In the U.K. between 1957 and 1992, herbicides used along hundreds of kilometres of railway track provided strong selection for plants with a chloroplast mutation known to confer resistance. When geneticist Pádraic Flood identified herbicide resistance genes in Arabidopsis thaliana sampled in Cornwall400 km from where they were first identified in 1988he and his colleagues at the Max Planck Institute for Plant Breeding and Research saw a rare opportunity to study indirect, genome-wide effects of adaptive selection on an organelle. Flood found that the adaptive mutation had arisen just once in railside populations of A. thaliana, and that these populations now had low genetic diversity across the entire genome. Theres been a massive increase [in the frequency of] that nuclear genomewhich is not doing anything particularly specialmerely because of its association with this organelle that provides resistance to herbicide, Flood explains. The nuclear genome isnt the only thing hitching a ride. Flood found the same genotype all along that 400 km of rail network, implying that trains and their passengers helped spread the herbicide-resistant plants. This spread will have a lasting consequence, Flood suspects, because now this nuclear genome and its associated alleles are at a high frequency. The scale of the impact is really quite striking, says Stephen Wright of the University of Toronto, adding that although the concept of nuclear genes hitchhiking on organellar mutations is uncontroversial in self-fertilising species like Arabidopsis, such processes are rarely observed in nature. Flood suspects the phenomenon may be more common than realised, and hopes to investigate it in outcrossing species with the same adaptive chloroplast mutation.
The Scientist, 1 August 2016 ;http://www.the-scientist.com ;