Oaks are an important part of our natural and cultural heritage. Not only are they ubiquitous in our most common landscapes but they have also supplied human societies with invaluable services, including food and shelter, since prehistoric times. With 450 species spread throughout Asia, Europe and America, oaks constitute a critical global renewable resource. The longevity of oaks (several hundred years) probably underlies their emblematic cultural and historical importance. Such long-lived sessile organisms must persist in the face of a wide range of abiotic and biotic threats over their lifespans. We investigated the genomic features associated with such a long lifespan by sequencing, assembling and annotating the oak genome. We then used the growing number of whole-genome sequences for plants (including tree and herbaceous species) to investigate the parallel evolution of genomic characteristics potentially underpinning tree longevity. A further consequence of the long lifespan of trees is their accumulation of somatic mutations during mitotic divisions of stem cells present in the shoot apical meristems. Empirical and modelling approaches have shown that intra-organismal genetic heterogeneity can be selected for and provides direct fitness benefits in the arms race with short-lived pests and pathogens through a patchwork of intra-organismal phenotypes. However, there is no clear proof that large-statured trees consist of a genetic mosaic of clonally distinct cell lineages within and between branches. Through this case study of oak, we demonstrate the accumulation and transmission of somatic mutations and the expansion of disease-resistance gene families in trees.
SummaryWe review here our own research and related work on hybridization between two widespread and largely sympatric European oak species ( Quercus petraea and Q. robur ). There is a near total lack of local differentiation in chloroplast DNA markers between them. A model is proposed to account for this lack of differentiation: invasion by one species of the range occupied by the other through pollen swamping.In support of this model, ecological, palaeoecological and population genetic studies indicate that one species ( Q. robur ) disperses its seeds better than the other one, that pollen flow is much more efficient than seed flow in oaks and that hybridization and introgression are asymmetric, hence reinforcing the ecological dynamics by facilitating the dispersal of Q. petraea in regions already colonized by Q. robur . 'Resurrection' of Q. petraea following this wave of hybridization appears to be rapid. More generally, available evidence indicates that hybridization could constitute an important mechanism of dispersal in both natural and human-induced plant invasions.
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