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Getting plants to clone themselves
Posted: Monday, June 15, 2009 3:06 pm
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Left to their own devices, plants reproduce sexually -- by combining genetic information from male pollen and female egg cells. What if it were possible to get plants to clone themselves, instead? This would allow a superior crop plant to be replicated, in quantity. A team of researchers in France and Austria is closing in on how to reproduce a plant that produces perfect potatoes, maize or rice, without the lottery of reassortment that each meiotic division and ensuing fertilization introduces.

New research published in PLoS Biology and recounted in esciencenews brings clonal reproduction of crop species a step closer to reality by modifying plant cell division.

There are two types of cell division: mitosis and meiosis.

Meiosis occurs in all species that reproduce sexually, from microorganisms such as yeast to plants, animals and human beings. This form of cell division is used specifically for the creation of pollen and egg cells, each of which contains only one-half of the parental DNA. During sexual reproduction, the two halves recombine to generate a seed for a new plant.

Mitosis is considered "normal" cell division, because it occurs during plant growth. As the plant matures, it adds more cells, all of which contain the same DNA.

The first steps of both meiosis and mitosis are the replication of the dividing cell's DNA. Once replication has occurred, the chromosomes condense into tightly bound structures.

In mitosis, these form an X shape in which each half of the X is a chromatid, comprising one complete copy of the chromosome. The double-chromatid then chromosomes line up along the centre of the cell. Then the two chromatids are pulled apart, and these then pass into two genetically identical daughter cells.

In meiosis, there are two lining up and dividing phases. The first lining up is of homologous chromosomes—all chromosomes in an adult cell have a partner, members of the partnership coming from the mother and father of the cell—and these homologous chromosomes are each made up of two chromatids. The first division divides homologous pairs of chromosomes while the second meiotic division divides the chromatids at the center of the X. However, they stay divided, resulting in pollen and egg cells with one-half of the parental DNA.

The new work, led by Raphael Mercier, identifies a gene that controls entry into the second meiotic division. By combining a mutation in this gene with two other previously described mutations—one that eliminates recombination and another that modifies chromosome segregation—the authors have created a strain of plant (called MiMe for 'mitosis instead of meiosis') in which meiosis is totally replaced by mitosis.

MiMe plants produce pollen and eggs that are genetically identical to their parent. If MiMe eggs are self-fertilized by MiMe pollen, the offspring plant has twice as much DNA as the parent generation, and has all the genes from this single parent.

Thus the authors have made a form of asexual reproduction possible in a normally sexual species. Turning meiosis into mitosis is not enough to reach clonal reproduction, but it's a giant leap towards it. This has potential revolutionary applications in crop improvement and propagation.
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GMO (noun) 1. An acronym for the term, 'Genetically Modified Organism'. 2. A plant, animal or microbe with a genetic code which has been intentionally altered, such as through forced evolution (see selective breeding); through random mutations induced with radiation or chemicals (see mutagenesis); or through the selective transfer, promoting or silencing of specific genes (see genetic engineering). 3. Most generally, any descendant of a primordial urbiote. (See evolution.)

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