Collaborative Research: Causes of Parallel Molecular Evolution

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Molecular evolution starts from random changes in DNA, but then natural selection eliminates some of the changes and retains the others. An important question concerns the adaptation of different species to a similar environmental challenge; would they do it through similar or different changes to the DNA? This project will address the causes of evolution, and the possibility to predict evolutionary change by studying birds living at high altitude in the Andes, and their close relatives living at low altitudes. The project will explore the ways in which the hemoglobins (the oxygen carrying proteins of the blood) have changed to enable the uptake and release of oxygen under different atmospheric conditions (high and low altitude). This project will provide interdisciplinary training for postdoctoral and graduate students and includes a public outreach component that will focus on 'adaptation to extreme environments' to teach elementary school students how evolution by natural selection works. All data and samples will be linked to specimen-vouchers at the Museum of Southwestern Biology (U. New Mexico) and will be accessible through an open-access data repository. This project is designed to assess the pervasiveness of parallel molecular evolution and to test hypotheses about its causes by examining mechanisms of hemoglobin (Hb) adaptation in multiple species that have independently colonized high-altitude environments. Experimental analyses of Hb function will be used to identify and characterize the molecular basis of evolved changes in Hb-O2 affinity in multiple bird species that have contrasting altitudinal range limits in the Andes. The comparisons will involve multiple phylogenetically replicated 'taxon pairs' (pairs of conspecific populations or closely related species that are native to high- or low-altitude). In cases where functionally distinct Hbs of high- and low-altitude taxa are distinguished by multiple amino acid substitutions, a combination of site-directed mutagenesis and ancestral protein resurrection will be used to examine the functional effects of sequential mutational steps in all possible pathways that connect the ancestral (low-affinity) Hb and the derived (high-affinity) Hb. In addition to measuring oxygenation properties of each engineered Hb mutant, additional functional and structural properties that can potentially trade-off with Hb-O2 affinity will also be measured. By relating the results of the site-directed mutagenesis experiments to phylogenetically replicated changes in Hb function, this protein-engineering analysis will illuminate the role of pleiotropic constraints in promoting parallel molecular evolution.