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Press release.
29/01/2009
Simple genetic mechanism may be behind the origin of species
A genetic study conducted by the team of Olivier Loudet at INRA-Versailles, in collaboration with the University of Nottingham, may have unlocked some of the secrets behind the reproductive barriers that isolate species from one another. The study revealed one of the aspects of their establishment in the genome during evolution. The study explains why the offspring of some cross-breeds are not viable in the model plant Arabidopsis thaliana, and indicates a potential mechanism for the formation of sub-species in supposedly homogeneous populations. These results are published in the 30 January 2009 issue of Science.
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The researchers, specialists in the genetics of the model plant Arabidopsis thaliana, first noted that offspring of the cross between two of the plant's many natural strains, Columbia (Col) and Cape Verde Island (Cvi), did not fully obey Mendel's classic laws of heredity. Individuals bearing a specific genetic combination of the two parent genomes were missing.
They discovered that the missing offspring corresponded to an incompatibility between two chromosome regions, carried by chromosome 1 in Col and chromosome 5 in Cvi, which were never found together in a homozygous state in the genome of plants resulting from their cross-breeding.
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The model plant Arabidopsis is indispensable for research. In the photo: Olivier Loudet.
©Jean Weber, INRA
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A more sophisticated genetic study enabled them to attribute this incompatibility to a single gene, the one coding for histidinol phosphate aminotransferase (HPA). The gene is carried by chromosome 1 in the Cvi strain and is present twice, on chromosomes 1 and 5, in the Col strain, due to the duplication and translocation of a chromosome 1 fragment to chromosome 5 in the latter. The HPA copy in Col chromosome 1 became inactivated as the plant evolved. As a result, in Arabidopsis, the functional gene is carried by different chromosomes in Cvi and Col. The HPA gene codes for an enzyme needed in the synthesis of histidine, an essential amino acid. The embryos of offspring that inherit both the inactive HPA genes carried by Col chromosome 1 and Cvi chromosome 5 cannot develop, as they have no functional HPA gene.
Proof that the lack of this gene is the cause of the observed chromosomal incompatibility came when the researchers observed that plant embryos developed normally when the plants carrying them were watered with a histidine-containing solution.
"Almost all plant species are known to have cross-breeds that sometimes produce infertile offspring", said Olivier Loudet. "For the first time, we've identified a simple mechanism that may explain why this happens within a single species." The duplication and dispersion of essential genes within the genome, followed by the inactivation of some copies of these genes during evolution, may progressively whittle down the possibilities for fertile cross-breeds between different strains and, in time, lead to their separation into distinct species. This simple, rapid mechanism may be part of the explanation for the genetic origin of species.
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Arabidopsis: research's indispensable model plant
In the 1980s, the international scientific community decided to focus its efforts on Arabidopsis thaliana, a plant with a small genome and several advantages owing to its ease of cultivation in the laboratory, its rapid development, and its prolific nature. INRA has contributed to the flourishing body of knowledge about the plant with its large collection of Arabidopsis mutants in Versailles, thanks to an original method that is now used the world over.
Today, the full Arabidopsis genome has been sequenced. Most of the genes have been identified and localised, and researchers are working on discovering the function of each gene. The study of Arabidopsis has helped speed up the acquisition of plant knowledge in general, opening up new research possibilities and agricultural applications. Recent years have been marked by the success of INRA scientists.
They have contributed to deciphering hidden functions such as the mechanisms behind the epigenetic regulation of gene expression, the role of plant hormones, how meiosis takes places, cellulose synthesis in cell walls, how plant shape is regulated and how plant organs are formed, from leaves and roots to flowers and seeds.
These scientists have uncovered the regulation of metabolic pathways, including for sulphur, flavonoids and oilseed, and described the mechanisms behind plant resistance to salt. Researchers have also discovered disease-fighting mechanisms, such as resistance to the viral disease sharka, Salmonella plant infections or plant sensitivity to aphids. |
Reference:
Divergent Evolution of Duplicate Genes Leads to Genetic Incompatibilities within A. thaliana
SCIENCE – 30/01/09
Authors:
David Bikard 1, Dhaval Patel 2, Claire Le Metté 1, Veronica Giorgi 1, Christine Camilleri 1, Malcolm Bennett 2 & Olivier Loudet 1
1.INRA Genetics and Plant Breeding SGAP UR254 F-78026 Versailles, France
2.Division of Plant Sciences, Centre for Plant Integrative Biology, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough, UK LE12 5RD
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Written by :
INRA press service, phone: +33 (0)1 42 75 91 69
Sylvie Colleu, tel +33 1 42 75 95 55 or presse@inra.fr
Contacts :
Scientific contact
Olivier Loudet
UR254 Plant Breeding and Genetics Unit
Tel : +33 6.32.34.77.98 or +33 1 30 83 33 16
Olivier.Loudet@versailles.inra.fr
Division: Plant Breeding and Genetics
INRA Versailles-Grignon
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