June 22, 2012
Using the model plant Arabidopsis thaliana, INRA scientists in Versailles-Grignon have identified a factor that limits exchanges of DNA fragments between chromosomes (crossover) during the formation of gametes (sex cells). Mutation of this gene results in a number of crossovers that is three times higher than normal, and hence an increase in genetic recombination during reproduction. These findings give great promise to plant breeding, in that they may facilitate easier production of novel combinations of traits of interest. They are published in the online edition of Science of the 22nd of June 2012.
Meiosis is a specific type of cell division that generates gametes (sexual cells) in all living beings – animals, plants, fungi, etc. – which reproduce sexually. It consists of two successive cell divisions after which each of the four daughter cells (future gametes) only contain half of the chromosomes of the parent that produced them. Just before the first division, chromosomes from the same pair then pair up and some parts cross over, which is when fragments of genetic material can be exchanged between the chromosomes. This natural phenomenon, called crossing over, contributes to the recombination of genetic information at the level of an individual and a species and produces chromosomes that are unique from the parental chromosomes. It also plays a mechanical role, because it is essential for the correct distribution of chromosomes to daughter cells. In cultivated plants, it may be interesting to exploit such genetic recombination in order to group essential traits of agronomic interest in new varieties.
Tetrads from meiosis in fancm, showing a high rate of
recombination ©INRA/ Wayne Crismani and Raphael Mercier
The INRA researchers in Versailles-Grignon, and their Spanish and American colleagues, focused on the mechanisms that regulate crossovers in the model plant Arabidopsis thaliana. They set out to identify factors of this regulation in a zmm mutant of Arabidopsis that has a very low number of crossovers, a poor distribution of chromosomes to its gametes and a marked reduction in its fertility (which notably results in abnormally small fruit). Working with these plants, the scientists searched for new mutations that would be capable of restoring crossovers and fertility, under the hypothesis that this would enable them to identify the genes whose function is to limit the number of crossovers.
They thus identified an enzyme in the helicase family, FANCM, which limits the development of crossovers during meiosis in A. thaliana. A mutation of the gene coding for FANCM was able to restore the crossovers in zmm mutants. Furthermore, when a fancm mutant was compared with a wild-type plant, despite the fact that the number of crossovers is normally strictly regulated, a single mutation of the FANCM gene led to a tripling in the number of crossovers, without this having any effect on the fertility or health of the plant.
This major discovery is the first in the world of its type: until now, FANCM was known to intervene in DNA repair (notably in humans). Also in plants, only factors promoting the formation of crossovers had been discovered, but now a major factor limiting crossovers is known. Overall, this work opens promising perspectives in cultivated plants where an increase in genetic recombination during reproduction, via regulation of the number of crossovers, would provide access to hitherto unknown combinations of traits of interest.
Finally, it should be noted that these findings also question the role of genetic recombination in evolutionary terms. Although the frequency of crossovers was increased markedly by the researchers without this having any consequence on the mechanisms of meiosis and plant fertility, the reason why the number of crossovers is very small in practically all species still remains unknown.
Crismani W. et al. 2012. FANCM limits meiotic crossovers. Science, 22 June 2012. DOI: 10.1126/science.1220381