Mutation and Evolution Within Individual Trees Living in Chernobyl

This month I am helping some young(er) colleagues in France with an exciting project to assess mutation accumulation in trees.

As an evolutionary biologist, I have long been inspired by the idea that different branches of a tree (literally) could be genetically dissimilar as a result of mutation accumulation over time and thus could generate genetically dissimilar offspring given that, unlike most animals, plants have no separation of germ (e.g. sperm or pollen and eggs or ovules) and somatic cell lines (i.e. the body). In other words, the flowering bits of a plant are derived from somatic cells each reproductive season whereas in most animals, germ line cells are differentiated early in development of the embryo and are kept separate (genetically) and thus protected from mutations induced in somatic cells throughout the individual's life. The significance of this difference between plants and animals is that it opens the possibility for the inheritance of acquired mutations in plants. Discussions related to Leo Buss's seminal book, "The Evolution of Individuality", while I was a postdoc at UC Davis, inspired this idea in my thinking related to the possibility of Lamarckian-like evolution in plants.

In macroevolutionary processes (e.g. speciation) different branches (e.g. population lineages) on a phylogenetic tree accumulate genetic differences over multiple generations. For a group of populations, such change within lineages is called evolution and sometimes generates new species.

Some plants (e.g. many types of trees) can have relatively long life spans thus providing the opportunity for significant mutation accumulation within cell lineages of an individual tree. If mutation rates are significant, this could lead to genetically based variation among seeds produced in different parts of the individual tree. This provides the opportunity for a different sort of adaptive evolutionary response than those observed via typical Darwinian processes which generally assume that mutations acquired by somatic tissues throughout an individual's life (e.g. cancerous cells) are not passed onto offspring (although a predisposition towards cancer obviously can be inherited).

Research on the trees of Chernobyl offers a unique and exciting opportunity to address fundamental questions related to the process of evolution because of the known acceleration of the mutation accumulation of organisms exposed to ionizing radiation. One prediction from our hypothesis is that the degree of genetic heterogeneity among branches of individual trees should be higher in areas of high radioactivity than in control areas.

This radiation-augmented genetically based variation among propagules of individual trees could provide the raw material for significantly enhanced adaptive evolutionary responses to this environmental contaminant. Alternatively, since most mutations are either neutral or deleterious with respect to fitness, this process could significantly impede adaptive evolutionary responses. Either way, the proposed study will provide important insights to both evolutionary processes and how environmental contaminants are affecting natural populations in unexpected ways.

Tim Mousseau, Columbia, SC