Decoding the dance of the chromosomes

Sex is old.

In Scottish Country dancing, couples can swap sides during a dance. During meiosis, pairs of chromosomes line up and exchange genetic information. Image copyright Furlongs Travel/Crieff Hydro. Used with permission.

More correctly, sexual reproduction as a biological phenomenon has been around for over a billion years. Birds, bees, flowers and trees all reproduce sexually. For an organism to reproduce sexually it needs to produce gametes, or sex cells, and this occurs during a specialised form of cell division known as meiosis.

During meiosis, two cycles of cell division occur to produce gametes with only half of the number of chromosomes of the parent that produced them. Just before the first division, the maternal and paternal pairs of chromosomes line up and cross over, like couples swapping places in a formation dance. This recombination leads to gametes with a unique combination of genetic material from both parents and is responsible for genetic diversity within populations. It?s why two sisters (or two brothers) from the same parents don?t look the same.

Like in all formation dances with particular steps, the process of crossing over is tightly controlled. For researchers keen to unlock genetic diversity in agricultural crops, understanding the genetic control of crossing over is of particular interest. For Dr Wayne Crismani, this interest in the genes controlling recombination has led him from a PhD in his home town at the University of Adelaide, to a three-year stint in the meiosis lab of INRA in Versailles, France.

A fluorescent in situ hybridisation image of wild-type Arabidopsis just prior to the first meiotic division. Each of the five pairs of chromosomes (blue) have found their partners and are involved in reciprocal exchanges of their DNA. Labels (green and red) are used identify the different chromosome pairs so that the behaviour of each chromosome during meiosis can be analysed. Image copyright Wayne Crismani. Used with permission.

When asked where his interest came from, Wayne explained ?It was by chance at the beginning. A friend introduced me to my PhD supervisor, Dr Jason Able and I was fascinated by the complicated yet very impressive genetics of bread wheat and other crops. Later I made a very deliberate decision to come to France for my post-doc. I was lucky enough to visit many excellent overseas laboratories during my PhD including the opportunity to spend four months here on a Marie Curie fellowship from the EU. I developed a taste for genetics and microscopy and I knew this was the place to be and that I wanted to come back.?

And now, Dr Crismani, his fellow researchers at INRA and their Spanish and American colleagues have made an exciting new discovery, recently published in Science (DOI: 10.1126/science.1220381). ?Many genes that promote the formation of meiotic crossovers in plants have been discovered. But we have found a gene that actually limits the number of crossovers that occur? said Wayne.

Composite image showing fluorescent pollen demonstrating the increase in meiotic recombination. Each of the groups of four cells (known as a tetrad) is derived from a single cell which underwent meiosis. Three different labels (red, yellow and blue) are placed on the same chromosome. More colours spread across the four cells indicate more recombination along the chromsome. Image copyright Wayne Crismani. Used with permission.

Dr Crismani and his colleagues worked with a mutant of the model plant Arabidopsis thaliana (thale cress) with very low numbers of crossovers and, as a consequence, poor fertility. By looking for new mutations that had restored fertility due to increased crossovers, the researchers were able to work backwards and identify the genes responsible.

They discovered that a single mutation in a gene known as FANCM led to a tripling in the numbers of crossovers compared to what usually occurs in Arabidopsis, without any negative effects on the fertility or health of the plant. Wayne explained ?This was really exciting for us. Until now, FANCM was known to have a role in DNA repair and the human version of the protein has an essential role in genome stability. But we are the first to show that it limits meiotic crossing over in any species.?

Because combining traits in a breeding program is limited by meiotic crossovers, this discovery is likely to be welcomed by crop breeders world-wide. However, it will still take some time before this discovery may be applied to species of agricultural interest. ?The gene exists in essentially all species. However the important question for plant breeders is does a loss of function of the gene have the same effect in other species, notably in the world?s major crop species? If so it could deliver huge benefits by reducing the amount of time required to produce a new variety? said Wayne.

Dr Wayne Crismani inspects his Arabidopsis thaliana plants. Image copyright Wayne Crismani. Used with permission.

Although Wayne and his fellow researchers hope that this work will ultimately help to accelerate the production of new plant varieties needed to feed a growing population, their findings also question the role of genetic recombination in evolutionary terms. ?We were able to markedly increase the frequency of crossovers without negatively affecting the fertility, but we still don?t really know why in nature genetic recombination is typically quite limited? said Wayne, adding ?The findings have opened up many avenues of future research. I have lot of projects in mind, particularly since my boss Rapha?l Mercier is someone who just has idea after idea. I think it is rubbing off on me.?

Dr Crismani?s work is funded by the EU-FP7 program Meiosys-KBBE-2009-222883

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