The Patterns and Causes of Variation in Plant Nucleotide Substitution Rates

Early studies of protein evolution established that evolutionary rates vary among sites, genes, and lineages. Here, we review rate variation within each of these three components, with an emphasis on plant nuclear genes. Evolutionary rates vary among nucleotide sites as a consequence of selection and mutational biases. Mutation varies among sites as a function of the base, its neighboring bases, and its proximity to insertion-deletion polymorphisms. Nonsynonymous rate variation among genes correlates most strongly with gene expression, perhaps owing to selection for translational robustness. Among lineages, perennial plants evolve more slowly than annuals, but the mechanism driving this effect remains unclear. There are also interactions among these three main components; we discuss the patterns of interaction and their significance for understanding the evolutionary forces that shape nucleotide substitution rates in plants. Finally, we highlight gaps in our knowledge and future opportunities to answer fundamental questions in plant molecular evolution.