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The different steps of meiosis of pollen master cells of Arabidopsis thaliana wild ecotype.. © INRA, VEZON Daniel

MIME, a plant line producing pollen grains and ovules that are genetically identical to the parent plant

In MiMe, a line of Arabidopsis thaliana bred by INRA researchers, mitosis (cell division giving rise to two daughter cells identical to the original cell) replaces meiosis (cell division that produces reproductive cells and ensures genetic mixing).  In order to replace meiosis by mitosis and generate gametes carrying genetic information identical to that of the mother plant, INRA researchers bred plants with modifications to three traits: recombination (exchange of genetic material between pairs of chromosomes), chromosome segregation (separation of pairs of chromosomes) and the presence of a second cell division during meiosis.  Breeding of the MiMe line constitutes a major step towards clonal reproduction using seeds (apomixis), a potentially revolutionary technology for plant breeding.

Updated on 02/12/2019
Published on 08/05/2009

MiMe, a fertile line where mitosis replaces meiosis

One of the three major components of apomixis is apomeiosis (mitosis replacing meiosis). The team led by Raphaël Mercier was able to introduce apomeiosis in a sexual species thanks to the characterisation of a new gene called OSD1 (for "omission of second division"). The scientists observed that the progeny of osd1 plants (with a mutation in the OSD1 gene) carried four copies of chromosomes in their cells (4n, tetraploid) instead of two (2n, diploid). This phenomenon was due to the unusual constitution of sexual cells (gametes: pollen grains and ovules) in osd1 plants. These were diploid (2n), while normally gametes only possess one copy of each chromosome (n). The encounter between two osd1 gametes during fertilization thus generated a tetraploid individual (2n+2n=4n) instead of a diploid individual (n+n=2n). The observation of meiosis (cell division giving rise to gametes) made it possible to identify the origin of the diploid nature of osd1 gametes. Normally, gametes with n chromosomes arise from mother cells with 2n chromosomes which, during meiosis, undergo a doubling and then two successive separations of chromosomes. In osd1 plants, the second separation of chromosomes did not occur, thus generating 2n gametes.

Incomplete meiosis is thus the reason for the formation of 2n gametes in osd1 plants. However, this is not apomeiosis because the first division of meiosis differs from mitosis in two respects: the presence of recombination and the mode of chromosome distribution to daughter cells. The researchers thus took advantage of two mutations (already known) to inhibit recombination and modify the mode of chromosome distribution during meiosis. By combining them with the osd1 mutation in the same plant, they created the triple MiMe mutant in which meiosis is replaced by mitosis (Mitosis instead of Meiosis). This fertile line produces diploid gametes that are genetically and wholly identical to the mother plant (apomeiosis).

MiMe, a major step towards apomixis

The replacement of meiosis by mitosis, achieved here in the MiMe line, is one of the three essential components of apomixis, the other two components being development without fertilisation of the albumen (tissue nourishing the embryo) and of the embryo (parthenogenesis). Mutants spontaneously developing the albumen were isolated in Arabidopsis more than ten years ago. Apomixis has not yet been introduced in a sexual plant, but we have never been nearer to achieving this. These results are the subject of an INRA patent, and research projects are under way at the Versailles Research Centre on both the model plant Arabidopsis thaliana and on cultivated plants.

Left to right: products of meiosis in a wild plant and in MiMe:

 © Import
© Import

  

Reference

Isabelle d'Erfurth1, Sylvie Jolivet1, Nicole Froger1, Olivier Catrice2, Maria Novatchkova3 and Raphaël Mercier11. Turning Meiosis into Mitosis. PLoS BIOLOGY, June 2009 

1INRA UR254. IJPB. Route de Saint-Cyr. 78026, Versailles, France
2 CNRS. UPR2355. Avenue de la Terrasse. 91198, Gif sur Yvette, France
3 Research Institute of Molecular Pathology (IMP), Dr Bohr-Gasse 7 A-1030 Vienna, Austria
Article mentioned in Nature (Nature News 9 June 20009) and Science (Editor's choice 26 June 2009)

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What is apomixis?

Apomixis is a specific type of reproduction observed in more than 400 species of wild plants. The progeny of a plant that reproduces by apomixis are genetically identical to the mother plant. By contrast, progeny obtained by sexual reproduction carry original genetic information that is a mixture of part of the father's genome and part of the mother's genome. Almost all cultivated plants produce seeds as a result of sexual reproduction, and not by apomixis. The apomictic reproduction of a cultivated species would be an extremely efficient means of obtaining and propagating new, elite varieties to meet the needs of consumers and producers. Indeed, the most interesting plants, that combine a large number of traits, often have a complex genetic composition. Because sexual reproduction mixes genetic information at each generation, their progeny do not retain targeted traits. However, the progeny of these same plants obtained by apomixis could conserve these traits and be reproduced and distributed endlessly. Although the agronomic and economic stakes are considerable, apomixis has not yet been introduced in species of major agricultural interest.