| Summary: | <p>Little is known about the evolution and ecology of crassulacean acid metabolism (CAM) in the genus <em>Puya</em> Molina. CAM is a photosynthetic pathway typified by nocturnal CO2 fixation and is regarded as a water-saving mechanism. <em>Puya</em> is one of the largest genera in the pineapple family (Bromeliaceae), with 226 species distributed across the Andes to Costa Rica and the Guiana Shield, and from sea level to 5000 m. About 21% of <em>Puya</em> species are CAM and at least 10 of these CAM species occur above 3000 m. The main aim of this thesis was to uncover new evidence to understand the ecophysiology and evolution of CAM in the montane genus <em>Puya</em>. The prevalence of CAM and C<sub>3</sub> species in <em>Puya</em> was estimated from carbon isotope values of 161 species. The climatic niche of constitutive CAM species and C<sub>3</sub> species of <em>Puya</em> was modelled using georeferenced herbarium records and climatic variables to evaluate the differences between their niches.</p> <p>The evolution of CAM in <em>Puya</em> was investigated by reconstructing the ancestral photosynthetic pathway on an AFLP phylogeny and by studying positive selection in the genes encoding the key enzyme phospho<em>eno</em>lpyruvate carboxylase (PEPC). The coldresistance and the thermal lability of PEPC was investigated for high- and low- elevation CAM species of <em>Puya</em> to explore the potential molecular adaptations of CAM plants in high-elevation environments. The present study concludes that the common ancestor of <em>Puya</em> was a cold-resistant plant. This is suggested to explain the prevalence of <em>Puya</em> at highelevations.</p> <p>The evolution of CAM was correlated with changes in the climatic niche, and occurred multiple times in <em>Puya</em>. These multiple origins were not independent because the common ancestor of <em>Puya</em> was likely to be a weak CAM plant (based on a diagnostic Arg679 residue in the PEPC sequence). It is likely that populations of <em>P. chilensis</em> gained CAM by introgression with <em>P. alpestris</em> ssp. <em>zoellneri</em>. Weak CAM photosynthesis and coldxv resistance allowed <em>Puya</em> to colonise the Andes from the south to the north; and, in the process, constitutive CAM and C<sub>3</sub> evolved. The later-evolving species in the genus are suggested to have lost their capacity for CAM as they radiated into more mesic habitats during their colonisation of the northern Andes.</p>
|
|---|