Methylbutenol (MBO) is a 5 carbon alcohol produced and emitted in large quantities (as much as 2% of photosynthesis) by some but not all species in the genus Pinus. The large cost associated with MBO emission suggests that MBO emission may confer a benefit upon those pines that are emitters. While the physiological benefit conferred by MBO emission has not been conclusively demonstrated, the numerous similarities between MBO emission and emission of the related compound isoprene suggest that MBO emission may play a thermoprotective role specifically against temperature fluctuations. However, it is unclear whether thermal protection provided the selective pressure favoring the evolution of MBO emission. A threefold approach was taken to investigate the evolution of MBO emission. First, ancestral character state reconstruction was employed to reconstruct the probable pattern of gains and losses of MBO emission. Second a model of correlated evolution was employed to examine the relationship between MBO emission and habitat characteristics. Third, the genome of P. taeda was searched for evidence of gene loss in this non-emitting species that is predicted to have evolved from an emitting ancestor.
Results/Conclusions
Reconstructing the evolutionary history of MBO emission and examining the correlated evolution of MBO emission and habitat of origin suggests that MBO emission evolved during the Eocene when pines were restricted to a mountainous refugium in North America. Following the Eocene, taxa that remained in mountainous habitats retained MBO emission while those that diversified into lowland habitats lost the ability to produce MBO. This model is supported by the discovery of a functional MBO synthase in the genome of the non-emitting species P. taeda. Examining modeled rates of evolution further show that MBO emission was more likely to evolve in mountainous habitats than in lowland habitats, and more likely to be lost in lowland habitats than in mountainous habitats. Since mountainous habitats have climates characterized by large daily temperature fluctuations, this pattern of evolution is consistent with the model of MBO emission evolving to provide thermal protection, and that in the absence of temperature fluctuations the expression of MBO synthase was lost in species inhabiting lowlands.