Group leader: Raphael Mercier

Research project:

What prevents Centromeric crossovers?

The distribution (localization and number) of meiotic crossovers (COs)  along the genome is far from random. However, the mechanisms and the evolutionary forces that impose associated constraints are very poorly understood. First, in most eukaryotes, there is at least one CO per chromosome at each meiosis, which is required for the balanced segregation of chromosomes. Second, the average number of COs is surprisingly low, typically from 1 to 3 per chromosome. This raises the question of the selective pressure that limits CO numbers. Third, the density of COs is extremely inhomogeneous at both large (chromosomal) and small scales (kb). Centromeric and pericentromeric regions are notably universally depleted in COs. In some extreme cases, such as wheat, up to 80% of the genome hardly ever experiences any COs. These regions contain ~30% of the genes, which are thus out of reach for plant breeding.  The mechanisms behind CO depletion in pericentromeres are still elusive.

This PhD project aims to answer an important question about meiosis:  What mechanisms prevent COs in pericentromeric regions? What would be then the consequences of unlocking Centromeric regions for COs? The student will run an original screen to identify factors that prevent COs in peri-centromeres. We have previously identified major factors limiting COs (RECQ4, FANCM, FIGL1, ZYP1, HEI10). However, strikingly, COs in the corresponding mutants are still largely absent in peri-centromere regions, whereas there are massively increased along the arms. This suggests that some processes are at work, specifically in these regions, to limit COs at pericentromeres. To identify these processes, we will make use of specific genetic markers available in Arabidopsis: This system relies on transgenes that confer green and red fluorescence to seeds, allowing immediate detection and measurement of CO frequency in the defined interval. We have carefully selected pairs of these markers that flank the peri-centromeric regions. The candidate will use this tool to test a series of candidate mutations that affect chromosome structure and will, in parallel, run a forward genetic screen looking for increased recombination. The feasibility of the screen is ensured by the possibility of growing thousands of plants, each producing thousands of seeds, associated with the automatized analysis of seed fluorescence. The mutants/genes identified in this screen will be functionally characterized through a combination of cell biology, genetics, and molecular biology studies, leading to an understanding of the mechanisms preventing crossovers at the centromere.

In addition, the student will analyze with super-resolution microscopy the behavior of recombination proteins during meiosis, in parallel to the genomic organization of the pericentromeres, to determine the sources of recombination inhibition.

Key publication

https://www.pnas.org/doi/full/10.1073/pnas.2023613118

Potential collaborations with other research groups

More information