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The genome of oak lifts a veil on longevity of trees
A research reveals that they have a particularly well-equipped genome in resistance genes against pests.

Robles de 150 años del Bosque Nacional francés de Bercé.

A French national consortium led by INRA and CEA sequenced the pedunculate oak genome. Their work, published in the journal Nature Plants on June 18, 2018, reveals two genomic features of the longevity of this emblematic species. The first concerns the establishment of a particularly rich and diversified set of resistance genes, allowing trees to cope with their large predators (pathogenic fungi, oomycetes, insects, bacteria and viruses) throughout their lives. The second reveals the presence of somatic mutations that can be transmitted to the next generation, a result that raises questions about the evolutionary importance of this diversity engine.

Trees are an important part of our natural and cultural heritage. Omnipresent in our most common landscapes, they also provide invaluable services to human societies. Their longevity and resilience to heterogeneous environmental variations have contributed to their symbolic representations in all past and present human societies. These representations, ranging from sacred, mystical to popular, invoke stability, resistance and permanence of life.

A reference genome for one of the 400 species of oaks

Scientists from INRA and CEA have investigated whether the longevity of trees, whose oaks are emblematic, was based on genetic bases. They began their investigation by attacking the genome of pedunculate oak. High-throughput sequencing technologies have enabled them to sequence and assemble the 750 million nucleotides of this species, which is widespread in Europe, whose genetic diversity is ten times greater than that of humans.

A particularly well-equipped genome in resistance genes against pests... which could explain their longevity

The notation of this genome reveals that it contains 51% of transposable elements (DNA sequences capable of moving in a genome) and 26,000 genes, 36% of which are organized into groups of contiguous genes, while this proportion is only 15% in plants. Researchers have shown that the oak resistance genes have particularly benefited from these duplications in tandem. The comparison of genomes of annual herbaceous species (Arabidopsis, soybean, potato, watermelon, etc.) and perennial woody species (oak, poplar, eucalyptus, peach, etc.) has shown that this mechanism of resistance gene expansion does not was not specific to oak but shared with all the trees in the study. Trees are continually exposed to pests that can evolve rapidly compared to these long-lived organisms. Under these conditions, this wealth of genes in the immune system of plants, would allow them to cope with a wide range of biotic interactions with microorganisms (including pathogens) throughout their existence.

Trees: genetic mosaics?

Multicellular organisms accumulate somatic mutations during their growth. As oaks are particularly long-lived species (usually several hundred years), researchers wondered about the importance of these mutations in such a tree. By comparing the genomes of samples collected at the end of branches of different age of a centennial pedunculate oak, they identified rare mutations that appeared over time. They also showed that these mutations could be transmitted to the offspring. It remains to be seen whether this diversity engine could confer a selective advantage on the individuals who wear them.

The oak reference genome is now used to study the evolutionary processes involved in the adaptation and speciation of European white oaks, especially since the last postglacial recolonization. Like what is done in humans, it will help to reconstruct the evolutionary trajectories that have accompanied the history of these species, thanks to the analysis of ancient DNA that can now be extracted from fossil wood remains.

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