Benoit Lahouze Role of DNA Methylation in meiotic recombination in Arabidopsis thaliana
14h à 16h30
La soutenance de thèse de Benoit Lajouze aura lieu dans la salle Dussane, ENS - 45 rue d’Ulm 75005 Paris
During meiosis, the cellular division that gives rise to haploid cells, homologous
chromosomes inherited from each parent are paired and are subjected to reciprocal exchanges of chromosome segments called crossing-overs (COs). COs are not randomly distributed in the genome. Some of the involved mechanisms have recently been described in mammals and yeast but they are not conserved in plants. Repeat-rich heterochromatin is suppressed for COs. The high level of DNA methylation associated with repeats could be an inhibitor of COs. This was clearly demonstrated in the fungus Ascobolus immersus and recent studies have shown that the loss of DNA methylation also affects COs in Arabidopsis thaliana. The aim of my thesis was to describe more precisely the role of DNA methylation in the control of CO distribution in the absence of any DNA sequence polymorphism which are known to affect recombination. For this purpose, I measured recombination in different plants where DNA methylation has been partially or completely removed thanks to the mutation of the DDM1 gene. To test the opposed effect of a gain of DNA methylation. I also tried to target DNA methylation at a known recombination hotspot. My results show that the loss of DNA methylation induces a global increase of recombination.
Paradoxically, the normally highly methylated heterochromatin is less affected by this loss than the rest of the chromosome, probably because DNA methylation has distal effects. The increased recombination is exacerbated in successive generations of the hypomethylated ddm1 mutants. However, the strongest effect is seen in the heterozygotes where only half of the genome is hypomethylated, suggesting a complex role in the control of CO distribution. Finally, I show that DNA sequence polymorphism affects mainly recombination in the heterochromatin but not in the expected sense, since homozygous plants recombine less than heterozygous.
Keywords : meiosis ; crossing-overs ; DNA methylation