Molecular adaptation in Rubisco: discriminating between convergent evolution and positive selection using mechanistic and classical codon models

Abstract

Rubisco (Ribulose-1, 5-biphosphate carboxylase/oxygenase) is the most important enzyme on earth, catalyzing the first step of CO2 fixation in photosynthesis. Its molecular adaptation to C4 photosynthetic pathway has attracted a lot of attention. C4 plants, which comprise less than 5% of land plants, have evolved more efficient photosynthesis compared to C3 plants. Interestingly, a large number of independent transitions from C3 to C4 phenotype have occurred. Each time, the Rubisco enzyme has been subject to similar changes in selective pressure, thus providing an excellent model for convergent evolution at the molecular level. Molecular adaptation is often identified with positive selection and is typically characterized by an elevated ratio of non-synonymous over synonymous substitution rates (dN/dS). However, convergent adaptation is expected to leave a different molecular signature, taking the form of repeated transitions toward identical or similar amino acids.\n\nHere, we use a previously introduced codon-based differential selection model to detect and quantify consistent patterns of convergent adaptation in Rubisco in Amaranthaceae. We further contrast the results thus obtained with those obtained under classical codon models based on the estimation of dN/dS. We find that the two classes of models tend to select distinct, although overlapping, sets of positions. This discrepancy in the results illustrates the conceptual difference between these models, while emphasizing the need to better discriminate between qualitatively different selective regimes, by using a broader class of codon models than those currently considered in molecular evolutionary studies.