal chromosomes require homologous recombination and crossing-over. As observed over a century ago, crossovers occur at different chromosomal positions in different meiotic nuclei – however, the incidence of a crossover in a given location reduces the probability of a neighbouring crossover event. As a result, crossovers tend to be widely and evenly spaced along chromosomes, a phenomenon termed crossover interference. Work in the last 30 years has led to remarkable progress in the delineation of the sequence of molecular events that lead to crossing-over. However, how cells spatially regulate the deployment and assembly of molecular determinants to accomplish crossover patterning remains largely unknown. Here, I propose to tackle this fundamental question through the development of two novel approaches tailored to explore central aspects of meiotic recombination with unprecedented resolution:  

i) to understand how chromosomal context shapes crossing-over, we will develop novel methodology (HJmap) to achieve genome-wide mapping of Holliday junctions: central recombination intermediates which mark future crossover sites.  

ii) to explore how local chromosomal features influence crossing-over, we will visualise the architecture of crossover-designated recombination intermediates in situ, and in 3D, using electron cryotomography.  

By understanding how cells implement genetic exchange through crossing-over we will shed light on the molecular basis of heredity: the passing of traits from parents to their offspring.