Evolution of the chromatin organization in plants

Genome activities are regulated by the chromatin, which consists largely of basic proteins histones that form the nucleosomes supporting the DNA. Genome wide studies have shown that combinations of histone modifications contribute to the chromatin landscape that correlates with gene transcription and silencing. More recently variants of histones H3 and H2A were also shown to affect gene expression and their proper function is required to prevent several types of cancers. Surprisingly, the combination of profiles of variants maps the position of major components of the genome such as protein-coding genes, non-protein-coding heterochromatic domains. During the evolution of plants, histone variants have diversified tremendously while the number of histone modifications remained constant, providing a unique opportunity to question whether genome organization has been shaped by a diversifying repertoire of histone variants. To address this question we propose to use a filamentous alga and a moss as models representing ancestors of land plants. Using these species, we will establish the ancestral plant chromatin landscape using a combination of genomics and high-resolution microscopy. This will enable to establish the origin of the relationship between histone variants and major genomic features during plant evolution. In addition we identified the new class of H2A.M variant present only in mosses and ferns. This class may represent an attempt to evolve a variant with properties similar to H2A.W variants that are present in flowering plants and responsible for higher order organization of heterochromatin. Hence we will study properties and functions of H2A.M in Marchantia to find clues that explain the origins of H2A.W and the cause of its selection in relation with genome three-dimensional organization. We thus plan to obtain an overview of the evolution of chromatin and genome organization in one major kingdom of eukaryotes. Phylogenetic analyses suggests a degree of convergence during histone diversification between flowering plants and mammals. Our findings related will likely find parallels between plant and animal evolution of chromatin structure, thereby broadening the impact of our study.

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