Genetic expansion underpins oak tree longevity
Genome analysis reveals why species can grow very old without getting very sick. Jeff Glorfeld reports.
The oak tree has symbolised strength and endurance for thousands of years and across many cultures. Researchers writing in the journal Nature Plants believe the success of the species might be explained by a sudden expansion of disease-resistance genes, allowing individual trees to survive for centuries despite exposure to various threats.
The team of researchers from Europe and the US sequenced the genome of Quercus robur, the common English or pedunculate oak, which is widely distributed around the world.
They then compared the result with existing whole-genome sequences for other plants, including trees and shrubs. They found that the oak experienced a recent burst of tandem gene duplication, which appears to have contributed to a large proportion of its total gene family expansion.
These expanded families are largely associated with disease-resistance genes and exhibit what geneticists call “positive selection signatures” – meaning that they are highly likely to be dominant and thus spread rapidly through a population.
One of the study’s chief authors, Christophe Plomion, from the French National Institute for Agricultural Research, says the team searched for genomic features specific to oak that might contribute to its longevity by first reconstructing its ancient history within the rosid clade, the broad evolutionary group to which it belongs. The clade contains about 70,000 flowering plant species.
They compared oak’s ancestral relatives, including peach, grape, and cocoa, to reveal that the species underwent five fissions and 14 fusions to reach its present 12 chromosome structure. They also found that oak experienced a “recent burst of local gene duplications”, accounting for 35.6% of its gene repertoire, after the oak-peach lineage diverged.
The eucalyptus genome is the only other plant genome shown to date to display such high levels of tandem duplication (in its case, 34%), contrasting strongly with the other genomes investigated, the report says.
The researchers cite a 2014 study in which it is suggested that three mechanisms could allow plants to grow old without antibodies. These comprise “numerous and highly diversified defence genes”, the expansion of resistance gene families, and beneficial mutations transmitted to the next generation.
Plomion and his colleagues find support for all three suggested mechanisms in the oak.
They say that in sequencing the genome, they revealed its considerable genetic diversity, to which heritable somatic mutations may contribute.
“This work poses new research questions about the contribution of this mutational load in adaptation, in particular with regard to defences against new pests and pathogens,” they write.
“We also showed that the genome of this iconic tree went through a single event … followed by a massive burst of recent local gene duplication. These duplications have amplified families of genes involved in defence against pathogens.
“We observed a parallel expansion of resistance gene-related gene families across multiple tree species, suggesting that the immune system makes an essential contribution to the survival of long-lived plants over several centuries.”
The authors also found similar disease-resistance gene expansions in various other tree species, suggesting that they might play an important role in ensuring longevity.