Migration to colder climates in grasses involves pre-existing and adaptive traits

Abstract

A large number of transitions between climates have been found in phylogenetic trees, but some clades with particular features are more likely to shift to new climates than others. Several traits have previously been associated with these multiple transitions. In order to understand how traits are involved in transitions between climates, the enabling effects of traits on transitions and the evolution of traits in new climates need to be identified. The diversity of species that have made multiple transitions between climates in grasses allows the phylogenetic comparative method to be used to address this issue. In this thesis, I investigated the morphological, physiological and genomic traits which have been hypothesised to be enablers of, or adaptations to, the transitions between climates. I provide evidence that pre-existing traits are important factors in facilitating the migration between climates. Using a biogeographical analysis, I first showed that the evolution of C4 photosynthesis in tropical climates facilitates transitions into cooler climates and expansion into warmer climates. This is consistent with modelling analyses which show that the benefits of C4 photosynthesis for canopy carbon gain are maximised at high temperatures, but remain significant at low temperatures if leaves can resist chilling and freezing. Using an experimental approach, I next showed that transitions into cold climates were facilitated by additional pre-existing traits that provide constitutive chilling and freezing resistances. These also arise initially in tropical species. Freezing resistance was determined by osmotic pressure, moisture content and cold acclimation, which influence the transition to cold climates, while chilling resistance was associated with culm height and leaf width. Therefore the combined facilitating effects of pre-existing traits determined the initiation of transitions into new climates. However, the adaptive evolution of traits to improve the efficiency of a plant after migration to new environments may also be required for specialism in cold climates. I found that cold acclimation to increase freezing tolerance evolved after migration to cold climates. This suggests that cold acclimation may enhance the efficiency of freezing resistance. In particular, I found signatures of adaptive evolution to cold climates in chloroplast genes encoding proteins which function in the structural stability of the photosystems. These findings suggest that pre-existing traits facilitate migration to new climates, but their efficiency depends on the total effect of related traits. Once in new climates, the additional adaptive evolution of multiple traits is required for a plant to become successful