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Joint Research Unit 1095 Genetics, Diversity and Ecophysiology of Cereals

Task 2 : Organization

Principle : Polyploidization is followed by a diploidization process at the genome and gene levels.

Duplication of chromosomes, or polyploidy at large, are followed by a return to the diploid state through a process called diploidization that we investigated at two levels of resolution: gene and chromosome. At the chromosome level, we observed that successive rounds of polyploidy were followed by ancestral chromosome fusions. The genomes of modern plants and animals appear as a mosaic of fused ancestral chromosomes. Our work demonstrated that the appearance of new plant species was related to the fusion/fission of chromosomes and each modern species could then be associated with a specific pattern of ancestral chromosome fusions/fissions. However, while the chromosomal fusions in cereals are centromeric (CCF for 'Centromeric Chromosome Fusion'), chromosomal fusions appear telomeric (TCF for 'telomeric Chromosome Fusion') in animals. These regions of ancestral chromosomes fusion regions become over time the preferential location of intense rearrangements via the insertion of repeats and tandem gene duplications.

At the gene level, the diploidization process leads to a loss and/or differential transposition of duplicated genes from paralogous blocks. We have established that this deletion / transposition of duplicated genes is not random and preferentially observed on one of the subgenomes (following genome duplication). This dominance and / or sensitivity of the subgenomes (or intra-genomic) following a duplication event is more pronounced in plants which have undergone recurrent and recent polyploidies unlike animals.


Plant genome diploidization processes (Salse 2012). [Left]- Diploidization at the chromosome level. The model begins with WGD, followed by ancestral chromosome fusions, producing a diploidized paleopolyploid karyotype. A derived duplicated region with different gene contents (colored bars) is illustrated in the centre. [Right]- Diploidization at the gene level. The figure illustrates the reduction in duplicated gene redundancy after WGD through non-functionalization (gene deletion), sub-functionalization (new expression pattern via regulatory sequence shuffling shown as a pink bar), neo-functionalization (new function through functional domain shuffling shown as a pink bar), concerted evolution (expression and functional redundancy retained) mechanisms. The consequences for duplicated gene structures and expression patterns are schematically illustrated in colored boxes at the right.